How post Increment ++ Operator works while initialization

Question

I have written following code to reverse the given String:

String str = "abcdef";
char[] strToChar = str.toCharArray();
char[] outString = new char[strToChar.length];

for(int j = 0; j < strToChar.length; ) {
    int len = strToChar.length-j;
    outString[j++] = strToChar[strToChar.length-j];
}

As per my understanding initialization happens from Right to Left. Hence, strToChar[strToChar.length-j] here should throw ArrayIndexOutOfBoundException.

But it runs fine. How is that happening? Shouldn't it evaluate to this?

outString[0] = strToChar[6];    //This should throw exception

well, you assign `outString[0] = strToChar[6-1]` here, which will result in no error. The key here is the `j++` which will lead to the right part not throwing an exception, as the left hand assignment variable will increase the `j` value due to the postincrement — SomeJavaGuy, Apr 19 '17 at 11:27
The [JLS](https://docs.oracle.com/javase/specs/jls/se8/html/jls-15.html#jls-15.7) states: "The Java programming language guarantees that the operands of operators appear to be evaluated in a specific evaluation order, namely, from _left to right_." — Thomas, Apr 19 '17 at 11:28

score 1 · Answer 1 · answered Apr 19 '17 at 11:31

1

If I look at your code, then you have written your condition to be

for(int j = 0; j < strToChar.length;)

here strToChar.length will return you the actual length of the array which is 6.

The correct way to iterate an array with for loop would be:

for(int j = 0; j < strToChar.length;j++)

The index starting from 0 and going to < strToChar.length

answered Apr 19 '17 at 11:31

Satish Saini

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well the missing last part of the for loop is the major part why there is no exception. So with the current implementation he´d either have to change it to `strToChar[strToChar.length-j-1]` or he´d be running into the exception he was curious about not beeing raised. – SomeJavaGuy Apr 19 '17 at 11:33
@SomeJavaGuy That's true! – Satish Saini Apr 19 '17 at 11:36
"The correct way to iterate an array with for loop [...]" no, this way is not "the correct way", it is the most common way. OP's code is somewhat uncommon but works just fine. – Gabriel Apr 19 '17 at 13:11

dsharew · Answer 2 · 2017-04-19T11:48:23.153

Acording to the java doc:

The Java programming language guarantees that the operands of operators appear to be evaluated in a specific evaluation order, namely, from left to right.

Which means j is incremented before evaluating the right side of the expression.

So accordingly this code:

outString[j++] = strToChar[strToChar.length-j];

is evaluated to:

outString[0] = strToChar[6-1];

instead of:

outString[0] = strToChar[6-0];

How post Increment ++ Operator works while initialization

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