15.25.1 Simple Assignment Operator =

A compile-time error occurs if the type of the right-hand operand cannot be converted to the type of the variable by assignment conversion (§5.2).

At run time, the expression is evaluated in one of two ways. If the left-hand operand expression is not an array access expression, then three steps are required:

• First, the left-hand operand is evaluated to produce a variable. If this evaluation completes abruptly, then the assignment expression completes abruptly for the same reason; the right-hand operand is not evaluated and no assignment occurs.
• Otherwise, the right-hand operand is evaluated. If this evaluation completes abruptly, then the assignment expression completes abruptly for the same reason and no assignment occurs.
• Otherwise, the value of the right-hand operand is converted to the type of the left-hand variable and the result of the conversion is stored into the variable.

If the left-hand operand expression is an array access expression (§15.12), then many steps are required:

• First, the array reference subexpression of the left-hand operand array access expression is evaluated. If this evaluation completes abruptly, then the assignment expression completes abruptly for the same reason; the index subexpression (of the left-hand operand array access expression) and the right-hand operand are not evaluated and no assignment occurs.
• Otherwise, the index subexpression of the left-hand operand array access expression is evaluated. If this evaluation completes abruptly, then the assignment expression completes abruptly for the same reason and the right-hand operand is not evaluated and no assignment occurs.
• Otherwise, the right-hand operand is evaluated. If this evaluation completes abruptly, then the assignment expression completes abruptly for the same reason and no assignment occurs.
• Otherwise, if the value of the array reference subexpression is null, then no assignment occurs and a NullPointerException is thrown.
• Otherwise, the value of the array reference subexpression indeed refers to an array. If the value of the index subexpression is less than zero, or greater than or equal to the length of the array, then no assignment occurs and an IndexOutOfBoundsException is thrown.
• Otherwise, the value of the index subexpression is used to select a component of the array referred to by the value of the array reference subexpression. This component is a variable; call its type SC. Also, let TC be the type of the left-hand operand of the assignment operator as determined at compile time.
  • If TC is a primitive type, then SC is necessarily the same as TC. The value of the right-hand operand is converted to a value of type TC and stored into the selected array component.
  • If T is a reference type, then SC may not be the same as T, but rather a type that extends or implements TC. Let RC be the class of the object referred to by the value of the right-hand operand at run time. The compiler may be able to prove at compile time that the array component will be of type TC exactly (for example, TC might be final). But if the compiler cannot prove at compile time that the array component will be of type TC exactly, then a check must be performed at run time to ensure that the class RC is assignment compatible (§5.2) with the actual type SC of the array component. This check is similar to a narrowing cast (§5.4, §15.15), except that if the check fails, an ArrayStoreException is thrown rather than a ClassCastException. Therefore:
    • If class RC is not assignable to type SC, then no assignment occurs and an ArrayStoreException is thrown.
    • Otherwise, the reference value of the right-hand operand is stored into the selected array component.

The rules for assignment to an array component are illustrated by the following example program:

class ArrayReferenceThrow extends RuntimeException { }


class IndexThrow extends RuntimeException { }


class RightHandSideThrow extends RuntimeException { }

class IllustrateSimpleArrayAssignment {

	static Object[] objects = { new Object(), new Object() };


	static Thread[] threads = { new Thread(), new Thread() };


	static Object[] arrayThrow() {
		throw new ArrayReferenceThrow();
	}


	static int indexThrow() { throw new IndexThrow(); }


	static Thread rightThrow() {
		throw new RightHandSideThrow();
	}


	static String name(Object q) {
		String sq = q.getClass().getName();
		int k = sq.lastIndexOf('.');
		return (k < 0) ? sq : sq.substring(k+1);
	}

	static void testFour(Object[] x, int j, Object y) {
		String sx = x == null ? "null" : name(x[0]) + "s";
		String sy = name(y);
		System.out.println();
		try {
			System.out.print(sx + "[throw]=throw => ");
			x[indexThrow()] = rightThrow();
			System.out.println("Okay!");
		} catch (Throwable e) { System.out.println(name(e)); }
		try {
			System.out.print(sx + "[throw]=" + sy + " => ");
			x[indexThrow()] = y;
			System.out.println("Okay!");
		} catch (Throwable e) { System.out.println(name(e)); }
		try {
			System.out.print(sx + "[" + j + "]=throw => ");
			x[j] = rightThrow();
			System.out.println("Okay!");
		} catch (Throwable e) { System.out.println(name(e)); }
		try {
			System.out.print(sx + "[" + j + "]=" + sy + " => ");
			x[j] = y;
			System.out.println("Okay!");
		} catch (Throwable e) { System.out.println(name(e)); }
	}


