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Reverse an Array in Java

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Reverse an Array in Java

Reverse an Array is about Java arrays. An array is a fixed-size row of the same type. Index from 0. Length is a.length (a field, not a method). Last valid index is length - 1.

Create Reverse an Array: int[] a = new int[5]; or int[] a = {10, 20, 30};. A 2D array is an array of arrays: int[][] m = {{1,2},{3,4}}; m[1][0] is 3. Jagged means rows can have different lengths.

Reverse an Array size cannot grow. If you need grow/shrink, use ArrayList. Out of range → ArrayIndexOutOfBoundsException.

Let's take this on the board with one tiny Main class — no extra files. Reverse an Array in Java — two-pointer swap reverses to 4 3 2 1. Start from main, go line by line, and stop at each print. That output is the proof for Reverse an Array.

Diagram

index →  0    1    2
  array → [10,  20,  30]
           ↑         ↑
         first   length-1

Exam tip

Contrast array vs ArrayList in one line.

Example

public class Main {
  public static void main(String[] args) {
    int[] a = {1, 2, 3, 4};
    for (int i = 0, j = a.length - 1; i < j; i++, j--) {
      int t = a[i];
      a[i] = a[j];
      a[j] = t;
    }
    for (int x : a) System.out.print(x + " ");
  }
}

Reverse an Array in Java — two-pointer swap reverses to 4 3 2 1.

Short notes

DefReverse an Array — array = fixed-size, same type, index from 0.
RuleReverse an Array — a.length is a field. Last index = length - 1.
RememberReverse an Array — 2D = array of arrays. Jagged = rows of different length.
TrapReverse an Array — a[a.length] crashes. Size cannot grow — use ArrayList.

public class Main { public static void main(String[] args) { int[] a = {1, 2, 3, 4}; for (int i = 0, j = a.length - 1; i < j; i++, j--) { int t = a[i]; a[i] = a[j]; a[j] = t; } for (int x : a) System.out.print(x + " "); } }

Reverse an Array in Java

Example

Short notes

Questions

Reverse an Array in Java

Example

Short notes

Questions