Reverse the order of array in java?

Question

I am trying to write a function to reverse array, but it doesn't work like I want to.

public class ReverseArray{
    public static int[] reverseArray(int[] array){
        int[] revArray=array;
        int i=0;
        int j=revArray.length-1;
        while(i<=j){
            swap(i,j,revArray);
            j--;
            i++;
        }
        return revArray;
    }
    
    public static void swap(int i, int j, int[] revArray){
        int temp;
        temp=revArray[i];
        revArray[i]=revArray[j]; 
        revArray[j]=temp;
    }

    public static void main(String []args){
         
        int[] array={2,4,6,8,10,12,14,16};
         
         
        int[] revArray=reverseArray(array);
         
        for(int i=0;i<array.length;i++){
            System.out.print(array[i]+" ");
        }
        System.out.println();
         
         
        for(int i=0;i<revArray.length;i++){
            System.out.print(revArray[i]+" ");
        }
    }
} 

Output:16 14 12 10 8 6 4 2   
       16 14 12 10 8 6 4 2

But when I do it like this:

int[] array={2,4,6,8,10,12,14,16};
         
         for(int i=0;i<array.length;i++){
             System.out.print(array[i]+" ");
         }
         
         int[] revArray=reverseArray(array);
         
         
         System.out.println();
         
         
         for(int i=0;i<revArray.length;i++){
             System.out.print(revArray[i]+" ");
         }
Output:2 4 6 8 10 12 14 16   
       16 14 12 10 8 6 4 2

I don't understand why both arrays are reversed if I call the function revArray before I print them.

`int[] revArray=array;` is not making a copy of the array. It just makes 2 references to the same object. — takendarkk, Oct 20 '20 at 15:14
@takendarkk Or to be more precise it creates another *reference type* variable `revArray` and assigns to it same reference (object identifier) as held in `array` reference type variable. More info: [What is the difference between a variable, object, and reference?](https://stackoverflow.com/q/32010172) — Pshemo, Oct 20 '20 at 15:19
Well, you reverse the array in-place. You never keep a copy of the original array, anywhere. — Polygnome, Oct 20 '20 at 15:23

score 0 · Accepted Answer · answered Oct 20 '20 at 15:18

0

Your reverseArray() method assigns array to the variable revArray, thus the 2 variables pointing the same array in memory.

To avoid that, try something like this:

public static int[] reverseArray(int[] array){
    int[] revArray= Arrays.copyOf(array, array.length);
    int i=0;
    int j=revArray.length-1;
    while(i<=j){
        swap(i,j,revArray);
        j--;
        i++;
    }
    return revArray;
}

answered Oct 20 '20 at 15:18

Usagi Miyamoto

6,196
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Thank you good sir, why so many dislikes on my question tho? – Viktor Dimitrioski Oct 20 '20 at 20:50

score 0 · Answer 2 · answered Oct 20 '20 at 15:27

As it has been mentioned in the comments, the problem is you are refering to the same array in. So, when you change it in the methods, it changes the original. But, to reverse an array you don't need this much of a complexity. check this:

int[] array={2,4,6,8,10,12,14,16};
int length = array.length;
int[] array1 = new int[length];
for(int i = 0 ; i < length; i++) {
  array1[length - 1 - i] = array[i];
}
System.out.println(Arrays.toString(array1)); // prints [16, 14, 12, 10, 8, 6, 4, 2]

Reverse the order of array in java?

2 Answers2