Table of Contents
Every expression written in the Java programming language either produces no result (§15.1) or has a type that can be deduced at compile time (§15.3). When an expression appears in most contexts, it must be compatible with a type expected in that context; this type is called the target type. For convenience, compatibility of an expression with its surrounding context is facilitated in two ways:
First, for some expressions, termed poly expressions (§15.2), the deduced type can be influenced by the target type. The same expression can have different types in different contexts.
Second, after the type of the expression has been deduced, an implicit conversion from the type of the expression to the target type can sometimes be performed.
If neither strategy is able to produce the appropriate type, a compile-time error occurs.
The rules determining whether an expression is a poly expression, and if so, its type and compatibility in a particular context, vary depending on the kind of context and the form of the expression. In addition to influencing the type of the expression, the target type may in some cases influence the run time behavior of the expression in order to produce a value of the appropriate type.
Similarly, the rules determining whether a target type allows an implicit conversion vary depending on the kind of context, the type of the expression, and, in one special case, the value of a constant expression (§15.28). A conversion from type S to type T allows an expression of type S to be treated at compile time as if it had type T instead. In some cases this will require a corresponding action at run time to check the validity of the conversion or to translate the run-time value of the expression into a form appropriate for the new type T.
Example 5.0-1. Conversions at Compile Time and Run Time
A conversion from type Object
to type Thread
requires a run-time check to make sure that the run-time value
is actually an instance of class Thread
or one of its
subclasses; if it is not, an exception is thrown.
A conversion from type Thread
to type Object
requires no run-time action; Thread
is a subclass of Object
,
so any reference produced by an expression of type Thread
is a
valid reference value of type Object
.
A conversion from type int
to type long
requires run-time sign-extension of a 32-bit integer value to
the 64-bit long
representation. No information is lost.
A conversion from type double
to type long
requires a non-trivial translation from a 64-bit floating-point
value to the 64-bit integer representation. Depending on the
actual run-time value, information may be lost.
The conversions possible in the Java programming language are grouped into several broad categories:
There are six kinds of conversion contexts in which poly expressions may be influenced by context or implicit conversions may occur. Each kind of context has different rules for poly expression typing and allows conversions in some of the categories above but not others. The contexts are:
Assignment contexts (§5.2, §15.26), in which an expression's value is bound to a named variable. Primitive and reference types are subject to widening, values may be boxed or unboxed, and some primitive constant expressions may be subject to narrowing. An unchecked conversion may also occur.
Strict invocation contexts (§5.3, §15.9, §15.12), in which an argument is bound to a formal parameter of a constructor or method. Widening primitive, widening reference, and unchecked conversions may occur.
Loose invocation contexts (§5.3, §15.9, §15.12), in which, like strict invocation contexts, an argument is bound to a formal parameter. Method or constructor invocations may provide this context if no applicable declaration can be found using only strict invocation contexts. In addition to widening and unchecked conversions, this context allows boxing and unboxing conversions to occur.
String contexts (§5.4,
§15.18.1), in which a value of any type is
converted to an object of type String
.
Casting contexts (§5.5), in which an expression's value is converted to a type explicitly specified by a cast operator (§15.16). Casting contexts are more inclusive than assignment or loose invocation contexts, allowing any specific conversion other than a string conversion, but certain casts to a reference type are checked for correctness at run time.
Numeric contexts (§5.6), in which the operands of a numeric operator may be widened to a common type so that an operation can be performed.
The term "conversion" is also used to describe, without being specific, any conversions allowed in a particular context. For example, we say that an expression that is the initializer of a local variable is subject to "assignment conversion", meaning that a specific conversion will be implicitly chosen for that expression according to the rules for the assignment context.
Example 5.0-2. Conversions In Various Contexts
class Test { public static void main(String[] args) { // Casting conversion (5.4) of a float literal to // type int. Without the cast operator, this would // be a compile-time error, because this is a // narrowing conversion (5.1.3): int i = (int)12.5f; // String conversion (5.4) of i's int value: System.out.println("(int)12.5f==" + i); // Assignment conversion (5.2) of i's value to type // float. This is a widening conversion (5.1.2): float f = i; // String conversion of f's float value: System.out.println("after float widening: " + f); // Numeric promotion (5.6) of i's value to type // float. This is a binary numeric promotion. // After promotion, the operation is float*float: System.out.print(f); f = f * i; // Two string conversions of i and f: System.out.println("*" + i + "==" + f); // Invocation conversion (5.3) of f's value // to type double, needed because the method Math.sin // accepts only a double argument: double d = Math.sin(f); // Two string conversions of f and d: System.out.println("Math.sin(" + f + ")==" + d); } }
This program produces the output:
(int)12.5f==12 after float widening: 12.0 12.0*12==144.0 Math.sin(144.0)==-0.49102159389846934
Specific type conversions in the Java programming language are divided into 13 categories.
A conversion from a type to that same type is permitted for any type.
This may seem trivial, but it has two practical consequences. First, it is always permitted for an expression to have the desired type to begin with, thus allowing the simply stated rule that every expression is subject to conversion, if only a trivial identity conversion. Second, it implies that it is permitted for a program to include redundant cast operators for the sake of clarity.
19 specific conversions on primitive types are called the widening primitive conversions:
A widening primitive conversion does not lose information about the overall magnitude of a numeric value in the following cases, where the numeric value is preserved exactly:
from float
to double
in a strictfp
expression (§15.4)
A widening primitive conversion
from float
to double
that is not strictfp
may lose information
about the overall magnitude of the converted value.
A
widening primitive conversion from int
to float
, or from long
to
float
, or from long
to double
, may result in loss of
precision - that is, the result may lose some of the least
significant bits of the value. In this case, the resulting
floating-point value will be a correctly rounded version of the
integer value, using IEEE 754 round-to-nearest mode
(§4.2.4).
A widening conversion of a signed integer value to an integral type T simply sign-extends the two's-complement representation of the integer value to fill the wider format.
A
widening conversion of a char
to an integral type T zero-extends
the representation of the char
value to fill the wider
format.
Despite the fact that loss of precision may occur, a widening primitive conversion never results in a run-time exception (§11.1.1).
Example 5.1.2-1. Widening Primitive Conversion
class Test { public static void main(String[] args) { int big = 1234567890; float approx = big; System.out.println(big - (int)approx); } }
This program prints:
-46
thus indicating that information was lost during the
conversion from type int
to type float
because values of type
float
are not precise to nine significant digits.
22 specific conversions on primitive types are called the narrowing primitive conversions:
A narrowing primitive conversion may lose information about the overall magnitude of a numeric value and may also lose precision and range.
