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One can resolve basic types and character string Text scanner for , For example, the following code enables the user to from System.in Read a number from

java Specified in the , Strings enclosed in double quotes are immutable

Scanner sc = new Scanner(System.in);
int i = sc.nextInt();

Create objects

 data type   Variable name   =  new   data type (System.in);

String

String Represents string type , Belong to Reference data type , Not a basic data type .

stay java Use casually in In double quotation marks The are String object .

java Specified in the , A string in double quotation marks , Is immutable ,

Comparisons between string objects cannot use “== ”,"==" No insurance . Should be called String class Of equals Method .

leaght(): Get string length
concat(): String concatenation
equals(): String comparison

loop

while loop

grammar ：
while（ The loop condition ）{
The loop body ;
}

do...while loop

grammar ：
do{
The loop body ;
}while( The loop condition );

for loop

grammar ：
for( Loop variable ; The loop condition ; Change the loop variables ）{
The loop body
}

foreach() ( enhance for loop )

 int[][] sparsArray1= new int[11][11];
        sparsArray1[2][1]=3;
        sparsArray1[3][5]=4;
        for (int[] row:sparsArray1) {
            for (int item:
                 row) {
                System.out.print("\t"+item);
            }
            System.out.println();
        }

Dead cycle

for(;;){}
while(true){}

Array

Containers ： It's storing multiple data together , Each data is called the element of the container .

Array concept ： An array is a container that stores data of fixed length , Ensure that the data types of multiple data are consistent .

Definition

The way 1： The data type stored in the array [] Array name = new The data type stored in the array [ length ];

The way 2： data type [] Array name = new data type []{ Elements 1, Elements 2, Elements 3…};

The way 3： data type [] Array name = { Elements 1, Elements 2, Elements 3…};

Array access

                 Indexes ： Every element stored in an array , Will automatically have a number , from 0 Start , This automatic number is called an array index (index), You can access the elements of the array through the index of the array .
                 Format ：
                 Array name [ Indexes ]
                 The length property of an array ： Each array has length , And it's fixed ,Java An attribute of the array is given in , You can get the of the array length , Statement for ： Array name .length , attribute length The result is the length of the array ,int Type the results . It can be inferred from this that , Count The maximum index value for a group is Array name .length-1
                 Index access to elements in an array ：
                 Array name [ Indexes ]= The number , Assign values to elements in an array
                 Variable = Array name [ Indexes ], Get the elements in the array

Array null pointer exception

public static void main(String[] args) { 
	int[] arr = {1,2,3,4,5}; 
	arr = null; 
	System.out.println(arr[0]); 
｝

arr = null This line of code , It means variable arr The memory address of the array will not be saved , You are not allowed to manipulate arrays anymore , So when running Will throw out NullPointerException Null pointer exception . In development , Array out of bounds exception is not allowed , Once it appears , It must be repaired Change the code we write .

Array traversal

Is to get each element of the array separately , It's traversal . Traversal is also the cornerstone of array operations .

 for (int[] row:sparsArray1) {
            for (int item:
                 row) {
                System.out.print("\t"+item);
            }
            System.out.println();
        }

for (int i = 0; i < sparsArray2.length; i++) {
            System.out.printf("%d\t%d\t%d\t",sparsArray2[i][0],sparsArray2[i][1],sparsArray2[i][2]);
            System.out.println();
        }

for (int i = 0; i <sparsArray1.length ; i++) {
            for (int j = 0; j < sparsArray1.length; j++) {
                System.out.println(sparsArray1[i][j])
            }
            System.out.println();
}

Process control

Sequential structure

Refers to the structure of each statement executed by the program from top to bottom .

Selection structure

It is to choose and execute different codes according to the result of condition judgment . Usually by if sentence 、switch Statement to implement the selection structure .

Simple choice structure

if The format of the statement is as follows :

		 if( expression ){
             // Code block 
         }

Loop structure

It is to execute a piece of code repeatedly according to the judgment conditions . Usually by while sentence 、do-while Statement and for Statement to realize the loop structure .