Java How to declare and initialize an Array
摘要: Few Java examples to declare, initialize and manipulate Array in Java
Few Java examples to declare, initialize and manipulate Array in Java
1. Declares Array
1.1 For primitive types.
ArrayExample1.java
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.
ArrayExample1.java
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.
ArrayExample2.java
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
ArrayExample3.java
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.
ArrayExample4.java
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
ArrayExample5.java
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);
ArrayExample6.java
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.
ArrayExample7.java
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.
ArrayExample8.java
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