A narrowing
primitive conversion from double
to float
is governed by the IEEE
754 rounding rules (§4.2.4). This conversion can
lose precision, but also lose range, resulting in a float
zero from
a nonzero double
and a float
infinity from a finite double
. A
double
NaN is converted to a float
NaN and a double
infinity is
converted to the same-signed float
infinity.
A narrowing conversion of a signed integer to an integral type T simply discards all but the n lowest order bits, where n is the number of bits used to represent type T. In addition to a possible loss of information about the magnitude of the numeric value, this may cause the sign of the resulting value to differ from the sign of the input value.
A
narrowing conversion of a char
to an integral type T likewise
simply discards all but the n lowest order bits, where n is
the number of bits used to represent type T. In addition to a
possible loss of information about the magnitude of the numeric value,
this may cause the resulting value to be a negative number, even
though chars represent 16-bit unsigned integer values.
A narrowing conversion of a floating-point number to an integral type T takes two steps:
In
the first step, the floating-point number is converted either to
a long
, if T is long
, or to an int
, if T is byte
,
short
, char
, or int
, as follows:
If the floating-point number is
NaN (§4.2.3), the result of the first
step of the conversion is an int
or long
0
.
Otherwise, if the floating-point
number is not an infinity, the floating-point value is
rounded to an integer value V
, rounding
toward zero using IEEE 754 round-toward-zero mode
(§4.2.3). Then there are two
cases:
Otherwise, one of the following two cases must be true:
The value must be too small
(a negative value of large magnitude or negative
infinity), and the result of the first step is the
smallest representable value of type int
or
long
.
The value must be too large
(a positive value of large magnitude or positive
infinity), and the result of the first step is the
largest representable value of type int
or
long
.
If T is int
or long
, the
result of the conversion is the result of the first
step.
If T is byte
, char
, or
short
, the result of the conversion is the result of a
narrowing conversion to type T
(§5.1.3) of the result of the first
step.
Despite the fact that overflow, underflow, or other loss of information may occur, a narrowing primitive conversion never results in a run-time exception (§11.1.1).
Example 5.1.3-1. Narrowing Primitive Conversion
class Test { public static void main(String[] args) { float fmin = Float.NEGATIVE_INFINITY; float fmax = Float.POSITIVE_INFINITY; System.out.println("long: " + (long)fmin + ".." + (long)fmax); System.out.println("int: " + (int)fmin + ".." + (int)fmax); System.out.println("short: " + (short)fmin + ".." + (short)fmax); System.out.println("char: " + (int)(char)fmin + ".." + (int)(char)fmax); System.out.println("byte: " + (byte)fmin + ".." + (byte)fmax); } }
This program produces the output:
long: -9223372036854775808..9223372036854775807 int: -2147483648..2147483647 short: 0..-1 char: 0..65535 byte: 0..-1
The results for char
, int
, and long
are
unsurprising, producing the minimum and maximum representable values
of the type.
The results for byte
and short
lose information
about the sign and magnitude of the numeric values and also lose
precision. The results can be understood by examining the low order
bits of the minimum and maximum int
. The minimum int
is, in
hexadecimal, 0x80000000
, and the maximum int
is 0x7fffffff
. This explains the short
results,
which are the low 16 bits of these values,
namely, 0x0000
and 0xffff
; it
explains the char results, which also are the low 16 bits of these
values, namely, '\u0000'
and '\uffff'
; and it explains the byte results,
which are the low 8 bits of these values,
namely, 0x00
and 0xff
.
Example 5.1.3-2. Narrowing Primitive Conversions that lose information
class Test { public static void main(String[] args) { // A narrowing of int to short loses high bits: System.out.println("(short)0x12345678==0x" + Integer.toHexString((short)0x12345678)); // An int value too big for byte changes sign and magnitude: System.out.println("(byte)255==" + (byte)255); // A float value too big to fit gives largest int value: System.out.println("(int)1e20f==" + (int)1e20f); // A NaN converted to int yields zero: System.out.println("(int)NaN==" + (int)Float.NaN); // A double value too large for float yields infinity: System.out.println("(float)-1e100==" + (float)-1e100); // A double value too small for float underflows to zero: System.out.println("(float)1e-50==" + (float)1e-50); } }
This program produces the output:
(short)0x12345678==0x5678 (byte)255==-1 (int)1e20f==2147483647 (int)NaN==0 (float)-1e100==-Infinity (float)1e-50==0.0
The following conversion combines both widening and narrowing primitive conversions:
First,
the byte
is converted to an int
via widening primitive conversion
(§5.1.2), and then the resulting int
is
converted to a char
by narrowing primitive conversion
(§5.1.3).
A widening reference conversion exists from any reference type S to any reference type T, provided S is a subtype (§4.10) of T.
Widening reference conversions never require a special action at run time and therefore never throw an exception at run time. They consist simply in regarding a reference as having some other type in a manner that can be proved correct at compile time.
Six kinds of conversions are called the narrowing reference conversions:
From any reference type S to any reference type T, provided that S is a proper supertype of T (§4.10).
An
important special case is that there is a narrowing reference
conversion from the class type Object
to any other reference
type (§4.12.4).
From
any class type C to any non-parameterized interface type K,
provided that C is not final
and does not implement
K.
From
any interface type J to any non-parameterized class type C
that is not final
.
From any interface type J to any non-parameterized interface type K, provided that J is not a subinterface of K.
From
the interface types Cloneable
and java.io.Serializable
to any array
type T[]
.
From
any array type SC[]
to any array type TC[]
,
provided that SC and TC are reference types and there is a
narrowing reference conversion from SC to TC.
Such
conversions require a test at run time to find out whether the actual
reference value is a legitimate value of the new type. If not, then a
ClassCastException
is thrown.
Boxing conversion converts expressions of primitive type to corresponding expressions of reference type. Specifically, the following nine conversions are called the boxing conversions:
From the null type to the null type
This rule is necessary because the conditional operator (§15.25) applies boxing conversion to the types of its operands, and uses the result in further calculations.
At run time, boxing conversion proceeds as follows:
If
p
is a value of type boolean
, then boxing conversion converts
p
into a reference r
of class and type Boolean
, such
that r
.booleanValue() == p
If
p
is a value of type byte
, then boxing conversion converts p
into a reference r
of class and type Byte
, such
that r
.byteValue() == p
If
p
is a value of type char
, then boxing conversion converts p
into a reference r
of class and type Character
, such
that r
.charValue() == p
If
p
is a value of type short
, then boxing conversion converts
p
into a reference r
of class and type Short
, such
that r
.shortValue() == p
If
p
is a value of type int
, then boxing conversion converts p
into a reference r
of class and type Integer
, such
that r
.intValue() == p
If
p
is a value of type long
, then boxing conversion converts p
into a reference r
of class and type Long
, such
that r
.longValue() == p
If
p
is a value of any other type, boxing conversion is equivalent
to an identity conversion (§5.1.1).
