1. Array overview

1. Understanding of array: an array is a collection of multiple data of the same type arranged in a certain order and named with a name,
   These data are uniformly managed by numbering.
2. Array related concepts:

• Array name
• element
• Corner mark, subscript, index
• Length of array: number of elements

1. Array features:

• Arrays are ordered
• Array is a variable of reference data type. The element of an array can be either a basic data type or a reference data type
• Creating an array object opens up a whole contiguous space in memory
• Once the length of the array is determined, it cannot be modified.

1. Classification of arrays:
   ① Illumination dimension: one-dimensional array, two-dimensional array...
   ② According to the type of array elements: array of basic data type elements and array of reference data type elements

2. One dimensional array

2.1 declaration and initialization

//The right way:
	int num;//statement
		num = 10;//initialization
		int id = 1001;//Declaration + initialization
		
		int[] ids;//statement
		//1.1 static initialization: array initialization and array element assignment are performed simultaneously
		ids = new int[]{1001,1002,1003,1004};
		//1.2 dynamic initialization: array initialization and array element assignment are performed separately
		String[] names = new String[5];

	int[] arr4 = {1,2,3,4,5};//Type inference

//Wrong way:
//		int[] arr1 = new int[];
//		int[5] arr2 = new int[5];
//		int[] arr3 = new int[3]{1,2,3};

2.2 reference of one-dimensional array elements: called by corner markers.

//The subscript (or index) of the array starts from 0 and ends with the length of the array - 1.
names[0] = "Wang Ming";
names[1] = "Wang He";
names[2] = "placed under house arrest";
names[3] = "Sun Julong";
names[4] = "Wang Hongzhi";//charAt(0)

2.3 array attribute: length

System.out.println(names.length);//5
System.out.println(ids.length);

explain:
Once the array is initialized, its length is determined. arr.length
Once the array length is determined, it cannot be modified.

2.4 traversal of one-dimensional array

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

2.5 default initialization value of one-dimensional array elements

1. Array element is an integer: 0
2. Array elements are floating point: 0.0
3. Array elements are char type: 0 or '\ u0000', not '0'
4. Array elements are boolean: false
5. Array element is a reference data type: null

2.6 memory parsing of one-dimensional array

3. Two dimensional array

3.1 how to understand two-dimensional array?

Array is a reference data type
The elements of an array can also be reference data types
If the elements of a one-dimensional array a are of one-dimensional array type, the array A is called a two-dimensional array.

3.2 declaration and initialization of two-dimensional array

//The right way:

	int[] arr = new int[]{1,2,3};//One dimensional array
		//initiate static
		int[][] arr1 = new int[][]{{1,2,3},{4,5},{6,7,8}};
		//Dynamic initialization 1
		String[][] arr2 = new String[3][2];
		//Dynamic initialization 2
		String[][] arr3 = new String[3][];
	//It is also the correct 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}};//Type inference
//Wrong way:
//		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}};

3.3 how to call 2D array elements:

	System.out.println(arr1[0][1]);//2
	System.out.println(arr2[1][1]);//null
	
	arr3[1] = new String[4];
	System.out.println(arr3[1][0]);
System.out.println(arr3[0]);//

3.4 properties of two-dimensional array:

System.out.println(arr4.length);//3
System.out.println(arr4[0].length);//3
System.out.println(arr4[1].length);//4

3.5 traversing two-dimensional array elements

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();
}

3.6 default initialization value of two-dimensional array elements

Regulation: the two-dimensional array is divided into the elements of the outer array and 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

Default initialization value for array elements
For initialization method 1: for example: int[][] arr = new int[4][3];
The initialization value of the outer element is: address value
The initialization value of the inner element is: the same as that of the one-dimensional array

For initialization mode 2: for example: int[][] arr = new int[4] [];
The initialization value of the outer element is null
The initialization value of the inner element is: it cannot be called, otherwise an error will be reported.

3.7 memory structure of two-dimensional array

4. Practice

• Write two ways to initialize a one-dimensional array

int[] arr = new int[5];// dynamic initialization
String[] arr1 = new String[]{“Tom”,“Jerry”,“Jim”};// initiate static

Once the array is initialized, its length is determined. arr.length
Once the array length is determined, it cannot be modified.

1. Write out two ways to initialize a two-dimensional array

int[][] arr = new int[4][3];// Dynamic initialization 1
int[][] arr1 = new int[4][];// Dynamic initialization 2

1. How to traverse the following two-dimensional array
   int[] arr = new int[][]{{1,2,3},{4,5},{6,7,8}};

int[] arr = new int[][]{{1,2,3},{4,5},{6,7,8}};

for(int i = 0;i < arr.length;i++){
	for(int j = 0;j < arr[i].length;j++){
		System.out.print(arr[i][j] + "\t");
	}
	System.out.println();
}

1. What are the default initialization values for different types of one-dimensional array elements

Integer: 0
Floating point: 0.0
char:0
boolean :false
Reference type: null

1. Memory parsing of one-dimensional array:
   String[] strs = new String[5];
   strs[2] = "Tom";
   strs = new String[3];