	public static void main(String[] args) {
		try {
			System.out.print("throw[throw]=throw => ");
			arrayThrow()[indexThrow()] = rightThrow();
			System.out.println("Okay!");
		} catch (Throwable e) { System.out.println(name(e)); }
		try {
			System.out.print("throw[throw]=Thread => ");
			arrayThrow()[indexThrow()] = new Thread();
			System.out.println("Okay!");
		} catch (Throwable e) { System.out.println(name(e)); }
		try {
			System.out.print("throw[1]=throw => ");
			arrayThrow()[1] = rightThrow();
			System.out.println("Okay!");
		} catch (Throwable e) { System.out.println(name(e)); }
		try {
			System.out.print("throw[1]=Thread => ");
			arrayThrow()[1] = new Thread();
			System.out.println("Okay!");
		} catch (Throwable e) { System.out.println(name(e)); }

		testFour(null, 1, new StringBuffer());
		testFour(null, 1, new StringBuffer());
		testFour(null, 9, new Thread());
		testFour(null, 9, new Thread());
		testFour(objects, 1, new StringBuffer());
		testFour(objects, 1, new Thread());
		testFour(objects, 9, new StringBuffer());
		testFour(objects, 9, new Thread());
		testFour(threads, 1, new StringBuffer());
		testFour(threads, 1, new Thread());
		testFour(threads, 9, new StringBuffer());
		testFour(threads, 9, new Thread());
	}

}

This program prints:

throw[throw]=throw => ArrayReferenceThrow
throw[throw]=Thread => ArrayReferenceThrow
throw[1]=throw => ArrayReferenceThrow
throw[1]=Thread => ArrayReferenceThrow


null[throw]=throw => IndexThrow
null[throw]=StringBuffer => IndexThrow
null[1]=throw => RightHandSideThrow
null[1]=StringBuffer => NullPointerException


null[throw]=throw => IndexThrow
null[throw]=StringBuffer => IndexThrow
null[1]=throw => RightHandSideThrow
null[1]=StringBuffer => NullPointerException


null[throw]=throw => IndexThrow
null[throw]=Thread => IndexThrow
null[9]=throw => RightHandSideThrow
null[9]=Thread => NullPointerException


null[throw]=throw => IndexThrow
null[throw]=Thread => IndexThrow
null[9]=throw => RightHandSideThrow
null[9]=Thread => NullPointerException


Objects[throw]=throw => IndexThrow
Objects[throw]=StringBuffer => IndexThrow
Objects[1]=throw => RightHandSideThrow
Objects[1]=StringBuffer => Okay!


Objects[throw]=throw => IndexThrow
Objects[throw]=Thread => IndexThrow
Objects[1]=throw => RightHandSideThrow
Objects[1]=Thread => Okay!


Objects[throw]=throw => IndexThrow
Objects[throw]=StringBuffer => IndexThrow
Objects[9]=throw => RightHandSideThrow
Objects[9]=StringBuffer => IndexOutOfBoundsException


Objects[throw]=throw => IndexThrow
Objects[throw]=Thread => IndexThrow
Objects[9]=throw => RightHandSideThrow
Objects[9]=Thread => IndexOutOfBoundsException


Threads[throw]=throw => IndexThrow
Threads[throw]=StringBuffer => IndexThrow
Threads[1]=throw => RightHandSideThrow
Threads[1]=StringBuffer => ArrayStoreException


Threads[throw]=throw => IndexThrow
Threads[throw]=Thread => IndexThrow
Threads[1]=throw => RightHandSideThrow
Threads[1]=Thread => Okay!


Threads[throw]=throw => IndexThrow
Threads[throw]=StringBuffer => IndexThrow
Threads[9]=throw => RightHandSideThrow
Threads[9]=StringBuffer => IndexOutOfBoundsException


Threads[throw]=throw => IndexThrow
Threads[throw]=Thread => IndexThrow
Threads[9]=throw => RightHandSideThrow
Threads[9]=Thread => IndexOutOfBoundsException

The most interesting case of the lot is the one thirteenth from the end:

Threads[1]=StringBuffer => ArrayStoreException

which indicates that the attempt to store a reference to a StringBuffer into an array whose components are of type Thread throws an ArrayStoreException. The code is type-correct at compile time: the assignment has a left-hand side of type Object[] and a right-hand side of type Object. At run time, the first actual argument to method testFour is a reference to an instance of "array of Thread" and the third actual argument is a reference to an instance of class StringBuffer.