Java exhibiting pass-by-reference behavior

Question

Writing code to reverse the elements in an array. Both method1 and method2 perform the same operations, but are changing the values of the array in main, despite scope and no assignment.

import java.util.Arrays;
public class ReversingMatrix {
    public static void main(String[] args) {
        int values[] = {1, 2, 3, 4, 5, 6, 7};
        method1(values, 0, values.length - 1);
            // Values array is now : {7, 6 ... 1} --> why?
        method2(values);
    }
    public static void method1(int[] arr, int lPos, int rPos) {
        if(lPos >= rPos) System.out.println("1D Recursion Reverse: " + Arrays.toString(arr));
        else {
            int temp = arr[lPos];
            arr[lPos] = arr[rPos];
            arr[rPos] = temp;
            method1(arr, lPos + 1, rPos - 1);
        }
    }
    public static void method2(int[] arr) {
        int temp;
        for(int a = 0; a < arr.length / 2; a++) {
            temp = arr[a];
            arr[a] = arr[arr.length - (1+a)];
            arr[arr.length - (1+a)] = temp;
        }
        System.out.println("1D For Loop Reverse: " + Arrays.toString(arr));
    }
}

Why doesn't values array remain {1, 2, 3, 4, 5, 6, 7}, so I can use it multiple times?

Because `values`, `method1.arr` and `method2.arr` all reference the same array object. — Sotirios Delimanolis, Mar 24 '15 at 23:06
Am I passing the array incorrectly as a parameter? This issue would not be happening with simple Objects, what is the proper way to pass arrays by value? — , Mar 24 '15 at 23:08
There is no pass-by-reference in Java. `arr[lpos]` is dereferencing the reference stored in `arr` which is equal to the one stored in `values` in the calling code in the `main` method. — Sotirios Delimanolis, Mar 24 '15 at 23:09
Then how do I use the value stored in `arr` with a position `lPos` in the called method, without modifying `values` in main? — , Mar 24 '15 at 23:12
You make a copy of the array stored in `values` and pass that when invoking `method1`. — Sotirios Delimanolis, Mar 24 '15 at 23:14

Java exhibiting pass-by-reference behavior

0 Answers0