1. Sequential structure

The sequential structure is relatively simple. For example, the code we wrote before is a sequential structure, which is executed line by line in the order of code writing
If you adjust the writing order of the code, the execution order also changes

2. Branch structure

2.1 if statement

Basic grammatical form 1

if(Boolean expression){
  //Execute code when conditions are met
}	

Basic grammatical form 2

if(Boolean expression){
  //Execute code when conditions are met
}else{
  //Execute code when conditions are not met
}

Basic grammatical form 3 multi branch case

if(Boolean expression){
  //Execute code when conditions are met
}else if(Boolean expression){
  //Execute code when conditions are met
}else{
  //Execute code when none of the conditions are met
}

2.2 switch statement

Basic grammar

switch(integer|enumeration|character|character string){
case Content 1 : {
Execute statement when content is satisfied;
[break;]
}
case Content 2 : {
Execute statement when content is satisfied;
[break;]
}
...
default:{
Execute the statement when the content is not satisfied;
[break;]
}
}

Note 1: depending on the value of the switch, the corresponding case statement will be executed. The case statement will end when a break is encountered. If the value in the switch does not match the case, the statement in default will be executed
We suggest that a switch statement should preferably carry default
Note 2 do not omit break, otherwise the effect of "multi branch selection" will be lost

int day = 1;
switch(day) {
  case 1:
    System.out.println("Monday");
    // break;
  case 2:
    System.out.println("Tuesday");
    break;
}
// Operation results
 Monday
 Tuesday

We find that when we don't write break, the case statements will be executed downward in turn, thus losing the effect of multiple branches
Note 3 the type of value in switch can only be byte, int, short, char, string and enumeration.

double num = 1.0;
switch(num) {
  case 1.0:
    System.out.println("hehe");
    break;
  case 2.0:
    System.out.println("haha");
    break;
}
// Compilation error
Test.java:4: error: Incompatible types: from double Convert to int There may be losses switch(num) {
           ^
1 Errors

Note 4 switch cannot express complex conditions

// For example, if the value of num is between 10 and 20, print hehe
// Such code is easy to express using if, but it cannot be expressed using switch
if (num > 10 && num < 20) {
System.out.println("hehe");
}

Note 5 although switch supports nesting, it is ugly~

int x = 1;
int y = 1;
switch(x) {
    case 1:
       switch(y) {
           case 1:
              System.out.println("hehe");
              break;
              }
   break;
   case 2:
      System.out.println("haha");
   break;
}

The beauty of the code is also an important standard. After all, this is the face world

3. Circulation structure

3.1 while loop

Basic syntax format:

while(Cycle condition){
Circular statement;
}

If the loop condition is true, the loop statement is executed; Otherwise end the cycle
matters needing attention

1. Similar to if, the statement under while may not write {}, but only one statement can be supported when it is not written. It is recommended to add {}
2. Similar to if, the {suggestion after while is written on the same line as while
3. Similar to if, do not write more semicolons after while, otherwise the loop may not execute correctly

3.2 break

The function of break is to end the cycle ahead of time
Code example: find the multiple of the first 3 in 100 - 200

int num = 100;
while (num <= 200) {
if (num % 3 == 0) {
System.out.println("A multiple of 3 was found, by:" + num);
break;
}
num++;
}
// results of enforcement
 A multiple of 3 was found, by:102

Executing break will end the loop

3.3 continue

The function of continue is to skip this cycle and immediately enter the next cycle
Code example: find multiples of all 3 in 100 - 200

int num = 100;
while (num <= 200) {
if (num % 3 != 0) {
num++; // Don't forget the + + here! Otherwise it will loop
continue;
}
System.out.println("A multiple of 3 was found, by:" + num);
num++;
}

When the continue statement is executed, it will immediately enter the next cycle (determine the cycle conditions), so it will not execute the following print statement

3.4 for loop

Basic grammar

for(Expression 1;Expression 2;Expression 3){
Circulatory body;
}

• Expression 1: used to initialize a loop variable
• Expression 2: loop condition
• Expression 3: update loop variable