If the value
p
being boxed is an integer literal of type int
between -128
and 127
inclusive
(§3.10.1), or the boolean literal true
or
false
(§3.10.3), or a character literal
between '\u0000'
and '\u007f'
inclusive (§3.10.4), then let a
and b
be the
results of any two boxing conversions of p
. It is always the case
that a
==
b
.
Ideally, boxing a primitive value would always yield
an identical reference. In practice, this may not be feasible using
existing implementation techniques. The rule above is a pragmatic
compromise, requiring that certain common values always be boxed into
indistinguishable objects. The implementation may cache these, lazily
or eagerly. For other values, the rule disallows any assumptions about
the identity of the boxed values on the programmer's part. This allows
(but does not require) sharing of some or all of these
references. Notice that integer literals of type long
are allowed,
but not required, to be shared.
This ensures that in most common cases, the behavior
will be the desired one, without imposing an undue performance
penalty, especially on small devices. Less memory-limited
implementations might, for example, cache all char
and short
values, as well as int
and long
values in the range of -32K to
+32K.
A boxing
conversion may result in an OutOfMemoryError
if a new instance of one of the
wrapper classes (Boolean
, Byte
, Character
, Short
, Integer
,
Long
, Float
, or Double
) needs to be allocated and insufficient
storage is available.
Unboxing conversion converts expressions of reference type to corresponding expressions of primitive type. Specifically, the following eight conversions are called the unboxing conversions:
At run time, unboxing conversion proceeds as follows:
If
r
is a reference of type Boolean
, then unboxing conversion
converts r
into r
.booleanValue()
If
r
is a reference of type Byte
, then unboxing conversion
converts r
into r
.byteValue()
If
r
is a reference of type Character
, then unboxing conversion
converts r
into r
.charValue()
If
r
is a reference of type Short
, then unboxing conversion
converts r
into r
.shortValue()
If
r
is a reference of type Integer
, then unboxing conversion
converts r
into r
.intValue()
If
r
is a reference of type Long
, then unboxing conversion
converts r
into r
.longValue()
If
r
is a reference of type Float
, unboxing conversion converts
r
into r
.floatValue()
If
r
is a reference of type Double
, then unboxing conversion
converts r
into r
.doubleValue()
If
r
is null
, unboxing conversion throws a NullPointerException
A type is said to be convertible to a numeric type if it is a numeric type (§4.2), or it is a reference type that may be converted to a numeric type by unboxing conversion.
A type is said to be convertible to an integral type if it is an integral type, or it is a reference type that may be converted to an integral type by unboxing conversion.
Let G name a generic type declaration with n type parameters.
There is
an unchecked conversion from the
raw class or interface type
(§4.8) G to any parameterized type of the form
G<
T_{1},...,T_{n}>
.
There is
an unchecked conversion from the raw array type
G[]
^{k} to any array type of the
form
G<
T_{1},...,T_{n}>
[]
^{k}.
(The notation []
^{k} indicates an
array type of k dimensions.)
Use of an unchecked conversion
causes a compile-time unchecked
warning unless all type arguments T_{i} (1 ≤ i
≤ n) are unbounded wildcards (§4.5.1),
or the unchecked warning is suppressed by the SuppressWarnings
annotation (§9.6.4.5).
Unchecked conversion is used to enable a smooth
interoperation of legacy code, written before the introduction of
generic types, with libraries that have undergone a conversion to use
genericity (a process we call generification). In such circumstances
(most notably, clients of the Collections Framework
in java.util
), legacy code uses raw types
(e.g. Collection
instead
of Collection<String>
). Expressions of raw
types are passed as arguments to library methods that use
parameterized versions of those same types as the types of their
corresponding formal parameters.
Such calls cannot be shown to be statically safe under the type system using generics. Rejecting such calls would invalidate large bodies of existing code, and prevent them from using newer versions of the libraries. This in turn, would discourage library vendors from taking advantage of genericity. To prevent such an unwelcome turn of events, a raw type may be converted to an arbitrary invocation of the generic type declaration to which the raw type refers. While the conversion is unsound, it is tolerated as a concession to practicality. An unchecked warning is issued in such cases.
Let G name a generic type declaration (§8.1.2, §9.1.2) with n type parameters A_{1},...,A_{n} with corresponding bounds U_{1},...,U_{n}.
There exists a capture conversion
from a parameterized type
G<
T_{1},...,T_{n}>
(§4.5)
to a parameterized type
G<
S_{1},...,S_{n}>
, where, for 1 ≤ i ≤
n :
If
T_{i} is a wildcard type argument (§4.5.1) of
the form ?
, then S_{i} is a fresh type variable whose upper
bound is U_{i}[A_{1}:=S_{1},...,A_{n}:=S_{n}]
and
whose lower bound is the null type
(§4.1).
If
T_{i} is a wildcard type argument of the form ?
extends
B_{i}, then S_{i} is a fresh type variable whose upper bound is
glb(B_{i}, U_{i}[A_{1}:=S_{1},...,A_{n}:=S_{n}]
) and
whose lower bound is the null type.
glb(V_{1},...,V_{m})
is defined as V_{1} &
... &
V_{m}.
It is a compile-time error if, for any two classes (not interfaces) V_{i} and V_{j}, V_{i} is not a subclass of V_{j} or vice versa.
If
T_{i} is a wildcard type argument of the form ?
super
B_{i},
then S_{i} is a fresh type variable whose upper bound is
U_{i}[A_{1}:=S_{1},...,A_{n}:=S_{n}]
and whose lower
bound is B_{i}.
Capture conversion on any type other than a parameterized type (§4.5) acts as an identity conversion (§5.1.1).
Capture conversion is not applied recursively.
Capture conversion never requires a special action at run time and therefore never throws an exception at run time.
Capture conversion is designed to make wildcards
more useful. To understand the motivation, let's begin by looking at
the method java.util.Collections.reverse()
:
public static void reverse(List<?> list);
The method reverses the list provided as a
parameter. It works for any type of list, and so the use of the
wildcard type List<?>
as the type of the
formal parameter is entirely appropriate.
Now consider how one would
implement reverse()
:
public static void reverse(List<?> list) { rev(list); } private static <T> void rev(List<T> list) { List<T> tmp = new ArrayList<T>(list); for (int i = 0; i < list.size(); i++) { list.set(i, tmp.get(list.size() - i - 1)); } }
The implementation needs to copy the list, extract
elements from the copy, and insert them into the original. To do this
in a type-safe manner, we need to give a name, T
,
to the element type of the incoming list. We do this in the private
service method rev()
. This requires us to pass the
incoming argument list, of type List<?>
, as
an argument to rev()
. In
general, List<?>
is a list of unknown
type. It is not a subtype of List<T>
, for any
type T. Allowing such a subtype relation would be unsound. Given the
method:
public static <T> void fill(List<T> l, T obj)
the following code would undermine the type system:
List<String> ls = new ArrayList<String>(); List<?> l = ls; Collections.fill(l, new Object()); // not legal - but assume it was! String s = ls.get(0); // ClassCastException - ls contains // Objects, not Strings.
