Java process control
Scanner object
We use the Scanner class to get the user's input
Basic syntax: Scanner s=new Scanner(System.in);
Get the input string through the next() and nextLine() methods of Scanner class. Before reading, we generally need to use hasNext() and hasNextLine() to judge whether there is still input data
Scanner object
next():
- Be sure to read valid characters before you can end the input
- The next () method will automatically remove the blank space encountered before entering valid characters
- Only after a valid character is entered, the blank space entered after it will be used as a delimiter or terminator
- next() cannot get a string with spaces
package Javal technological process;
import java.util.Scanner;
public class Demo01 {
public static void main(String[] args) {
//Create a scanner object to accept keyboard data
Scanner scanner = new Scanner(System.in);//alt + ENTER
System.out.println("use next Method receiving: ");
//Judge whether the user has entered a string
if(scanner.hasNext())
{
String str=scanner.next();
System.out.println("The input content is:"+str);
}
//If the class using IO stream is not closed, it will always occupy all resources. Form a good habit and close it after use
scanner.close();
}
}
nextLine()
- With enter as the terminator, that is, the nextLine() method returns all characters before the input carriage return
- Can get blank
package Javal technological process;
import java.util.Scanner;
public class Demo02 {
public static void main(String[] args) {
Scanner scanner = new Scanner(System.in);
System.out.println("use nextline Acceptance method: ");
if(scanner.hasNextLine())
{
String str=scanner.nextLine();
System.out.println("The data entered is"+str);
}
scanner.close();
}
}
Case 1
package Javal technological process.Scanner Use of;
import java.util.Scanner;
public class Demo03 {
public static void main(String[] args) {
System.out.println("Please enter an integer:");
Scanner scanner = new Scanner(System.in);
//Accept data from keyboard
int i=0;
float f=0.1f;
if(scanner.hasNextInt())
{
i=scanner.nextInt();
System.out.println("Integer data"+i);
}
else
{
System.out.println("The input is not integer data");
}
System.out.println("Please enter a decimal:");
Scanner scanner1 = new Scanner(System.in);
if(scanner1.hasNextFloat())
{
f=scanner1.nextFloat();
System.out.println("Decimal data"+f);
}
else
{
System.out.println("The input is not decimal data");
}
scanner.close();
scanner1.close();
}
}
Case 2
package Javal technological process.Scanner Use of;
import java.util.Scanner;
public class Demo04 {
public static void main(String[] args) {
//We need to input multiple numbers and find the sum and average of them. Each number is determined by entering. The input is ended by non numbers and the execution result is output
Scanner scanner = new Scanner(System.in);
//and
double sum=0;
//Calculate how many numbers are entered
int m=0;
System.out.println("Please enter data:");
//Through the loop to determine whether there is still input, and sum each time
while(scanner.hasNextDouble()) {
double x = scanner.nextDouble();
//m++;
m = m + 1;
sum = sum + x;
}
System.out.println("The sum of the current number is"+sum);
System.out.println("The average of the current number is"+sum/m);
}
}
Sequential structure
The basic structure of JAVA is sequential structure. Unless otherwise specified, it will be executed sentence by sentence in order
Sequential structure is the simplest algorithm structure
Between statements and between boxes, it is carried out from top to bottom. It is composed of several steps executed in turn. It is the basic algorithm structure that every algorithm is inseparable from
package Javal technological process.struct;
public class ShuXuDome {
public static void main(String[] args) {
//Sequential structure
System.out.println("hello1");
System.out.println("hello2");
System.out.println("hello3");
System.out.println("hello4");
System.out.println("hello5");
System.out.println("hello6");
}
}
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Select structure
if single selection structure
If (Boolean expression)
{/ / statement to be executed if Boolean expression is true
}
package Javal technological process.struct;
import java.util.Scanner;
public class ifDemo01 {
public static void main(String[] args) {
Scanner scanner = new Scanner(System.in);
System.out.println("Please enter the content");
String str=scanner.nextLine();
//equals: compare strings for consistency
if(str.equals("hello"))
{
System.out.println(str);
}
System.out.println("end");
scanner.close();
}
}
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if double selection structure
Syntax:
If (Boolean expression)
{/ / if Boolean expression is true}
