JavaSE笔记02

JavaSE笔记02

一：用户交互Scanner

基本语法

import java.util.Scanner;

public class Demo02 {
    public static void main(String[] args) {
//        创建一个扫描器对象，用于接收键盘数据
        Scanner scanner = new Scanner(System.in);
        System.out.println("请输入你想输入的内容：");
        
//        判断用户用户是否还有输入
        if (scanner.hasNext()){   
//            String str = scanner.next();
            String str = scanner.nextLine();
            System.out.println("输出的内容为：" + str);
        }

//        凡是属于IO流的类如果不关闭会一直占用资源
        scanner.close();
    }
}

1. next()：不能输入带有空格的字符串；
2. nextLine():以enter为结束符；

import java.util.Scanner;

public class Demo03 {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        int i = 0;
        float f = 0.0f;

        System.out.println("请输入整数：");
      
        if (scanner.hasNextInt()){
            i = scanner.nextInt();
            System.out.println("整数数据：" + i);
        }else {
            System.out.println("你输入的不是整数");
        }

        System.out.println("请输入小数：");
        if (scanner.hasNextFloat()){
            f = scanner.nextFloat();
            System.out.println("小数数据：" + f);
        }else {
            System.out.println("你输入的不是小数");
        }

        scanner.close();
    }
}

二：顺序结构(java基本结构就是顺序结构)

选择结构

if单选择结构

import java.util.Scanner;

public class Demo04 {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        System.out.println("请输入内容：");

        String s = scanner.nextLine();
        
        if (s.equals("hello")){
            System.out.println(s);
        }
            
      	System.out.println("END");
        scanner.close();
    }
}

if双选择结构

import java.util.Scanner;

public class Demo05 {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        System.out.println("请输入考试分数：");

        int i = scanner.nextInt();
        
        if (i > 60){
            System.out.println("及格");
        }else{
            System.out.println("不及格");
        }
        scanner.close();
    }
}

if多选择结构

import java.util.Scanner;

public class Demo06 {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        System.out.println("请输入考试分数：");

        int i = scanner.nextInt();

        if (i == 100){
            System.out.println("满分");
        }else if(i < 100 && i >=90){
            System.out.println("y优秀");
        }else if(i < 90 && i >=80){
            System.out.println("良好");
        }else if(i < 80 && i >=60){
            System.out.println("及格");
        }else {
            System.out.println("不及格");
        }
        scanner.close();
    }
}

嵌套的if结构

import java.util.Scanner;

public class Demo07 {

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        System.out.println("请输入0到100之间到数字");

        int i = scanner.nextInt();
        
        if( i >= 0 && i <= 100){
            if(i > 50){
                if(i >75){
                }else {
                }
            }else {
                if(i > 25){
                }else {
                }
            }
        }else {
            System.out.println("输入错误");
        }
    }
}

switch多选择结构

1. switch语句中的变量类型是byte、short、int、char
2. switch支持字符串String类型了
3. 同时case标签必须为字符串常量或字面量

public class Demo08 {
    public static void main(String[] args) {
        
//        case 穿透
        char a = 'c';
        
        switch (a){
            case 'A':
                System.out.println("A");
                break;
            case 'B':
                System.out.println("B");
                break;
            case 'C':
                System.out.println("C");
                break;
            case 'D':
                System.out.println("D");
                break;
            default:
                System.out.println("未知");
        }
    }
}

三：循环结构

while循环(不满足条件不循环)

public class Demo09 {
    public static void main(String[] args) {
        int i = 0;
        int sum = 0;
        while (i <= 100){
          	sum = sum + i;
            i++;
        }
      
        System.out.println(sum);
    }
}

do...while循环(最少循环一次)

public class Demo10 {
    public static void main(String[] args) {
        int i = 0;
        int sum = 0;
        
        do {
            sum = sum + i;
            i++;
        }while (i <= 100);

        System.out.println(sum);
    }
}

for循环(初始化；布尔表达式；更新)

public class Demo11 {
    public static void main(String[] args) {
        int sum = 0;
        for (int i = 0; i <=100; i++){
            sum = sum + i;
        }
        System.out.println(sum);
    }
}

增强型for(遍历数组与集合)

public class Demo12 {
    public static void main(String[] args) {
        int [] a = {11,22,33,44,55,66,77,88,99};
        for (int x : a){
            System.out.println(x);
        }
    }
}

break(用于强制退出循环)

continue(退出本次循环，继续下次循环)