So, without some special dispensation, we can see
that the call from reverse()
to rev()
would be disallowed. If this were the
case, the author of reverse()
would be forced to
write its signature as:
public static <T> void reverse(List<T> list)
This is undesirable, as it exposes implementation information to the caller. Worse, the designer of an API might reason that the signature using a wildcard is what the callers of the API require, and only later realize that a type safe implementation was precluded.
The call from reverse()
to rev()
is in fact harmless, but it cannot be
justified on the basis of a general subtyping relation
between List<?>
and List<T>
. The call is harmless, because
the incoming argument is doubtless a list of some type (albeit an
unknown one). If we can capture this unknown type in a type
variable X
, we can infer T
to
be X
. That is the essence of capture
conversion. The specification of course must cope with complications,
like non-trivial (and possibly recursively defined) upper or lower
bounds, the presence of multiple arguments etc.
Mathematically sophisticated readers will want to relate capture conversion to established type theory. Readers unfamiliar with type theory can skip this discussion - or else study a suitable text, such as Types and Programming Languages by Benjamin Pierce, and then revisit this section.
Here then is a brief summary of the relationship of
capture conversion to established type theoretical notions. Wildcard
types are a restricted form of existential types. Capture conversion
corresponds loosely to an opening of a value of existential type. A
capture conversion of an expression e
can be
thought of as an open
of e
in a
scope that comprises the top level expression that
encloses e
.
The classical open
operation on
existentials requires that the captured type variable must not escape
the opened expression. The open
that corresponds to
capture conversion is always on a scope sufficiently large that the
captured type variable can never be visible outside that scope. The
advantage of this scheme is that there is no need for
a close
operation, as defined in the
paper On Variance-Based Subtyping for Parametric
Types by Atsushi Igarashi and Mirko Viroli, in the
proceedings of the 16th European Conference on Object Oriented
Programming (ECOOP 2002). For a formal account of wildcards,
see Wild FJ by Mads Torgersen, Erik Ernst and
Christian Plesner Hansen, in the 12th workshop on Foundations of
Object Oriented Programming (FOOL 2005).
Any type
may be converted to type String
by string
conversion.
A
value x
of primitive type T is first converted to
a reference value as if by giving it as an argument to an appropriate
class instance creation expression
(§15.9):
This
reference value is then converted to type String
by string
conversion.
Now only reference values need to be considered:
If
the reference is null
, it is converted to the string "null
"
(four ASCII characters n
, u
,
l
, l
).
Otherwise, the conversion is performed
as if by an invocation of the toString
method of the referenced
object with no arguments; but if the result of invoking the
toString
method is null
, then the string "null
" is used
instead.
The toString
method is defined by the
primordial class Object
(§4.3.2). Many
classes override it, notably Boolean
, Character
, Integer
,
Long
, Float
, Double
, and String
.
See §5.4 for details of the string context.
Value set conversion is the process of mapping a floating-point value from one value set to another without changing its type.
Within an expression that is not FP-strict (§15.4), value set conversion provides choices to an implementation of the Java programming language:
If the value is an element of the float-extended-exponent value set, then the implementation may, at its option, map the value to the nearest element of the float value set. This conversion may result in overflow (in which case the value is replaced by an infinity of the same sign) or underflow (in which case the value may lose precision because it is replaced by a denormalized number or zero of the same sign).
If the value is an element of the double-extended-exponent value set, then the implementation may, at its option, map the value to the nearest element of the double value set. This conversion may result in overflow (in which case the value is replaced by an infinity of the same sign) or underflow (in which case the value may lose precision because it is replaced by a denormalized number or zero of the same sign).
Within an FP-strict expression (§15.4), value set conversion does not provide any choices; every implementation must behave in the same way:
If
the value is of type float
and is not an element of the float
value set, then the implementation must map the value to the
nearest element of the float value set. This conversion may
result in overflow or underflow.
If
the value is of type double
and is not an element of the
double value set, then the implementation must map the value to
the nearest element of the double value set. This conversion may
result in overflow or underflow.
Within an FP-strict expression, mapping values from the float-extended-exponent value set or double-extended-exponent value set is necessary only when a method is invoked whose declaration is not FP-strict and the implementation has chosen to represent the result of the method invocation as an element of an extended-exponent value set.
Whether
in FP-strict code or code that is not FP-strict, value set conversion
always leaves unchanged any value whose type is neither float
nor
double
.
Assignment contexts allow the value of an expression to be assigned (§15.26) to a variable; the type of the expression must be converted to the type of the variable.
Assignment contexts allow the use of one of the following:
If, after the conversions listed above have been applied, the resulting type is a raw type (§4.8), an unchecked conversion (§5.1.9) may then be applied.
In addition,
if the expression is a constant expression
(§15.28) of type byte
, short
, char
, or
int
:
The compile-time narrowing of constant expressions means that code such as:
byte theAnswer = 42;
is allowed. Without the narrowing, the fact that the
integer literal 42
has type int
would mean that a
cast to byte
would be required:
byte theAnswer = (byte)42; // cast is permitted but not required
Finally, a value of the null type (the null reference is the only such value) may be assigned to any reference type, resulting in a null reference of that type.
It is a compile-time error if the chain of conversions contains two parameterized types that are not in the subtype relation (§4.10).
An example of such an illegal chain would be:
Integer, Comparable<Integer>, Comparable, Comparable<String>
The first three elements of the chain are related by
widening reference conversion, while the last entry is derived from
its predecessor by unchecked conversion. However, this is not a valid
assignment conversion, because the chain contains two parameterized
types, Comparable<Integer>
and Comparable<String>
, that are not
subtypes.
If the type of the expression cannot be converted to the type of the variable by a conversion permitted in an assignment context, then a compile-time error occurs.
If the type of an expression can be converted to the type of a variable by assignment conversion, we say the expression (or its value) is assignable to the variable or, equivalently, that the type of the expression is assignment compatible with the type of the variable.
If the type
of the variable is float
or double
, then value set conversion
(§5.1.13) is applied to the value v
that is the
result of the conversion(s):
If v
is of type float
and is an element of the
float-extended-exponent value set, then the implementation must
map v
to the nearest element of the float value set. This
conversion may result in overflow or underflow.
If v
is of type double
and is an element of the
double-extended-exponent value set, then the implementation must
map v
to the nearest element of the double value set. This
conversion may result in overflow or underflow.
