Java subtleties in the foreach loop

Question

If I have a 2d array of objects, and want to initialize them all, I call a loop, eg:

for(int i=0; i<len; i++)
    for(int j=0; j<len; j++)
        objects[i][j] = new MyObject();

Which works fine, but when I tried doing this with the for-each construct, it didn't work and the entire array remains null:

for(MyObject[] intermediate: objects)
    for(MyObject obj: intermediate)
        obj = new MyObject();

How come they behave differently?

@alain.janinm: Don't close, because the other question only handles simple Arrays, not nested Arrays. You can initialise the outer Array that way - see my answer below. — user unknown, Apr 28 '12 at 10:36

score 4 · Accepted Answer · answered Apr 28 '12 at 10:05

The assigment

obj = new MyObject();

just set a new object in the variable obj, and does not change the value in the array, it only changes the reference variable obj.

What happens is that objects[i][j] is assined to obj, and then you change the value of obj, without changing the actual array.

when you assign directly to objects[i][j] - it works as expected, since you change the value of objects[i][j], which is exactly what you want to do.

score 3 · Answer 2 · answered Apr 28 '12 at 10:05

3

References are passed by value, so obj = new MyObject(); only updates the local copy of your reference to objects[i][j].

answered Apr 28 '12 at 10:05

Alexander Pavlov

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score 1 · Answer 3 · answered Apr 28 '12 at 10:31

It only works for the outer loop, because there are new references being made, but not deep ones:

public static void main (String[] args ) {
    Integer [][] iaa = new Integer[3][4];
    for (Integer[] ia : iaa) {
        for (int i = 0; i < ia.length; ++i) {
            ia[i] = i; 
        }
    }
}

Java subtleties in the foreach loop

3 Answers3