Table of Contents
The Java® programming language is a general-purpose, concurrent, class-based, object-oriented language. It is designed to be simple enough that many programmers can achieve fluency in the language. The Java programming language is related to C and C++ but is organized rather differently, with a number of aspects of C and C++ omitted and a few ideas from other languages included. It is intended to be a production language, not a research language, and so, as C. A. R. Hoare suggested in his classic paper on language design, the design has avoided including new and untested features.
The Java programming language is strongly and statically typed. This specification clearly distinguishes between the compile-time errors that can and must be detected at compile time, and those that occur at run time. Compile time normally consists of translating programs into a machine-independent byte code representation. Run-time activities include loading and linking of the classes needed to execute a program, optional machine code generation and dynamic optimization of the program, and actual program execution.
The Java programming language is a relatively
high-level language, in that details of the machine representation are
not available through the language. It includes automatic storage
management, typically using a garbage collector, to avoid the safety
problems of explicit deallocation (as in C's free
or C++'s delete
). High-performance
garbage-collected implementations can have bounded pauses to support
systems programming and real-time applications. The language does not
include any unsafe constructs, such as array accesses without index
checking, since such unsafe constructs would cause a program to behave
in an unspecified way.
The Java programming language is normally compiled to the bytecode instruction set and binary format defined in The Java Virtual Machine Specification, Java SE 8 Edition.
Chapter 2 describes grammars and the notation used to present the lexical and syntactic grammars for the language.
Chapter 3 describes the lexical structure of the Java programming language, which is based on C and C++. The language is written in the Unicode character set. It supports the writing of Unicode characters on systems that support only ASCII.
Chapter 4 describes types, values, and variables. Types are subdivided into primitive types and reference types.
The primitive types are defined
to be the same on all machines and in all implementations, and are
various sizes of two's-complement integers, single- and
double-precision IEEE 754 standard floating-point numbers, a boolean
type, and a Unicode character char
type. Values of the primitive
types do not share state.
Reference types are the class
types, the interface types, and the array types. The reference types
are implemented by dynamically created objects that are either
instances of classes or arrays. Many references to each object can
exist. All objects (including arrays) support the methods of the class
Object
, which is the (single) root of the class hierarchy. A
predefined String
class supports Unicode character strings. Classes
exist for wrapping primitive values inside of objects. In many cases,
wrapping and unwrapping is performed automatically by the compiler (in
which case, wrapping is called boxing, and unwrapping is called
unboxing). Class and interface declarations may be generic, that is,
they may be parameterized by other reference types. Such declarations
may then be invoked with specific type arguments.
Variables are typed storage
locations. A variable of a primitive type holds a value of that exact
primitive type. A variable of a class type can hold a null reference
or a reference to an object whose type is that class type or any
subclass of that class type. A variable of an interface type can hold
a null reference or a reference to an instance of any class that
implements the interface. A variable of an array type can hold a null
reference or a reference to an array. A variable of class type
Object
can hold a null reference or a reference to any object,
whether class instance or array.
Chapter 5 describes conversions and numeric promotions. Conversions change the compile-time type and, sometimes, the value of an expression. These conversions include the boxing and unboxing conversions between primitive types and reference types. Numeric promotions are used to convert the operands of a numeric operator to a common type where an operation can be performed. There are no loopholes in the language; casts on reference types are checked at run time to ensure type safety.
Chapter 6 describes declarations and names, and how to determine what names mean (denote). The language does not require types or their members to be declared before they are used. Declaration order is significant only for local variables, local classes, and the order of initializers of fields in a class or interface.
The Java programming language provides control over the scope of names and supports limitations on external access to members of packages, classes, and interfaces. This helps in writing large programs by distinguishing the implementation of a type from its users and those who extend it. Recommended naming conventions that make for more readable programs are described here.
Chapter 7 describes the structure of a program, which is organized into packages similar to the modules of Modula. The members of a package are classes, interfaces, and subpackages. Packages are divided into compilation units. Compilation units contain type declarations and can import types from other packages to give them short names. Packages have names in a hierarchical name space, and the Internet domain name system can usually be used to form unique package names.
Chapter 8 describes
classes. The members of classes are classes, interfaces, fields
(variables) and methods. Class variables exist once per class. Class
methods operate without reference to a specific object. Instance
variables are dynamically created in objects that are instances of
classes. Instance methods are invoked on instances of classes; such
instances become the current object this
during their execution,
supporting the object-oriented programming style.
Classes support single
implementation inheritance, in which the implementation of each class
is derived from that of a single superclass, and ultimately from the
class Object
. Variables of a class type can reference an instance of
that class or of any subclass of that class, allowing new types to be
used with existing methods, polymorphically.
Classes support concurrent
programming with synchronized
methods. Methods declare the checked
exceptions that can arise from their execution, which allows
compile-time checking to ensure that exceptional conditions are
handled. Objects can declare a finalize
method that
will be invoked before the objects are discarded by the garbage
collector, allowing the objects to clean up their state.
For simplicity, the language has neither declaration "headers" separate from the implementation of a class nor separate type and class hierarchies.
A special form of classes, enums, support the definition of small sets of values and their manipulation in a type safe manner. Unlike enumerations in other languages, enums are objects and may have their own methods.
Chapter 9 describes interface types, which declare a set of abstract methods, member types, and constants. Classes that are otherwise unrelated can implement the same interface type. A variable of an interface type can contain a reference to any object that implements the interface. Multiple interface inheritance is supported.
