How to Initialize All Array Elements to Zero in Java

A newly allocated Java array of a primitive numeric type is initialized with zero-valued elements, so no extra step is needed for a fresh array. To reset an existing array, use Arrays.fill() or assign each element explicitly. Arrays of reference types, such as Integer[], instead start with null elements.
Array initialization is covered as part of the core data path in the Java Tutorial.

Initialize Array Elements to Zero by Using a for Loop

A for loop assigns a value to each index in an existing array. The loop starts at index 0 and stops before numbers.length, so every valid element is visited.

This complete Java program applies the techniques to short arrays and prints each result:

import java.util.Arrays;
import java.util.Collections;
import java.util.List;

public class ArrayInitialization {
  public static void main(String[] args) {
    int[] loopValues = {4, 8, 12};
    for (int i = 0; i < loopValues.length; i++) {
      loopValues[i] = 0;
    }
    System.out.println("for loop: " + Arrays.toString(loopValues));

    int[] filledValues = {4, 8, 12};
    Arrays.fill(filledValues, 0);
    System.out.println("fill: " + Arrays.toString(filledValues));

    List<Integer> repeatedValues = Collections.nCopies(3, 0);
    Integer[] boxedValues = repeatedValues.toArray(new Integer[0]);
    System.out.println("boxed values: " + Arrays.toString(boxedValues));

    int[] numbers = {4, 8, 12};
    int[] oldReference = numbers;
    numbers = new int[numbers.length];
    System.out.println("replacement: " + Arrays.toString(numbers));
    System.out.println("old reference: " + Arrays.toString(oldReference));

    int[] generatedValues = {4, 8, 12};
    Arrays.setAll(generatedValues, index -> 0);
    System.out.println("setAll: " + Arrays.toString(generatedValues));

    int[] newValues = new int[3];
    System.out.println("new array: " + Arrays.toString(newValues));
  }
}

The program prints:

for loop: [0, 0, 0]
fill: [0, 0, 0]
boxed values: [0, 0, 0]
replacement: [0, 0, 0]
old reference: [4, 8, 12]
setAll: [0, 0, 0]
new array: [0, 0, 0]

Initialize Array Elements to Zero by Using the fill() Method in Java

Arrays.fill() writes the supplied value into every element of the array. For a primitive numeric array, Arrays.fill(numbers, 0) resets the existing array in place. The same method also has overloads for filling a selected range when only part of an array should change.

Initialize Array Elements to Zero by Using the nCopies() Method

Collections.nCopies(length, 0) returns an immutable List<Integer> containing repeated boxed values. Converting that list with toArray(new Integer[0]) produces an Integer[]; it does not produce an int[] or reset an existing primitive array. Use it when a list or boxed array is the desired result, rather than as a substitute for modifying the original array.

Initialize Array Elements to Zero by Using Reassignment in Java

Assigning new int[numbers.length] makes numbers refer to a new array whose elements have their default values. Any other variable that refers to the original array continues to see its previous contents. Reassignment replaces one reference; it does not clear the old array for every reference to it.

Initialize Array Elements to Zero by Using Arrays.setAll()

Arrays.setAll() calls an index-based generator for each element and stores the generated value in the array. The example uses index -> 0 to assign the same value at every index. This method is useful when a value depends on its index; Arrays.fill() is more direct when every element receives one constant.

Conclusion

A new primitive numeric array is already initialized with zeros. For an existing array that must be reset, Arrays.fill() is a direct choice; a for loop makes the per-element assignment explicit, and Arrays.setAll() supports index-based values. Collections.nCopies() creates repeated boxed values in a list, while reassignment creates a new array and leaves any other references to the old one unchanged.