else
{/ / Boolean expression is false}
package Javal technological process.struct;
import java.util.Scanner;
public class IfDemo02 {
public static void main(String[] args) {
System.out.println("Please enter the test score");
Scanner scanner = new Scanner(System.in);
//If the test score is more than 60, you will pass, and if the test score is less than 60, you will fail
double i=scanner.nextDouble();
if(i>=60)
{
System.out.println("pass an examination");
}
else
{
System.out.println("Fail in the exam");
}
scanner.close();
}
}
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if multiple selection structure
Syntax:
If (Boolean expression 1)
{/ / execute code if Boolean expression 1 is true}
Else if (Boolean expression 2)
{/ / if Boolean expression 2 holds, execute the code}
Else if (Boolean expression 3)
{/ / if Boolean expression 3 is true, execute the code}
else
{/ / if none of the above is true, execute the code}
package Javal technological process.struct;
import java.util.Scanner;
public class IfDemo03 {
public static void main(String[] args) {
System.out.println("Please enter the student's grade:");
Scanner scanner = new Scanner(System.in);
double score=scanner.nextDouble();
System.out.println("Then the student's grade is:");
if(score==100)
{
System.out.println("excellent");
}
else if(score<100&&score>=90)
{
System.out.println("A");
}
else if(score<90&&score>=80)
{
System.out.println("B");
}
else if(score<80&&score>=70)
{
System.out.println("C");
}
else
{
System.out.println("D");
}
}
}
An IF statement can have at most one else statement. The else statement is after the if else. An IF statement can have several else if statements. They must be before the else. Once one else if statement is checked to be true, other else if and else statements will skip execution
switch selection structure
Another implementation method of multi selection structure is switch case statement
The switch case statement determines that a variable is equal to a value in a series of values, and each value is called a branch
The switch statement variable type can be
byte，short ,int ,char,String
At the same time, the case tag must be a string constant or literal
switch(expression)
{case value:
//Statement
break；// Optional
case value:
//Statement
break；// Optional
default: / / optional
//Statement
}
package Javal technological process.struct;
public class SwitchDemo01 {
public static void main(String[] args) {
//
char grade='C';
switch (grade)
{
case 'A':
System.out.println("excellent");
break;
case 'B':
System.out.println("good");
break;
case 'C':
System.out.println("pass");
break;
case 'D':
System.out.println("make persistent efforts");
break;
case 'E':
System.out.println("fail");
break;
default:
System.out.println("Unknown level");
break;
}
//switch is the matching value. If there is no break after the case, there will be case penetration, that is, execute down in turn
}
}
package Javal technological process.struct;
public class SwitchDemo02 {
public static void main(String[] args) {
String name="Mad God";
//The expression after JDK7 can also be a string
//String or number
//Decompile Java -- class (bytecode file) -- decompile (IDEA)
switch (name)
{
case "Qing Jiang":
System.out.println("qingjiang");
break;
case "Mad God":
System.out.println("Mad God");
break;
default:
System.out.println(" ");
break;
}
}
}
In bytecode mode: ctrl+shift+alt+s, find the copy path, find the file, find and open the java file from idea, and put the class file in it
Cyclic structure
while Loop
while is the most basic loop. Its structure is:
while (Boolean expression){
//Cyclic content
}
The loop continues as long as the Boolean expression is true
Most of us will stop the loop. We need an expression invalidation to end it
A small part of the loop needs to be executed all the time, such as the server listening for a response
package Javal technological process.XunHuan;
public class WhileDemo01 {
public static void main(String[] args) {
//Output 1-100
int i=0;
while(i<100)
{
i++;
System.out.println(i);
}
}
}
Do while
For a while loop, if the conditions are not met, the loop cannot be entered, and do while occurs at least once
do{
//Code statement
}While (Boolean expression)
The difference between while and do while
While judges recycling first, do while executes recycling first
package Javal technological process.XunHuan;
public class DoWhileDemo01 {
public static void main(String[] args) {
int i=1;
int sum=0;
do{
sum=sum+i;
i++;
}while(i<=100);
System.out.println(sum);
}
}
The difference between do while and while
package Javal technological process.XunHuan;
public class DoWhileDemo02 {
public static void main(String[] args) {
int a=0;
while(a<0)
{
System.out.println(a);
}