The only exceptions that may arise from conversions in an assignment context are:
A ClassCastException
if, after the conversions above have been applied, the
resulting value is an object which is not an instance of a
subclass or subinterface of the erasure
(§4.6) of the type of the variable.
This circumstance can only arise as a result of heap pollution (§4.12.2). In practice, implementations need only perform casts when accessing a field or method of an object of parameterized type when the erased type of the field, or the erased return type of the method, differ from its unerased type.
A NullPointerException
as a result of an unboxing conversion on a null
reference.
An ArrayStoreException
in special cases involving array elements or field
access (§10.5,
§15.26.1).
Example 5.2-1. Assignment Conversion for Primitive Types
class Test { public static void main(String[] args) { short s = 12; // narrow 12 to short float f = s; // widen short to float System.out.println("f=" + f); char c = '\u0123'; long l = c; // widen char to long System.out.println("l=0x" + Long.toString(l,16)); f = 1.23f; double d = f; // widen float to double System.out.println("d=" + d); } }
This program produces the output:
f=12.0 l=0x123 d=1.2300000190734863
The following program, however, produces compile-time errors:
class Test { public static void main(String[] args) { short s = 123; char c = s; // error: would require cast s = c; // error: would require cast } }
because not all short
values are char
values,
and neither are all char
values short
values.
Example 5.2-2. Assignment Conversion for Reference Types
class Point { int x, y; } class Point3D extends Point { int z; } interface Colorable { void setColor(int color); } class ColoredPoint extends Point implements Colorable { int color; public void setColor(int color) { this.color = color; } } class Test { public static void main(String[] args) { // Assignments to variables of class type: Point p = new Point(); p = new Point3D(); // OK because Point3D is a subclass of Point Point3D p3d = p; // Error: will require a cast because a Point // might not be a Point3D (even though it is, // dynamically, in this example.) // Assignments to variables of type Object: Object o = p; // OK: any object to Object int[] a = new int[3]; Object o2 = a; // OK: an array to Object // Assignments to variables of interface type: ColoredPoint cp = new ColoredPoint(); Colorable c = cp; // OK: ColoredPoint implements Colorable // Assignments to variables of array type: byte[] b = new byte[4]; a = b; // Error: these are not arrays of the same primitive type Point3D[] p3da = new Point3D[3]; Point[] pa = p3da; // OK: since we can assign a Point3D to a Point p3da = pa; // Error: (cast needed) since a Point // can't be assigned to a Point3D } }
The following test program illustrates assignment conversions on reference values, but fails to compile, as described in its comments. This example should be compared to the preceding one.
class Point { int x, y; } interface Colorable { void setColor(int color); } class ColoredPoint extends Point implements Colorable { int color; public void setColor(int color) { this.color = color; } } class Test { public static void main(String[] args) { Point p = new Point(); ColoredPoint cp = new ColoredPoint(); // Okay because ColoredPoint is a subclass of Point: p = cp; // Okay because ColoredPoint implements Colorable: Colorable c = cp; // The following cause compile-time errors because // we cannot be sure they will succeed, depending on // the run-time type of p; a run-time check will be // necessary for the needed narrowing conversion and // must be indicated by including a cast: cp = p; // p might be neither a ColoredPoint // nor a subclass of ColoredPoint c = p; // p might not implement Colorable } }
Example 5.2-3. Assignment Conversion for Array Types
class Point { int x, y; } class ColoredPoint extends Point { int color; } class Test { public static void main(String[] args) { long[] veclong = new long[100]; Object o = veclong; // okay Long l = veclong; // compile-time error short[] vecshort = veclong; // compile-time error Point[] pvec = new Point[100]; ColoredPoint[] cpvec = new ColoredPoint[100]; pvec = cpvec; // okay pvec[0] = new Point(); // okay at compile time, // but would throw an // exception at run time cpvec = pvec; // compile-time error } }
In this example:
The value of veclong
cannot
be assigned to a Long
variable, because Long
is a class type
other than Object
. An array can be assigned only to a variable
of a compatible array type, or to a variable of type Object
,
Cloneable
or java.io.Serializable
.
The value of veclong
cannot
be assigned to vecshort
, because they are
arrays of primitive type, and short
and long
are not the
same primitive type.
The value of cpvec
can be
assigned to pvec
, because any reference that
could be the value of an expression of
type ColoredPoint
can be the value of a
variable of type Point
. The subsequent
assignment of the new Point
to a component
of pvec
then would throw an ArrayStoreException
(if the
program were otherwise corrected so that it could be compiled),
because a ColoredPoint
array cannot have an
instance of Point
as the value of a
component.
The value of pvec
cannot be
assigned to cpvec
, because not every
reference that could be the value of an expression of
type ColoredPoint
can correctly be the value
of a variable of type Point
. If the value
of pvec
at run time were a reference to an
instance of Point[]
, and the assignment
to cpvec
were allowed, a simple reference to
a component of cpvec
,
say, cpvec[0]
, could return
a Point
, and a Point
is
not a ColoredPoint
. Thus to allow such an
assignment would allow a violation of the type system. A cast
may be used (§5.5,
§15.16) to ensure
that pvec
references
a ColoredPoint[]
:
cpvec = (ColoredPoint[])pvec; // OK, but may throw an // exception at run time
Invocation contexts allow an argument value in a method or constructor invocation (§8.8.7.1, §15.9, §15.12) to be assigned to a corresponding formal parameter.
Strict invocation contexts allow the use of one of the following:
Loose invocation contexts allow a more permissive set of conversions, because they are only used for a particular invocation if no applicable declaration can be found using strict invocation contexts. Loose invocation contexts allow the use of one of the following:
If, after the conversions listed for an invocation context have been applied, the resulting type is a raw type (§4.8), an unchecked conversion (§5.1.9) may then be applied.
A value of the null type (the null reference is the only such value) may be assigned to any reference type.
It is a compile-time error if the chain of conversions contains two parameterized types that are not in the subtype relation (§4.10).
If the type of the expression cannot be converted to the type of the parameter by a conversion permitted in a loose invocation context, then a compile-time error occurs.
If the type
of an argument expression is either float
or double
, then value
set conversion (§5.1.13) is applied after the
conversion(s):
If an argument value of type float
is an element of the
float-extended-exponent value set, then the implementation must
map the value to the nearest element of the float value
set. This conversion may result in overflow or underflow.
If an argument value of type double
is an element of the
double-extended-exponent value set, then the implementation must
map the value to the nearest element of the double value
set. This conversion may result in overflow or underflow.
The only exceptions that may arise in an invocation context are:
A ClassCastException
if, after the type conversions above have been applied,
the resulting value is an object which is not an instance of a
subclass or subinterface of the erasure
(§4.6) of the corresponding formal
parameter type.
A NullPointerException
as a result of an unboxing conversion on a null
reference.
