How to Fill Array in Java: Syntax and Examples

In Java, you can fill an array by assigning values in a loop or using Arrays.fill() to set all elements to the same value. For different values, use a for loop to assign each element individually.

Syntax

There are two common ways to fill an array in Java:

• Using a for loop: Assign values to each element by index.
• Using Arrays.fill(): Set all elements to the same value quickly.

java

int[] arr = new int[5];

// Using for loop
for (int i = 0; i < arr.length; i++) {
    arr[i] = i * 2;
}

// Using Arrays.fill()
java.util.Arrays.fill(arr, 10);

Example

This example shows how to fill an array with different values using a for loop and how to fill all elements with the same value using Arrays.fill().

java

import java.util.Arrays;

public class FillArrayExample {
    public static void main(String[] args) {
        int[] numbers = new int[5];

        // Fill with different values
        for (int i = 0; i < numbers.length; i++) {
            numbers[i] = i + 1;
        }
        System.out.println("Array after for loop: " + Arrays.toString(numbers));

        // Fill all elements with the same value
        Arrays.fill(numbers, 7);
        System.out.println("Array after Arrays.fill(): " + Arrays.toString(numbers));
    }
}

Output

Array after for loop: [1, 2, 3, 4, 5]
Array after Arrays.fill(): [7, 7, 7, 7, 7]

Common Pitfalls

Common mistakes when filling arrays include:

• Forgetting to initialize the array before filling it.
• Using Arrays.fill() when you want different values for each element.
• Accessing indexes outside the array bounds in a loop.

Always ensure the array is created with a size before filling it, and use the right method for your needs.

java

int[] arr;
// Wrong: arr not initialized
// Arrays.fill(arr, 5); // This causes NullPointerException

// Correct:
arr = new int[3];
java.util.Arrays.fill(arr, 5);

Quick Reference

Method	Description	Usage Example
for loop	Fill array with different values	for (int i=0; i
Arrays.fill()	Fill array with the same value	Arrays.fill(arr, 10);
Initialization	Create array before filling	int[] arr = new int[5];

Key Takeaways

Use a for loop to assign different values to each array element.

Use Arrays.fill() to quickly set all elements to the same value.

Always initialize the array with a size before filling it.

Avoid accessing indexes outside the array bounds to prevent errors.

Choose the filling method based on whether values are uniform or varied.