Java How to declare and initialize an Array

Few Java examples to declare, initialize and manipulate Array in Java

1. Declares Array

1.1 For primitive types.

package com.mkyong;
import java.util.Arrays;
public class ArrayExample1 {
    public static void main(String[] args) {
        //declares an array of integers
        int[] num1 = new int[5];
        int[] num2 = {1, 2, 3, 4, 5};
        int[] num3 = new int[]{1, 2, 3, 4, 5};
		// index starts with 0
        num1[0] = 1;
        num1[1] = 2;
        num1[2] = 3;
        num1[3] = 4;
        num1[4] = 5;
		// print array
        System.out.println(Arrays.toString(num1));
        System.out.println(Arrays.toString(num2));
        System.out.println(Arrays.toString(num3));

Output

[1, 2, 3, 4, 5]
[1, 2, 3, 4, 5]
[1, 2, 3, 4, 5]

1.2 For classes or objects, like String.class, it’s the same.

package com.mkyong;
import java.util.Arrays;
public class ArrayExample1 {
    public static void main(String[] args) {
        String[] str1 = new String[5];
        String[] str2 = {"a", "b", "c", "d", "e"};
        String[] str3 = new String[]{"a", "b", "c", "d", "e"};
        str1[0] = "a";
        str1[1] = "b";
        str1[2] = "c";
        str1[3] = "d";
        str1[4] = "e";
        System.out.println(Arrays.toString(str1));
        System.out.println(Arrays.toString(str2));
        System.out.println(Arrays.toString(str3));

Output

[a, b, c, d, e]
[a, b, c, d, e]
[a, b, c, d, e]

2. Return Array

2.1 A method to return an Array.

package com.mkyong;
import java.util.Arrays;
public class ArrayExample2 {
    public static void main(String[] args) {
        int[] resultNum = getArrayNumber();
        String[] resultStr = getArrayString();
        System.out.println(Arrays.toString(resultNum));
        System.out.println(Arrays.toString(resultStr));
    public static int[] getArrayNumber() {
        return new int[]{1, 2, 3, 4, 5};
    public static String[] getArrayString() {
        return new String[]{"a", "b", "c", "d", "e"};

Output

[1, 2, 3, 4, 5]
[a, b, c, d, e]

3. Access Array

3.1 Array index starts with 0

package com.mkyong;
public class ArrayExample3 {
    public static void main(String[] args) {
        // array, empty
        int[] num;
        // array of 5 elements
        num = new int[5];
        num[0] = 1;
        num[1] = 2;
        num[2] = 3;
        num[3] = 4;
        num[4] = 5;
        System.out.println("num[0] : " + num[0]);
        System.out.println("num[1] : " + num[1]);
        System.out.println("num[2] : " + num[2]);
        System.out.println("num[3] : " + num[3]);
        System.out.println("num[4] : " + num[4]);

Output

num[0] : 1
num[1] : 2
num[2] : 3
num[3] : 4
num[4] : 5

4. ArrayIndexOutOfBoundsException

4.1 If accessing with an index greater than the size of an array. Again, the index starts with 0.

package com.mkyong;
public class ArrayExample4 {
    public static void main(String[] args) {
        // array, empty
        int[] num;
        // array of 5
        num = new int[5];
        num[0] = 1;
        num[1] = 2;
        num[2] = 3;
        num[3] = 4;
        num[4] = 5;
        //num[5] = 6; //ArrayIndexOutOfBoundsException: 5
        System.out.println("num[0] : " + num[0]);
        System.out.println("num[1] : " + num[1]);
        System.out.println("num[2] : " + num[2]);
        System.out.println("num[3] : " + num[3]);
        System.out.println("num[4] : " + num[4]);
        System.out.println("num[5] : " + num[5]); //ArrayIndexOutOfBoundsException: 5

Output

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 5
	at com.mkyong.ArrayExample.main(ArrayExample.java:18)

