Array Declarations in Java (Single and Multidimensional) - GeeksforGeeks

21 Aug, 2017
Arrays in Java | Introduction
One Dimensional Array :
It is a collection of variables of same type which is used by a common name.
Examples:One dimensional array declaration of variable:
import java.io.*;  class GFG {    public static void main(String[] args)    {        int[] a; // valid declaration        int b[]; // valid declaration        int[] c; // valid declaration    }}
We can write it in any way.
Now, if you declare your array like below:
import java.io.*;  class GFG {    public static void main(String[] args)    {        // invalid declaration -- If we want to assign         // size of array at the declaration time,  it         // gives compile time error.        int a[5];          // valid declaration        int b[];    }}
Now, suppose we want to write multiple declaration of array variable then we can use it like this.
import java.io.*;  class GFG {     public static void main(String[] args)    {        // valid declaration, both arrays are        // one dimensional array.        int a[], b[];          // invalid declaration        int c[], [] d;          // invalid declaration        int[] e, [] f;    }}
When we are declaring multiple variable of same time at a time, we have to write variable first then declare that variable except first variable declaration. There is no restriction for the first variable.
Now, when we are creating array it is mandatory to pass the size of array; otherwise we will get compile time error.You can use new operator for creating an array.
import java.io.*;  class GFG {    public static void main(String[] args)    {        // invalid, here size of array is not given        int[] a = new int[];          // valid, here creating 'b' array of size 5        int[] b = new int[5];          // valid        int[] c = new int[0];          // gives runtime error        int[] d = new int[-1];    }}
Printing array :
/* A complete Java program to demonstrate working   of one dimensional arrays */class oneDimensionalArray {      public static void main(String args[])    {        int[] a; // one dimensional array declaration        a = new int[3]; // creating array of size 3        for (int i = 0; i < 3; i++) {            a[i] = 100;            System.out.println(a[i]);        }    }}
Output:
100
100
100

Two Dimensional Array
Suppose, you want to create two dimensional array of int type data. So you can declare two dimensional array in many of the following ways:
// Java program to demonstrate different ways// to create two dimensional array.import java.io.*;  class GFG {    public static void main(String[] args)    {        int a[][]; // valid        int[][] b; // valid        int[][] c; // valid        int[] d[]; // valid        int[][] e; // valid        int[] f[]; // valid        [][] int g; // invalid        [] int[] h; // invalid    }}
Now, Suppose we want to write multiple declarations of array variable then you can use it like this.
// Java program to demonstrate multiple declarations// of array variableimport java.io.*;  class GFG {public static void main(String[] args)    {        // Here, 'a' is two dimensional array, 'b'        // is two dimensional array        int[] a[], b[];          // Here, 'c' is two dimensional array, 'd'        // is two dimensional array         int[] c[], d[];           // Here, 'e' is two dimensional array, 'f'         // is three dimensional array        int[][] e, f[];           // Here, 'g' is two dimensional array,         // 'h' is one dimensional array        int[] g[], h;     }}
Creating one dimensional array and two dimensional array without new operator:
/* Java program for one and two dimensional arrays.   without new operator*/class oneTwoDimensionalArray {       public static void main(String args[])    {        int[] a[] = { { 1, 1, 1 }, { 2, 2, 2 },                     { 3, 3, 3 } }, b = { 20 };          // print 1D array        System.out.println(b[0]);          // print 2D array        for (int i = 0; i < 3; i++) {            for (int j = 0; j < 3; j++) {                a[i][j] = 100;                System.out.println(a[i][j]);            }        }    }}
Output:
20
100
100
100
100
100
100
100
100
100

Creating one dimensional array and two dimensional array using new operator:
/* Java program for one and two dimensional arrays.   using new operator*/class oneTwoDimensionalArray {      public static void main(String args[])    {        int[] a[], b = { 20 };        a = new int[3][3];        b = new int[3];          // print 1D array        for (int i = 0; i < 3; i++)            System.out.println(b[i]);          // print 2D array        for (int i = 0; i < 3; i++) {            for (int j = 0; j < 3; j++) {                a[i][j] = 100;                System.out.println(a[i][j]);            }        }    }}
Output:
0
0
0
100
100
100
100
100
100
100
100
100

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