How do I declare and initialize an array in Java?

Question

How do I declare and initialize an array in Java?

Answer 1

You can either use array declaration or array literal (but only when you declare and affect the variable right away, array literals cannot be used for re-assigning an array).

For primitive types:

int[] myIntArray = new int[3]; int[] myIntArray = {1, 2, 3}; int[] myIntArray = new int[]{1, 2, 3};  // Since Java 8. Doc of IntStream: https://docs.oracle.com/javase/8/docs/api/java/util/stream/IntStream.html  int [] myIntArray = IntStream.range(0, 100).toArray(); // From 0 to 99 int [] myIntArray = IntStream.rangeClosed(0, 100).toArray(); // From 0 to 100 int [] myIntArray = IntStream.of(12,25,36,85,28,96,47).toArray(); // The order is preserved. int [] myIntArray = IntStream.of(12,25,36,85,28,96,47).sorted().toArray(); // Sort  

For classes, for example String, it's the same:

String[] myStringArray = new String[3]; String[] myStringArray = {"a", "b", "c"}; String[] myStringArray = new String[]{"a", "b", "c"}; 

The third way of initializing is useful when you declare an array first and then initialize it, pass an array as a function argument, or return an array. The explicit type is required.

String[] myStringArray; myStringArray = new String[]{"a", "b", "c"}; 

Answer 2

There are two types of array.

One Dimensional Array

Syntax for default values:

int[] num = new int[5]; 

Or (less preferred)

int num[] = new int[5]; 

Syntax with values given (variable/field initialization):

int[] num = {1,2,3,4,5}; 

Or (less preferred)

int num[] = {1, 2, 3, 4, 5}; 

Note: For convenience int[] num is preferable because it clearly tells that you are talking here about array. Otherwise no difference. Not at all.

Multidimensional array

Declaration

int[][] num = new int[5][2]; 

Or

int num[][] = new int[5][2]; 

Or

int[] num[] = new int[5][2]; 

Initialization

 num[0][0]=1;  num[0][1]=2;  num[1][0]=1;  num[1][1]=2;  num[2][0]=1;  num[2][1]=2;  num[3][0]=1;  num[3][1]=2;  num[4][0]=1;  num[4][1]=2; 

Or

 int[][] num={ {1,2}, {1,2}, {1,2}, {1,2}, {1,2} }; 

Ragged Array (or Non-rectangular Array)

 int[][] num = new int[5][];  num[0] = new int[1];  num[1] = new int[5];  num[2] = new int[2];  num[3] = new int[3]; 

So here we are defining columns explicitly.
Another Way:

int[][] num={ {1}, {1,2}, {1,2,3,4,5}, {1,2}, {1,2,3} }; 

For Accessing:

for (int i=0; i<(num.length); i++ ) {     for (int j=0;j<num[i].length;j++)         System.out.println(num[i][j]); } 

Alternatively:

for (int[] a : num) {   for (int i : a) {     System.out.println(i);   } } 

Ragged arrays are multidimensional arrays.
For explanation see multidimensional array detail at the official java tutorials