5. Multidimensional Array

package com.mkyong;
public class ArrayExample5 {
    public static void main(String[] args) {
        int[][] num2d = new int[2][5];
        num2d[0][0] = 1;
        num2d[0][1] = 2;
        num2d[0][2] = 3;
        num2d[0][3] = 4;
        num2d[0][4] = 5;
        num2d[1][0] = 10;
        num2d[1][1] = 20;
        num2d[1][2] = 30;
        num2d[1][3] = 40;
        num2d[1][4] = 50;
        //or like this :
        int[][] num2dInit = {
                {1, 2, 3, 4, 5},
                {10, 20, 30, 40, 50}
        };
        // Accessing 2d array with index print
        int index1d, index2d = 0;
        for (int[] num1d : num2d) {
            index1d = 0;
            for (int num : num1d) {
                System.out.println("[" + index2d + "][" + index1d + "] = " + num);
                index1d++;
            index2d++;
        /* Simple print
        for (int[] num1d : num2d) {
            for (int num : num1d) {
                System.out.println(num);
        }*/

Output

[0][0] = 1
[0][1] = 2
[0][2] = 3
[0][3] = 4
[0][4] = 5
[1][0] = 10
[1][1] = 20
[1][2] = 30
[1][3] = 40
[1][4] = 50

6. Copy Array

We can use System.arraycopy to copy data from an Array into another.

arraycopy(Object src,  int  srcPos, 
	Object dest, int destPos,
	int length);

package com.mkyong;
import java.util.Arrays;
public class ArrayExample6 {
    public static void main(String[] args) {
        int[] arrayFrom = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
        int[] arrayTo = new int[5];
        int[] arrayTo2 = new int[10];
        System.out.println(Arrays.toString(arrayFrom));
        //[3, 4, 5, 6, 7]
        System.arraycopy(arrayFrom, 2, arrayTo, 0, 5);
        System.out.println(Arrays.toString(arrayTo));
        //[0, 0, 0, 0, 0, 1, 2, 3, 4, 5]
        System.arraycopy(arrayFrom, 0, arrayTo2, 5, 5);
        System.out.println(Arrays.toString(arrayTo2));

Output

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
[3, 4, 5, 6, 7]
[0, 0, 0, 0, 0, 1, 2, 3, 4, 5]

Read Java docs System.arraycopy

7. Sort Array

We can use Arrays.sort to sort an Array.

package com.mkyong;
import java.util.Arrays;
import java.util.Collections;
public class ArrayExample7 {
    public static void main(String[] args) {
        int[] num = {10, 5, 4, 3, 6, 9, 7, 8, 2, 1};
        System.out.println(Arrays.toString(num));
        // ascending order
        Arrays.sort(num);
        System.out.println(Arrays.toString(num));
        // descending order, int[] cant, need boxed to Integer[] first
        Integer[] numObjects = Arrays.stream(num).boxed().toArray(Integer[]::new);
        Arrays.sort(numObjects, Collections.reverseOrder());
        System.out.println(Arrays.toString(numObjects));

Output

[10, 5, 4, 3, 6, 9, 7, 8, 2, 1]
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
[10, 9, 8, 7, 6, 5, 4, 3, 2, 1]

8. Join Array

We can use Stream to join multiple Arrays.

package com.mkyong;
import java.util.Arrays;
import java.util.stream.IntStream;
import java.util.stream.Stream;
public class ArrayExample8 {
    public static void main(String[] args) {
        String[] s1 = new String[]{"a", "b", "c"};
        String[] s2 = new String[]{"d", "e", "f"};
        String[] result = Stream.of(s1, s2).flatMap(Stream::of).toArray(String[]::new);
        System.out.println(Arrays.toString(result));
        int[] num1 = new int[]{1, 2, 3};
        int[] num2 = new int[]{4, 5, 6};
        int[] result2 = IntStream.concat(Arrays.stream(num1), Arrays.stream(num2)).toArray();
        System.out.println(Arrays.toString(result2));

Output

[a, b, c, d, e, f]
[1, 2, 3, 4, 5, 6]

9. FAQs

9.1 Convert Array to a List

9.2 Convert Array to Stream