Neither strict nor loose invocation contexts include the implicit narrowing of integer constant expressions which is allowed in assignment contexts. The designers of the Java programming language felt that including these implicit narrowing conversions would add additional complexity to the rules of overload resolution (§15.12.2).
Thus, the program:
class Test { static int m(byte a, int b) { return a+b; } static int m(short a, short b) { return a-b; } public static void main(String[] args) { System.out.println(m(12, 2)); // compile-time error } }
causes a compile-time error because the integer
literals 12
and 2
have type
int
, so neither method m
matches under the rules
of overload resolution. A language that included implicit narrowing of
integer constant expressions would need additional rules to resolve
cases like this example.
String contexts
apply only to an operand of the binary +
operator which is not a
String
when the other operand is a String
.
The target type
in these contexts is always String
, and a string conversion
(§5.1.11) of the non-String
operand always
occurs. Evaluation of the +
operator then proceeds as specified
in §15.18.1.
Casting contexts allow the operand of a cast operator (§15.16) to be converted to the type explicitly named by the cast operator.
Casting contexts allow the use of one of:
an identity conversion (§5.1.1)
a widening primitive conversion (§5.1.2)
a narrowing primitive conversion (§5.1.3)
a widening and narrowing primitive conversion (§5.1.4)
a widening reference conversion (§5.1.5) optionally followed by either an unboxing conversion (§5.1.8) or an unchecked conversion (§5.1.9)
a narrowing reference conversion (§5.1.6) optionally followed by either an unboxing conversion (§5.1.8) or an unchecked conversion (§5.1.9)
a boxing conversion (§5.1.7) optionally followed by a widening reference conversion (§5.1.5)
an unboxing conversion (§5.1.8) optionally followed by a widening primitive conversion (§5.1.2).
Value set conversion (§5.1.13) is applied after the type conversion.
The compile-time legality of a casting conversion is as follows:
An expression of a primitive type may undergo casting conversion to another primitive type, by an identity conversion (if the types are the same), or by a widening primitive conversion, or by a narrowing primitive conversion, or by a widening and narrowing primitive conversion.
An expression of a primitive type may undergo casting conversion to a reference type without error, by boxing conversion.
An expression of a reference type may undergo casting conversion to a primitive type without error, by unboxing conversion.
An expression of a reference type may undergo casting conversion to another reference type if no compile-time error occurs given the rules in §5.5.1.
The following tables enumerate which conversions are used in certain casting conversions. Each conversion is signified by a symbol:
≈ signifies identity conversion (§5.1.1)
ω signifies widening primitive conversion (§5.1.2)
η signifies narrowing primitive conversion (§5.1.3)
ωη signifies widening and narrowing primitive conversion (§5.1.4)
⇑ signifies widening reference conversion (§5.1.5)
⇓ signifies narrowing reference conversion (§5.1.6)
⊕ signifies boxing conversion (§5.1.7)
⊗ signifies unboxing conversion (§5.1.8)
In the tables, a comma between symbols indicates that a
casting conversion uses one conversion followed by another. The type
Object
means any reference type other than the eight wrapper classes
Boolean
, Byte
, Short
, Character
, Integer
, Long
, Float
,
Double
.
Table 5.5-A. Casting conversions to primitive types
To → | byte |
short |
char |
int |
long |
float |
double |
boolean |
---|---|---|---|---|---|---|---|---|
From ↓ | ||||||||
byte |
≈ | ω | ωη | ω | ω | ω | ω | - |
short |
η | ≈ | η | ω | ω | ω | ω | - |
char |
η | η | ≈ | ω | ω | ω | ω | - |
int |
η | η | η | ≈ | ω | ω | ω | - |
long |
η | η | η | η | ≈ | ω | ω | - |
float |
η | η | η | η | η | ≈ | ω | - |
double |
η | η | η | η | η | η | ≈ | - |
boolean |
- | - | - | - | - | - | - | ≈ |
Byte |
⊗ | ⊗,ω | - | ⊗,ω | ⊗,ω | ⊗,ω | ⊗,ω | - |
Short |
- | ⊗ | - | ⊗,ω | ⊗,ω | ⊗,ω | ⊗,ω | - |
Character |
- | - | ⊗ | ⊗,ω | ⊗,ω | ⊗,ω | ⊗,ω | - |
Integer |
- | - | - | ⊗ | ⊗,ω | ⊗,ω | ⊗,ω | - |
Long |
- | - | - | - | ⊗ | ⊗,ω | ⊗,ω | - |
Float |
- | - | - | - | - | ⊗ | ⊗,ω | - |
Double |
- | - | - | - | - | - | ⊗ | - |
Boolean |
- | - | - | - | - | - | - | ⊗ |
Object |
⇓,⊗ | ⇓,⊗ | ⇓,⊗ | ⇓,⊗ | ⇓,⊗ | ⇓,⊗ | ⇓,⊗ | ⇓,⊗ |
Table 5.5-B. Casting conversions to reference types
To → | Byte |
Short |
Character |
Integer |
Long |
Float |
Double |
Boolean |
Object |
---|---|---|---|---|---|---|---|---|---|
From ↓ | |||||||||
byte |
⊕ | - | - | - | - | - | - | - | ⊕,⇑ |
short |
- | ⊕ | - | - | - | - | - | - | ⊕,⇑ |
char |
- | - | ⊕ | - | - | - | - | - | ⊕,⇑ |
int |
- | - | - | ⊕ | - | - | - | - | ⊕,⇑ |
long |
- | - | - | - | ⊕ | - | - | - | ⊕,⇑ |
float |
- | - | - | - | - | ⊕ | - | - | ⊕,⇑ |
double |
- | - | - | - | - | - | ⊕ | - | ⊕,⇑ |
boolean |
- | - | - | - | - | - | - | ⊕ | ⊕,⇑ |
Byte |
≈ | - | - | - | - | - | - | - | ⇑ |
Short |
- | ≈ | - | - | - | - | - | - | ⇑ |
Character |
- | - | ≈ | - | - | - | - | - | ⇑ |
Integer |
- | - | - | ≈ | - | - | - | - | ⇑ |
Long |
- | - | - | - | ≈ | - | - | - | ⇑ |
Float |
- | - | - | - | - | ≈ | - | - | ⇑ |
Double |
- | - | - | - | - | - | ≈ | - | ⇑ |
Boolean |
- | - | - | - | - | - | - | ≈ | ⇑ |
Object |
⇓ | ⇓ | ⇓ | ⇓ | ⇓ | ⇓ | ⇓ | ⇓ | ≈ |
Given a compile-time reference type S (source) and a compile-time reference type T (target), a casting conversion exists from S to T if no compile-time errors occur due to the following rules.
If
T is a class type, then either |S| <:
|T|, or |T|
<:
|S|. Otherwise, a compile-time error occurs.
