Memory usage/ reference in java

Question

I am going to ask a basic question about Java memory usage.

Imagine we have an array List and it is large enough and we don't like to use more memory. Now if I want to pass this array to another methods in this class, or other classes through their constructor or method, do I need additional memory/is there additional memory usage for this array?

If yes, could I just make this array package level, and therefore the other classes in this package could access it directly, without any memory need.

Thank you in advance.

Davide Lorenzo MARINO · Accepted Answer · 2016-01-31 09:10:45Z

2

No, no additional memory is necessary. The parameter of a function is passed by copy of the reference. It means that for any kind of object only 4 additional bytes are used.

If you pass an array as parameter and you modify it in the body of the method the changes will be exported outside of method.

Instead if you reassign the array variable, the difference is not visible externally.

This happens because the parameters are passed as copy of the reference and not by reference.

public void vsibleModification(int[] a) {
    for (int i = 0; i < a.length; i++) {
        // This change is visible outside of method because I change 
        // the content of a, not the reference
        a[i] = a[i] + 1;  
    }
}

public void nonVisibleModification(int[] a) {
    // Non visible modification because a is reassigned to a new value (reference modification)
    a = new int[2];
    a[0] = 1;
    a[1] = 2;
}

edited Jan 31, 2016 at 9:10

answered Jan 31, 2016 at 9:01

Davide Lorenzo MARINO

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3 Comments

jd466 Over a year ago

Hi, thanks. But if I change the array in the goal method/class , the original array will stay the same. doesn't it mean additional memory usage?

jd466 Over a year ago

Thanks, for my need, the goal methods or classes do not need to change the variable, only reading is enough. What would be your suggestion?

Davide Lorenzo MARINO Over a year ago

You can pass it without problem and no additional memory is needed.

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