Annotation types are specialized interfaces used to annotate declarations. Such annotations are not permitted to affect the semantics of programs in the Java programming language in any way. However, they provide useful input to various tools.
Chapter 10 describes
arrays. Array accesses include bounds checking. Arrays are dynamically
created objects and may be assigned to variables of type Object
. The
language supports arrays of arrays, rather than multidimensional
arrays.
Chapter 11 describes
exceptions, which are nonresuming and fully integrated with the
language semantics and concurrency mechanisms. There are three kinds
of exceptions: checked exceptions, run-time exceptions, and
errors. The compiler ensures that checked exceptions are properly
handled by requiring that a method or constructor can result in a
checked exception only if the method or constructor declares it. This
provides compile-time checking that exception handlers exist, and aids
programming in the large. Most user-defined exceptions should be
checked exceptions. Invalid operations in the program detected by the
Java Virtual Machine result in run-time exceptions, such as NullPointerException
. Errors result from
failures detected by the Java Virtual Machine, such as OutOfMemoryError
. Most simple programs
do not try to handle errors.
Chapter 12 describes activities that occur during execution of a program. A program is normally stored as binary files representing compiled classes and interfaces. These binary files can be loaded into a Java Virtual Machine, linked to other classes and interfaces, and initialized.
After initialization, class methods and class variables may be used. Some classes may be instantiated to create new objects of the class type. Objects that are class instances also contain an instance of each superclass of the class, and object creation involves recursive creation of these superclass instances.
When an object is no longer referenced, it may be reclaimed by the garbage collector. If an object declares a finalizer, the finalizer is executed before the object is reclaimed to give the object a last chance to clean up resources that would not otherwise be released. When a class is no longer needed, it may be unloaded.
Chapter 13 describes binary compatibility, specifying the impact of changes to types on other types that use the changed types but have not been recompiled. These considerations are of interest to developers of types that are to be widely distributed, in a continuing series of versions, often through the Internet. Good program development environments automatically recompile dependent code whenever a type is changed, so most programmers need not be concerned about these details.
Chapter 14 describes blocks and
statements, which are based on C and C++. The language has
no goto
statement, but includes labeled break
and
continue
statements. Unlike C, the Java programming language requires boolean
(or
Boolean
) expressions in control-flow statements, and does not
convert types to boolean
implicitly (except through unboxing), in
the hope of catching more errors at compile time. A synchronized
statement provides basic object-level monitor locking. A try
statement can include catch
and finally
clauses to protect against
non-local control transfers.
Chapter 15 describes expressions. This document fully specifies the (apparent) order of evaluation of expressions, for increased determinism and portability. Overloaded methods and constructors are resolved at compile time by picking the most specific method or constructor from those which are applicable.
Chapter 16 describes the precise way in which the language ensures that local variables are definitely set before use. While all other variables are automatically initialized to a default value, the Java programming language does not automatically initialize local variables in order to avoid masking programming errors.
Chapter 17 describes the semantics of threads and locks, which are based on the monitor-based concurrency originally introduced with the Mesa programming language. The Java programming language specifies a memory model for shared-memory multiprocessors that supports high-performance implementations.
Chapter 18 describes a variety of type inference algorithms used to test applicability of generic methods and to infer types in a generic method invocation.
Most of the example programs given in the text are ready to be executed and are similar in form to:
class Test { public static void main(String[] args) { for (int i = 0; i < args.length; i++) System.out.print(i == 0 ? args[i] : " " + args[i]); System.out.println(); } }
On a machine with the Oracle
JDK installed, this class, stored in the
file Test.java
, can be compiled and executed by
giving the commands:
javac Test.java java Test Hello, world.
Hello, world.
Throughout this specification
we refer to classes and interfaces drawn from the Java SE platform
API. Whenever we refer to a class or interface (other than those
declared in an example) using a single
identifier N, the intended reference is to the
class or interface named N in the package
java.lang
. We use the canonical name (§6.7) for
classes or interfaces from packages other than java.lang
.
Non-normative information, designed to clarify the specification, is given in smaller, indented text.
This is non-normative information. It provides intuition, rationale, advice, examples, etc.
The type system of the Java programming language occasionally relies on
the notion of a substitution. The notation
[F1:=T1,...,Fn:=Tn]
denotes substitution of
Fi by Ti for 1 ≤ i ≤ n.
As noted above, this
specification often refers to classes of the Java SE platform API. In
particular, some classes have a special relationship with the
Java programming language. Examples include classes such as Object
, Class
,
ClassLoader
, String
, Thread
, and the classes and interfaces in
package java.lang.reflect
, among others. This specification constrains
the behavior of such classes and interfaces, but does not provide a
complete specification for them. The reader is referred to the
Java SE platform API documentation.
Consequently, this
specification does not describe reflection in any detail. Many
linguistic constructs have analogs in the Core Reflection API
(java.lang.reflect
) and the Language Model API
(javax.lang.model
), but these are generally not
discussed here. For example, when we list the ways in which an object
can be created, we generally do not include the ways in which the Core
Reflection API can accomplish this. Readers should be aware of these
additional mechanisms even though they are not mentioned in the
text.
Readers are invited to report
technical errors and ambiguities in The Java® Language Specification
to jls-jvms-spec-comments@openjdk.java.net
.
Questions concerning the
behavior of javac
(the reference compiler for the
Java programming language), and in particular its conformance to this specification,
may be sent to compiler-dev@openjdk.java.net
.
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