Furthermore, if there exists a supertype X of T, and a supertype Y of S, such that both X and Y are provably distinct parameterized types (§4.5), and that the erasures of X and Y are the same, a compile-time error occurs.
If S is not a final
class
(§8.1.1), then, if there exists a
supertype X of T, and a supertype Y of S, such that
both X and Y are provably distinct parameterized types,
and that the erasures of X and Y are the same, a
compile-time error occurs.
Otherwise, the cast is always legal at compile time (because even if S does not implement T, a subclass of S might).
If S is a final
class
(§8.1.1), then S must implement T,
or a compile-time error occurs.
If T is a type variable, then this algorithm is applied recursively, using the upper bound of T in place of T.
If
T is an array type, then S must be the class Object
, or a
compile-time error occurs.
If T is an
intersection type, T_{1} &
... &
T_{n}, then it is a
compile-time error if there exists a T_{i} (1 ≤ i ≤
n) such that S cannot be cast to T_{i} by this
algorithm. That is, the success of the cast is determined by the
most restrictive component of the intersection type.
If T is an array type, then S must be the type java.io.Serializable
or
Cloneable
(the only interfaces implemented by
arrays), or a compile-time error occurs.
If T is a class or interface type that is not final
(§8.1.1), then if there exists a supertype
X of T, and a supertype Y of S, such that both X and
Y are provably distinct parameterized types, and that the
erasures of X and Y are the same, a compile-time error
occurs.
Otherwise, the cast is always legal at compile time (because even if T does not implement S, a subclass of T might).
If T is a class type that is final
, then:
If S is not a parameterized type or a raw type, then T must implement S, or a compile-time error occurs.
Otherwise, S is either a parameterized type that is an invocation of some generic type declaration G, or a raw type corresponding to a generic type declaration G. Then there must exist a supertype X of T, such that X is an invocation of G, or a compile-time error occurs.
Furthermore, if S and X are provably distinct parameterized types then a compile-time error occurs.
If T is a type variable, then this algorithm is applied recursively, using the upper bound of T in place of T.
If T is an intersection type, T_{1} &
... &
T_{n}, then
it is a compile-time error if there exists a T_{i} (1 ≤
i ≤ n) such that S cannot be cast to T_{i} by
this algorithm.
If S is a type variable, then this algorithm is applied recursively, using the upper bound of S in place of S.
If S is an
intersection type A_{1} &
... &
A_{n}, then it is a compile-time
error if there exists an A_{i} (1 ≤ i ≤ n) such that
A_{i} cannot be cast to T by this algorithm. That is, the success of
the cast is determined by the most restrictive component of the
intersection type.
If S is
an array type SC[]
, that is, an array of components of type
SC:
If
T is a class type, then if T is not Object
, then a
compile-time error occurs (because Object
is the only class
type to which arrays can be assigned).
If
T is an interface type, then a compile-time error occurs
unless T is the type java.io.Serializable
or the type Cloneable
(the only interfaces implemented by arrays).
If T is a type variable, then this algorithm is applied recursively, using the upper bound of T in place of T.
If
T is an array type TC[]
, that is, an array of
components of type TC, then a compile-time error occurs unless
one of the following is true:
If T is an
intersection type, T_{1} &
... &
T_{n}, then it is a
compile-time error if there exists a T_{i} (1 ≤ i ≤
n) such that S cannot be cast to T_{i} by this
algorithm.
Example 5.5.1-1. Casting Conversion for Reference Types
class Point { int x, y; } interface Colorable { void setColor(int color); } class ColoredPoint extends Point implements Colorable { int color; public void setColor(int color) { this.color = color; } } final class EndPoint extends Point {} class Test { public static void main(String[] args) { Point p = new Point(); ColoredPoint cp = new ColoredPoint(); Colorable c; // The following may cause errors at run time because // we cannot be sure they will succeed; this possibility // is suggested by the casts: cp = (ColoredPoint)p; // p might not reference an // object which is a ColoredPoint // or a subclass of ColoredPoint c = (Colorable)p; // p might not be Colorable // The following are incorrect at compile time because // they can never succeed as explained in the text: Long l = (Long)p; // compile-time error #1 EndPoint e = new EndPoint(); c = (Colorable)e; // compile-time error #2 } }
Here, the first compile-time error occurs because
the class types Long
and Point
are unrelated
(that is, they are not the same, and neither is a subclass of the
other), so a cast between them will always fail.
The second compile-time error occurs because a
variable of type EndPoint
can never reference a
value that implements the interface Colorable
. This
is because EndPoint
is a final
type, and a
variable of a final
type always holds a value of the same run-time
type as its compile-time type. Therefore, the run-time type of
variable e
must be exactly the
type EndPoint
, and type EndPoint
does not implement Colorable
.
Example 5.5.1-2. Casting Conversion for Array Types
class Point { int x, y; Point(int x, int y) { this.x = x; this.y = y; } public String toString() { return "("+x+","+y+")"; } } interface Colorable { void setColor(int color); } class ColoredPoint extends Point implements Colorable { int color; ColoredPoint(int x, int y, int color) { super(x, y); setColor(color); } public void setColor(int color) { this.color = color; } public String toString() { return super.toString() + "@" + color; } } class Test { public static void main(String[] args) { Point[] pa = new ColoredPoint[4]; pa[0] = new ColoredPoint(2, 2, 12); pa[1] = new ColoredPoint(4, 5, 24); ColoredPoint[] cpa = (ColoredPoint[])pa; System.out.print("cpa: {"); for (int i = 0; i < cpa.length; i++) System.out.print((i == 0 ? " " : ", ") + cpa[i]); System.out.println(" }"); } }
This program compiles without errors and produces the output:
cpa: { (2,2)@12, (4,5)@24, null, null }
A cast
from a type S to a type T is statically known to be
correct if and only if S <:
T
(§4.10).
A cast from a type S to a parameterized type (§4.5) T is unchecked unless at least one of the following is true:
All of the type arguments (§4.5.1) of T are unbounded wildcards
T <:
S and S has no subtype X other than
T where the type arguments of X are not contained in the
type arguments of T.
A cast from
a type S to a type variable T is unchecked unless S
<:
T.
A cast from a type
S to an intersection type T_{1} &
... &
T_{n} is unchecked if
there exists a T_{i} (1 ≤ i ≤ n) such that a cast
from S to T_{i} is unchecked.
An unchecked cast from S to a non-intersection type T is completely unchecked if the cast from |S| to |T| is statically known to be correct. Otherwise, it is partially unchecked.
An unchecked cast
from S to an intersection type T_{1} &
... &
T_{n}
is completely unchecked if, for all i (1
≤ i ≤ n), a cast from S to T_{i} is either
statically known to be correct or completely unchecked. Otherwise, it
is partially unchecked.
