Trail: The Reflection API
Lesson: Arrays and Enumerated Types
Section: Arrays

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The Java Tutorials have been written for JDK 8. Examples and practices described in this page don't take advantage of improvements introduced in later releases and might use technology no longer available.
See Dev.java for updated tutorials taking advantage of the latest releases.
See Java Language Changes for a summary of updated language features in Java SE 9 and subsequent releases.
See JDK Release Notes for information about new features, enhancements, and removed or deprecated options for all JDK releases.

Troubleshooting

The following examples show typical errors which may occur when operating on arrays.

IllegalArgumentException due to Inconvertible Types

The ArrayTroubleAgain example will generate an IllegalArgumentException. Array.setInt() is invoked to set a component that is of the reference type Integer with a value of primitive type int. In the non-reflection equivalent ary[0] = 1, the compiler would convert (or box) the value 1 to a reference type as new Integer(1) so that its type checking will accept the statement. When using reflection, type checking only occurs at runtime so there is no opportunity to box the value.


import java.lang.reflect.Array;
import static java.lang.System.err;

public class ArrayTroubleAgain {
    public static void main(String... args) {
	Integer[] ary = new Integer[2];
	try {
	    Array.setInt(ary, 0, 1);  // IllegalArgumentException

        // production code should handle these exceptions more gracefully
	} catch (IllegalArgumentException x) {
	    err.format("Unable to box%n");
	} catch (ArrayIndexOutOfBoundsException x) {
	    x.printStackTrace();
	}
    }
}

$ java ArrayTroubleAgain
Unable to box

To eliminate this exception, the problematic line should be replaced by the following invocation of Array.set(Object array, int index, Object value):

Array.set(ary, 0, new Integer(1));

Tip: When using reflection to set or get an array component, the compiler does not have an opportunity to perform boxing. It can only convert types that are related as described by the specification for Class.isAssignableFrom(). The example is expected to fail because isAssignableFrom() will return false in this test which can be used programmatically to verify whether a particular conversion is possible:

Integer.class.isAssignableFrom(int.class) == false

Similarly, automatic conversion from primitive to reference type is also impossible in reflection.

int.class.isAssignableFrom(Integer.class) == false

ArrayIndexOutOfBoundsException for Empty Arrays

The ArrayTrouble example illustrates an error which will occur if an attempt is made to access the elements of an array of zero length:


import java.lang.reflect.Array;
import static java.lang.System.out;

public class ArrayTrouble {
    public static void main(String... args) {
        Object o = Array.newInstance(int.class, 0);
        int[] i = (int[])o;
        int[] j = new int[0];
        out.format("i.length = %d, j.length = %d, args.length = %d%n",
                   i.length, j.length, args.length);
        Array.getInt(o, 0);  // ArrayIndexOutOfBoundsException
    }
}

$ java ArrayTrouble
i.length = 0, j.length = 0, args.length = 0
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException
        at java.lang.reflect.Array.getInt(Native Method)
        at ArrayTrouble.main(ArrayTrouble.java:11)

Tip: It is possible to have arrays with no elements (empty arrays). There are only a few cases in common code where they are seen but they can occur in reflection inadvertently. Of course, it is not possible to set/get the values of an empty array because an ArrayIndexOutOfBoundsException will be thrown.

IllegalArgumentException if Narrowing is Attempted

The ArrayTroubleToo example contains code which fails because it attempts perform an operation which could potentially lose data:


import java.lang.reflect.Array;
import static java.lang.System.out;

public class ArrayTroubleToo {
    public static void main(String... args) {
        Object o = new int[2];
        Array.setShort(o, 0, (short)2);  // widening, succeeds
        Array.setLong(o, 1, 2L);         // narrowing, fails
    }
}

$ java ArrayTroubleToo
Exception in thread "main" java.lang.IllegalArgumentException: argument type
  mismatch
        at java.lang.reflect.Array.setLong(Native Method)
        at ArrayTroubleToo.main(ArrayTroubleToo.java:9)

Tip: The Array.set*() and Array.get*() methods will perform automatic widening conversion but will throw an IllegalArgumentException if a narrowing conversion is attempted. For complete discussion of widening and narrowing conversions, see The Java Language Specification, Java SE 7 Edition, sections Widening Primitive Conversion and Narrowing Primitive Conversion respectively.

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