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I am trying to resize an array by adding the array size by 1 per method invoke.
I have created a static method and it takes array as its argument.

public static void addArray(int arrayName[]) {
    int tempNum[] = new int[arrayName.length]; // save the numbers before add array size
    for (int i = 0; i < arrayName.length; i++) { // because by adding/removing array size, it would clear element array
        tempNum[i] = arrayName[i];
    }
    arrayName = new int[arrayName.length + 1]; // adds array size by 1
    for (int i = 0; i < arrayName.length - 1; i++) { // sets all the saved numbers to the new element in the array
        arrayName[i] = tempNum[i];   // stops at (length - 1) because I want to leave it blank at the last element
    }
}

(sorry if the code is messed up, I don't know how to properly post code in here)

In the main, I do this;

public static void main(String[] args) {

    int num[] = {0, 1, 2, 3, 4};

    addArray(num);

    System.out.println(num.length);  
}

As you can see, the default array size (length) should be 5, but no matter how many times I invoke the method, it always print as 5.
Now I'm starting to think that static method does not allow the array from main to be resized ?
If it can't, do you have another way to resize an array by specifically using static method only ?

share|improve this question
6  
Java reference values don't work the way you think they do. –  Dave Newton Dec 9 '12 at 2:36
    
But, you can modify the reference values via static method when having the name of the array element as its argument. For example: public static void Modify(int a[]). If we edit the a[number], it would affect the main values. –  e-zard Yusof Dec 9 '12 at 2:47
    
Please consider accepting some answers for your previous questions. –  The111 Dec 9 '12 at 2:51
    
You can modify the object that the reference points to - but you cannot change TO WHAT the reference points at the caller, as the reference is copied when passed as an argument. –  Pyranja Dec 9 '12 at 2:51
    
And you just figured out that Java is call-by-value :) –  Brian Roach Dec 9 '12 at 3:32
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2 Answers

up vote 1 down vote

You need to return the array from the function:

public static int[] addArray(int arrayName[]) {
    ...
    arrayName = new int[arrayName.length + 1]; // adds array size by 1
    ...
    return arrayName;
}


public static void main(String[] args) {
    int num[] = {0, 1, 2, 3, 4};
    num = addArray(num);
    System.out.println(num.length);  
}
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up vote 0 down vote

You can simply do this:

int num[] = {0, 1, 2, 3, 4};
num = Arrays.copyOf(num, num.length + 1);
System.out.println(num.length);

This should then print 6.

The issue with your code is that when you call a method, the method receives a copy of the reference. Thus, the method cannot change what object is referenced by the variable in the calling method.

Another approach is to make num a static field::

static int num[] = {0, 1, 2, 3, 4};
public static void main(String[] args) {

    addArray(); // or use Arrays.copyOf() as above

    System.out.println(num.length);  
}

public static void addArray() {
    int tempNum[] = new int[num.length + 1];
    System.arraycopy(num, 0, tempNum, 0, num.length);
    num = tempNum;
}

What you can't do (without complex reflection code) is pass a variable name to the method and have it change the length of an array with that name.

share|improve this answer
    
It seems I learned new things :). By making the variable static, I can relate with the static method, LOL. I can do this fine with Class, I just want to do with static method, thanks :), yeah SOLVED. Also, what does this do ? System.arraycopy(num, 0, tempNum, 0, num.length); Can you tell what does each argument does (or refer to) ? –  e-zard Yusof Dec 9 '12 at 3:11
1  
@e-zardYusof The arraycopy method copies contents of num to tempNum. 1st argument - original array, 2nd argument - starting index of original array, 3rd argument - destination array, 4th argument - starting index of destination array, 5th argument - number of elements to be copied from original to destination array Refer this: docs.oracle.com/javase/1.4.2/docs/api/java/lang/…, int, java.lang.Object, int, int) –  Robin Chander Dec 9 '12 at 3:23
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