An
unchecked cast causes a compile-time unchecked warning, unless
suppressed by the SuppressWarnings
annotation
(§9.6.4.5).
A cast is checked if it is not statically known to be correct and it is not unchecked.
If a cast to a reference type is not a compile-time error, there are several cases:
The cast is a partially unchecked or checked cast to an intersection type.
Where the
intersection type is T_{1} &
... &
T_{n}, then for all
i (1 ≤ i ≤ n), any run-time check
required for a cast from S to T_{i} is also required for the
cast to the intersection type.
The cast is a partially unchecked cast to a non-intersection type.
Such a cast requires a run-time validity check. The check is performed as if the cast had been a checked cast between |S| and |T|, as described below.
The cast is a checked cast to a non-intersection type.
Such a cast requires a run-time
validity check. If the value at run time is null
, then the
cast is allowed. Otherwise, let R be the class of the object
referred to by the run-time reference value, and let T be the
erasure (§4.6) of the type named in the
cast operator. A cast conversion must check, at run time, that
the class R is assignment compatible with the type T, via
the algorithm in §5.5.3.
Note that R cannot be an interface when these rules are first applied for any given cast, but R may be an interface if the rules are applied recursively because the run-time reference value may refer to an array whose element type is an interface type.
Here is
the algorithm to check whether the run-time type R of an object is
assignment compatible with the type T which is the erasure
(§4.6) of the type named in the cast operator. If
a run-time exception is thrown, it is a ClassCastException
.
If R is an ordinary class (not an array class):
If
T is a class type, then T must be Object
(§4.3.2), or a run-time exception is
thrown.
If T is an interface type, then R must be either the same interface as T or a subinterface of T, or a run-time exception is thrown.
If R is
a class representing an array type RC[]
, that is, an array
of components of type RC:
If
T is a class type, then T must be Object
(§4.3.2), or a run-time exception is
thrown.
If
T is an interface type, then a run-time exception is thrown
unless T is the type java.io.Serializable
or the type Cloneable
(the only interfaces implemented by arrays).
This case could slip past the compile-time
checking if, for example, a reference to an array were stored in
a variable of type Object
.
If
T is an array type TC[]
, that is, an array of
components of type TC, then a run-time exception is thrown
unless one of the following is true:
Example 5.5.3-1. Incompatible Types at Run Time
class Point { int x, y; } interface Colorable { void setColor(int color); } class ColoredPoint extends Point implements Colorable { int color; public void setColor(int color) { this.color = color; } } class Test { public static void main(String[] args) { Point[] pa = new Point[100]; // The following line will throw a ClassCastException: ColoredPoint[] cpa = (ColoredPoint[])pa; System.out.println(cpa[0]); int[] shortvec = new int[2]; Object o = shortvec; // The following line will throw a ClassCastException: Colorable c = (Colorable)o; c.setColor(0); } }
This program uses casts to compile, but it throws exceptions at run time, because the types are incompatible.
Numeric contexts apply to the operands of an arithmetic operator.
Numeric contexts allow the use of:
A numeric promotion is a process by which, given an arithmetic operator and its argument expressions, the arguments are converted to an inferred target type T. T is chosen during promotion such that each argument expression can be converted to T and the arithmetic operation is defined for values of type T.
The two kinds of numeric promotion are unary numeric promotion (§5.6.1) and binary numeric promotion (§5.6.2).
Some operators apply unary numeric promotion to a single operand, which must produce a value of a numeric type:
If
the operand is of compile-time type Byte
, Short
,
Character
, or Integer
, it is subjected to unboxing
conversion (§5.1.8). The result is then
promoted to a value of type int
by a widening primitive
conversion (§5.1.2) or an identity
conversion (§5.1.1).
Otherwise, if the operand is of
compile-time type Long
, Float
, or Double
, it is subjected
to unboxing conversion (§5.1.8).
Otherwise, if the operand is of
compile-time type byte
, short
, or char
, it is promoted to
a value of type int
by a widening primitive conversion
(§5.1.2).
Otherwise, a unary numeric operand remains as is and is not converted.
After the conversion(s), if any, value set conversion (§5.1.13) is then applied.
Unary numeric promotion is performed on expressions in the following situations:
Each dimension expression in an array creation expression (§15.10.1)
The index expression in an array access expression (§15.10.3)
The operand of a unary plus operator +
(§15.15.3)
The operand of a unary minus operator -
(§15.15.4)
The operand of a bitwise complement operator ~
(§15.15.5)
Each operand,
separately, of a shift operator <<
, >>
, or
>>>
(§15.19).
A long
shift
distance (right operand) does not promote the value being
shifted (left operand) to long
.
Example 5.6.1-1. Unary Numeric Promotion
class Test { public static void main(String[] args) { byte b = 2; int a[] = new int[b]; // dimension expression promotion char c = '\u0001'; a[c] = 1; // index expression promotion a[0] = -c; // unary - promotion System.out.println("a: " + a[0] + "," + a[1]); b = -1; int i = ~b; // bitwise complement promotion System.out.println("~0x" + Integer.toHexString(b) + "==0x" + Integer.toHexString(i)); i = b << 4L; // shift promotion (left operand) System.out.println("0x" + Integer.toHexString(b) + "<<4L==0x" + Integer.toHexString(i)); } }
This program produces the output:
a: -1,1 ~0xffffffff==0x0 0xffffffff<<4L==0xfffffff0
When an operator applies binary numeric promotion to a pair of operands, each of which must denote a value that is convertible to a numeric type, the following rules apply, in order:
After the conversion(s), if any, value set conversion (§5.1.13) is then applied to each operand.
Binary numeric promotion is performed on the operands of certain operators:
The multiplicative
operators *
, /
, and %
(§15.17)
The addition and
subtraction operators for numeric types +
and -
(§15.18.2)
The numerical
comparison operators <
, <=
,
>
, and >=
(§15.20.1)
The numerical equality
operators ==
and !=
(§15.21.1)
The integer bitwise
operators &
, ^
, and |
(§15.22.1)
In certain cases, the
conditional operator ? :
(§15.25)
Example 5.6.2-1. Binary Numeric Promotion
class Test { public static void main(String[] args) { int i = 0; float f = 1.0f; double d = 2.0; // First int*float is promoted to float*float, then // float==double is promoted to double==double: if (i * f == d) System.out.println("oops"); // A char&byte is promoted to int&int: byte b = 0x1f; char c = 'G'; int control = c & b; System.out.println(Integer.toHexString(control)); // Here int:float is promoted to float:float: f = (b==0) ? i : 4.0f; System.out.println(1.0/f); } }
This program produces the output:
7 0.25
The example converts the ASCII
character G
to the ASCII control-G (BEL), by
masking off all but the low 5 bits of the
character. The 7
is the numeric value of this
control character.