EXPRESSIONS

Examples: Array Access Evaluation Order

15.12.2

If the array reference expression produces

null

 instead of a reference to an

array, then a

NullPointerException

 is thrown at run time, but only after all

parts of the array reference expression have been evaluated and only if these eval 

uations completed normally. Thus, the example:

class Test {

public static void main(String[] args) {

int index = 1;

try {

nada()[index=2]++;

} catch (Exception e) {

System.out.println(e + ", index=" + index);

}

}

static int[] nada() { return null; }

}

prints:

java.lang.NullPointerException, index=2

because the embedded assignment of

2

 to

index

 occurs before the check for a null

pointer. As a related example, the program:

class Test {

public static void main(String[] args) {

int[] a = null;

try {

int i = a[vamoose()];

System.out.println(i);

} catch (Exception e) {

System.out.println(e);

}

}

static int vamoose() throws Exception {

throw new Exception("Twenty three skidoo!");

}

}

always prints:

java.lang.Exception: Twenty three skidoo!

A

NullPointerException

 never occurs, because the index expression must be

completely evaluated before any part of the indexing operation occurs, and that

includes the check as to whether the value of the left hand operand is

null

.

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