What are the ways to reverse an array in Java?

1. Question

Input : 1, 2, 3, 4, 5 Output :5, 4, 3, 2, 1

Input : 10, 20, 30, 40 Output : 40, 30, 20, 10

2. Method

Method 1: Use temporary array

1. Enter the size of the array and the elements of the array.

2. Consider a function reverse, which accepts parameters - an array (such as arr) and the size of the array (such as n)

3. In Inside the function, a new array (with the array size of the first array, arr) is initialized.

4. Array arr[] starts iteration from the first element, and each element of array arr[] is put into a new array from the back, that is, the new array starts iteration from the last element.

5. This puts all the elements of the array arr[] into the new array in reverse order.

public class reverseArray {
    // function that reverses array and stores it 
    // in another array
    static void reverse(int a[], int n)
    {
        int[] b = new int[n];
        int j = n;
        for (int i = 0; i < n; i++) {
            b[j - 1] = a[i];
            j = j - 1;
        }
        // printing the reversed array
        System.out.println("Reversed array is: \n");
        for (int k = 0; k < n; k++) {
            System.out.println(b[k]);
        }
    }
    public static void main(String[] args)
    {
        int [] arr = {10, 20, 30, 40, 50};
        reverse(arr, arr.length);
    }
}

The results are as follows:

Reversed array is: 
50
40
30
20
10

Method 2: Use exchange

to exchange the elements of the array. The first element is swapped with the last element. The second element is swapped with the last element, and so on. For example, suppose you have an array with elements [1, 2, 3, ..., n-2, n-1, n]. Swap 1 and n, 2 and n-1, 3 and n-2, etc. one by one.

public class Test {
    public static void main(String[] args) {
        int [] arr = {10, 20, 30, 40, 50};
        reverse(arr,arr.length);
    }
    static void reverse(int[] a,int n) {
        int i, k, t;
        for (i = 0; i < n / 2; i++) {
            t = a[i];
            a[i] = a[n - i - 1];
            a[n - i - 1] = t;
        }
        System.out.println("Reversed array is: \n");
        for (k = 0; k < n; k++) {
            System.out.println(a[k]);
        }
    }
}

The results are as follows:

Reversed array is: 
50
40
30
20
10

Method 3: Use Collections.reverse() method

The method java.util.Collections.reverse(List list) reverses a given list .. Reverse an array by converting it to a list and then calling Collections.reverse(list).

public class Test {
    public static void main(String[] args) {
        Integer [] arr = {10, 20, 30, 40, 50};
        reverse(arr);
    }
    static void reverse(Integer[] a) {
        Collections.reverse(Arrays.asList(a));
        System.out.println(Arrays.asList(a));
    }
}

The results are as follows:

[50, 40, 30, 20, 10]

Method 4: Use StringBuilder.append() method

If you are using a String array, we can use StringBuilder and for each array A for loop is attached to the elements, starting from the length of the array and decrementing it, converting the StringBuilder to a string, and then splitting back into the array.

public class Test {
    public static void main(String[] args) {
        String[] arr = {"Hello", "World"};
        StringBuilder reversed = new StringBuilder();
        for (int i = arr.length-1; i >= 0; i--) {
            reversed.append(arr[i]).append(" ");
        };
        String[] reversedArray = reversed.toString().split(" ");
        System.out.println(Arrays.toString(reversedArray));
    }
}

The results are as follows:

[World, Hello]

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