Compared with the while loop, the for loop combines these three parts and is not easy to miss when writing code
Precautions (similar to while loop)

1. Similar to if, the statement below for can not write {}, but only one statement can be supported when it is not written. It is recommended to add {}
2. Similar to if, the {suggestion after for is written on the same line as while
3. Similar to if, do not write more semicolons after for, otherwise the loop may not execute correctly

3.5 do while loop (optional)

Basic grammar

do{
Circular statement;
}while(Cycle condition);

Execute the loop statement first, and then determine the loop condition
Code example: print 1 - 10

int num = 1;
do {
System.out.println(num);
num++;
} while (num <= 10)

matters needing attention

1. Don't forget the semicolon at the end of the do while loop
2. Generally, do while is rarely used, and for and while are recommended

4. Input and output

4.1 output to console

Basic grammar

System.out.println(msg); // Output a string with newline
System.out.print(msg); // Output a string without line breaks
System.out.printf(format, msg); // Format output

• println output content comes with \ n, print does not \ n
• The format output mode of printf is basically the same as that of C language

format string

4.2 input from keyboard

Read in a character (optional)
A character can be read directly using System.in.read, but it needs to be combined with exception handling (which will be highlighted later)

System.out.print("Enter a Char:");
char i = (char) System.in.read();
System.out.println("your char is :"+i);
// Compilation error
Test.java:4: error: Unreported exception error IOException; It must be captured or declared in order to be thrown
char i = (char) System.in.read();
^
1 Errors

Correct writing

import java.io.IOException; // IOException package needs to be imported
try {
    System.out.print("Enter a Char:");
    char i = (char) System.in.read();
    System.out.println("your char is :"+i);
} catch (IOException e) {
    System.out.println("exception");
}

This method is troublesome and we don't recommend it
Use Scanner to read string / integer / floating point number

import java.util.Scanner; // The util package needs to be imported
Scanner sc = new Scanner(System.in);
System.out.println("Please enter your name:");
String name = sc.nextLine();
System.out.println("Please enter your age:");
int age = sc.nextInt();
System.out.println("Please enter your salary:");
float salary = sc.nextFloat();
System.out.println("Your information is as follows:");
System.out.println("full name: "+name+"\n"+"Age:"+age+"\n"+"Salary:"+salary);
sc.close(); // Note that remember to call the close method
// results of enforcement
 Please enter your name:
Zhang San
 Please enter your age:
18
 Please enter your salary:
1000
 Your information is as follows:
full name: Zhang San
 Age: 18
 Salary: 1000.0

Use the Scanner loop to read N numbers

Scanner sc = new Scanner(System.in);
double sum = 0.0;
int num = 0;
while (sc.hasNextDouble()) {
double tmp = sc.nextDouble();
sum += tmp;
num++;
}
System.out.println("sum = " + sum);
System.out.println("avg = " + sum / num);
sc.close();
// results of enforcement
10
40.0
50.5
^Z
sum = 150.5
avg = 30.1

Note: when entering multiple data in a loop, use ctrl + z to end the input

5. Number guessing game

Rules of the game:
The system automatically generates a random integer (1-100), and then the user enters a guessing number. If the entered number is smaller than the random number, it will prompt "low". If the entered number is larger than the random number, it will prompt "high". If the entered number is equal to the random number, it will prompt "guessed right"
Reference code

import java.util.Random;
import java.util.Scanner;

class Test {
    public static void main(String[] args) {
        Random random = new Random(); // The default random seed is system time
        Scanner sc = new Scanner(System.in);
        int toGuess = random.nextInt(100);
        // System.out.println("toGuess: " + toGuess);
        while (true) {
            System.out.println("Please enter the number you want to enter: (1-100)");
            int num = sc.nextInt();
            if (num < toGuess) {
               System.out.println("Low");
            } else if (num > toGuess) {
            System.out.println("High");
            } else {
            System.out.println("You guessed right");
            break;
            }
     }
     sc.close();
     }
}