System.out.println("===================================================");
do{
System.out.println(a);
}while(a<0);//Password input is generally do while
}
}
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for loop
The for loop statement is a general structure that supports iteration and is the most effective. The most flexible cycle structure
The number of cycles is determined at the beginning of the for loop
For (initialization; Boolean expression; update){
//Code statement
}
package Javal technological process.XunHuan;
public class ForDemo01 {
public static void main(String[] args) {
int sum=0;
//Initialize / / condition judgment / / iteration
for(int a=1;a<=100;a++){
sum=sum+a;
}
System.out.println(sum);
}
}
package Javal technological process.XunHuan;
//Three numbers are required for one line. println is output line feed, and print is output line feed
public class ForDemo02 {
public static void main(String[] args) {
int a=1;
while(a<=1000)
{
if(a%5==0)
{
System.out.println(a);
}
a++;
}
System.out.println("=====================================");
for (int i = 1; i <= 1000; i++) {
if(i%5==0)
{
System.out.println(i);
}
if(i%(5*3)==0)
{
System.out.print("\n");
}
}
}
}
package Javal technological process.XunHuan;
public class ForDemo03 {
public static void main(String[] args) {
for(int i=1;i<=9;i++)
{
for(int a=1;a<=i;a++)
System.out.print(a+"*"+i+"="+(i*a)+"\t");
System.out.println();
}
}
}
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Enhanced for statement
java5 refers to an enhanced for loop that is primarily used for arrays or collections
The syntax format of Java enhanced loop for loop is as follows:
For (declaration statement: expression)
{
//Code sentence
}
Declaration statement: declare a new local variable. The type of the variable must match the type of the array element. Its scope is limited to the circular statement block, and its value is equal to the value of the array element at this time
Expression: the expression is the name of the array to be accessed, or the method whose return value is array
package Javal technological process.XunHuan;
public class ForDemo04 {
public static void main(String[] args) {
int[] numbers={10,20,30,40,50};//Defines an array
for (int i = 0; i < 5; i++) {
System.out.println(numbers[i]);
}
System.out.println("==================================================");
//The element of the variable array, which assigns the number in the numbers array to x
for(int x:numbers){
System.out.println(x);
}
}
}
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break continue
Break in the main part of any loop statement, you can use break to control the flow of the loop. Break is used to forcibly exit the loop without executing the remaining statements in the loop. (break statements are also used in switch statements)
The continue statement is used in a loop statement to terminate a loop process, that is, to skip the statement that has not been executed in the loop body, followed by the determination of the next loop
package Javal technological process.XunHuan;
public class BreakDemo {
public static void main(String[] args) {
int i=0;
while(i<100)
{
i++;
if(i==30)
break;
}
System.out.println(i);
}
}
package Javal technological process.XunHuan;
public class ContinueDemo {
public static void main(String[] args) {
int i=0;
while(i<100){
i++;
if(i%10==0)
{System.out.println(i);
continue;}
System.out.print(i+"\t");
}
}
}
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About goto keyword
This can be achieved through a labeled break or continue
"Label" refers to the identifier followed by a colon, for example: label;
For Java, the only place where the label is used is before the loop statement, and the only reason for using the label before the loop statement is that we want to nest another loop in it. Because the break and continue keywords usually only interrupt the current loop, but if they are used with the label, it will interrupt to the place of the label
package Javal technological process.XunHuan;
public class LabelDemo {
public static void main(String[] args) {
//Print all prime numbers between 101-150
int count=0;
outer:for (int i=101;i<150;i++)
{
for(int j=2;j<i/2;j++)
{ if(i%j==0)
continue outer;}
System.out.println(i+"\t");
}
}
}
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Text01
Print a triangle
package Javal technological process.XunHuan;
public class TextDome01 {
public static void main(String[] args) {
//Print five lines of triangles
for (int i = 1; i <=5; i++) {
for(int j=4;j>=i;j--)
{ System.out.print(" ");}
for(int j=1;j<=i;j++)
{
System.out.print("-");
}
for(int j=1;j<=i-1;j++)
{
System.out.print("-");
}
System.out.println();
}
}
}
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