The use of two-dimensional array (including the definition of two-dimensional array, the declaration and initialization of two-dimensional array (dynamic initialization, static initialization), common

The use of two-dimensional arrays （ Including the definition of two-dimensional array , Two dimensional array declaration and initialization （ dynamic initialization , initiate static ）, Common assignment methods for binary arrays （ dynamic initialization , Assignment of static initialization ）, Wrong definition, assignment method, etc ）

The details are in the code comments  , Very detailed , Read carefully and you will get something

package com.atcording.java;
/*
 *  The use of two-dimensional arrays 
 * 
 * 1. understand ：
 *  Understanding of two-dimensional arrays , We can think of it as a one-dimensional array array1 As another one-dimensional array array2 Of the elements of .
 *  Actually , From the operating mechanism of the underlying array , There's no multidimensional array .
 * 
 * 2.  The use of two-dimensional arrays :
 *   ①  Two dimensional array declaration and initialization 
 *   ②  How to call the element in the specified position of the array 
 *   ③  How to get the length of an array 
 *   ④  How to traverse arrays 
 *   ⑤  The default initialization value of an array element  : see  ArrayTest3.java
 *   ⑥  Memory parsing of arrays  : see  ArrayTest3.java
 * 
 * 
 */
public class ArrayTest2 {
	public static void main(String[] args) {
		//1. Two dimensional array declaration and initialization 
		int[] arr = new int[]{1,2,3};// Assignment of one-dimensional array 
		// initiate static ( Assign values directly after definition )
		int[][] arr1 = new int[][]{
   {1,2,3},{4,5},{6,7,8}};
        /* A two-dimensional array can be simply understood as a one-dimensional array 
         Nesting of , This two bit array is equivalent to defining a one-dimensional array with three large elements , And the three big elements contain 
         Different small elements , A pair of braces in braces is equivalent to a large element , The first big element is {1,2,3}, The small element is  
         1,2,3 Three ,3 In the third small element of the first large element arr[0][2]==3,arr[1][1]==5*/
		// dynamic initialization 1( Dynamic initialization refers to defining an empty array first , After the definition is completed, it can be assigned at any time )
		String[][] arr2 = new String[3][2];// When defining, both inner and outer elements define 
        /*  dynamic initialization 1 assignment 
          for example ：arr2[0][0]=2;arr2[0][1]=3;arr2[1][0]=6; etc. */

		// dynamic initialization 2
		String[][] arr3 = new String[3][];// Only outer elements are defined , Undefined inner layer 
       /* dynamic initialization 2 assignment ：
        arr3[1] = new String[4];( Equivalent to after assignment arr3[1] There are four empty elements inside, and then you can assign values one by one )
       arr3[1][0]="ni";arr3[1][1]="nih"; etc. 
        */
		// Wrong situation  
//		String[][] arr4 = new String[][4];
//		String[4][3] arr5 = new String[][];
//		int[][] arr6 = new int[4][3]{
   {1,2,3},{4,5},{6,7,8}};
		
		// It's also the right way to write ：
		int[] arr4[] = new int[][]{
   {1,2,3},{4,5,9,10},{6,7,8}};
		int[] arr5[] = {
   {1,2,3},{4,5},{6,7,8}};
		
		//2. How to call the element in the specified position of the array 
		System.out.println(arr1[0][1]);// Output results ：2
		System.out.println(arr2[1][1]);
        /* Output results ：null（ Because array arr2 Initialize for dynamic , Not assigned, so it is empty ）*/
		arr3[1] = new String[4];
		System.out.println(arr3[1][0]);
		
		//3. Gets the length of the array 
		System.out.println(arr4.length);// Output results ：3
       /*int[] arr4[] = new int[][]{
   {1,2,3},{4,5,9,10},{6,7,8}};arr4 There are three parentheses in the array braces  
        Number , There are three major elements */
		System.out.println(arr4[0].length);// Output results ：3（ It is equivalent to that the first big element has 3 A small element ）
		System.out.println(arr4[1].length);// Output results ：4（ It is equivalent to that the second big element has 4 A small element ）
		
		//4. How to traverse a two-dimensional array 
		for(int i = 0;i < arr4.length;i++){
			
			for(int j = 0;j < arr4[i].length;j++){
				System.out.print(arr4[i][j] + "  ");
			}
			System.out.println();
		}
		
	}
}









 Code 2：

/*
 *  The use of two-dimensional arrays ：
 * 	 Regulations ： The two-dimensional array is divided into the elements of the outer array , The elements of the inner array 
 * 		int[][] arr = new int[4][3];
 * 		 Outer elements ：arr[0],arr[1] etc. 
 * 		 Inner elements ：arr[0][0],arr[1][2] etc. 
 * 
 *   ⑤  The default initialization value of an array element  
 *    For initialization mode 1 ： such as ：int[][] arr = new int[4][3];
 *       The initialization value of the outer element is ： Address values 
 *       The initialization value of the inner element is ： Same as one-dimensional array initialization 
 *      
 *    For initialization mode 2 ： such as ：int[][] arr = new int[4][];
 *   	 The initialization value of the outer element is ：null
 *       The initialization value of the inner element is ： Cannot call , Otherwise, the report will be wrong .
 *   
 *   ⑥  Memory parsing of arrays  
 * 
 */
public class ArrayTest3 {
	public static void main(String[] args) {
		// Define an integer array 
		int[][] arr = new int[4][3];
		System.out.println(arr[0]);// Output results ：[[email protected] 
		System.out.println(arr[0][0]);// Output results ：0
		
//		System.out.println(arr);// Output results ：[[[email protected]
		// Define floating point arrays 
		System.out.println("*****************");
		float[][] arr1 = new float[4][3];
		System.out.println(arr1[0]);// Output results ： Address values 
		System.out.println(arr1[0][0]);// Output results ：0.0
		
		System.out.println("*****************");
		// Define a string array 
		String[][] arr2 = new String[4][2];
		System.out.println(arr2[1]);// Output results ： Address values 
		System.out.println(arr2[1][1]);// Output results ：null（ The array is empty because it is not assigned a value ）
		
		System.out.println("*****************");
		double[][] arr3 = new double[4][];
		System.out.println(arr3[1]);// Output results ：null（ The array is empty because it is not assigned a value ）
//		System.out.println(arr3[1][0]);// Report errors （ Only the outer element is defined, and the inner element is not defined, so an error is reported ）
		
	}
}