狂神说JavaSE篇Day05

一、用户交互 1.Scanner对象

java5新特性：java.util.Scanner,通过Scanner类获取用户输入
基本语法

Scanner s = new Scanner(System.in)
方法：next(),nextLine(),hasNext(),hasNextLine()

package com.zhu.scanner;

import java.util.Scanner;

public class Demo01 {
    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);
        //判断用户有没有输入字符串
//        if (scanner.hasNextLine()){
//            //使用nextLine方式接收
//            String s = scanner.nextLine();
//            System.out.println(s);
//        }
//        if (scanner.hasNext()){
//            //使用next方式接收
//            String s = scanner.next();
//            System.out.println(s);
//        }
        
        String s = scanner.next();
        System.out.println(s);
        //凡是属于IO流的类如果不关闭会一直占用资源，要养成好习惯用完就关闭
        scanner.close();
    }
}

next():必须有效字符、忽略有效字符空格后的内容
nextLine():可以为空，可以获得空白

2.scanner进阶用法

package com.zhu.scanner;

import java.util.Scanner;

public class Demo02 {
    public static void main(String[] args) {
        int i = 0;
        float f = 0.0F;
        Scanner scanner = new Scanner(System.in);
        System.out.println("请输入整数数据");
        if (scanner.hasNextInt()){
            i = scanner.nextInt();  //i变量在前面已经定义
            System.out.println("整数数据: "+i);
        }
        else{
            System.out.println("请输入整数");
        }
        System.out.println("请输入小数数据");
        if (scanner.hasNextFloat()){
            f = scanner.nextFloat();
            System.out.println("小数数据: "+f);
        }
        else{
            System.out.println("请输入小数");
        }
        scanner.close();
    }
}

二、顺序结构

三、if选择结构 1.if单选择

package com.zhu.struct;

import java.util.Scanner;

public class IfDemo01 {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.println("请输入内容：");
        String s = scanner.nextLine();
        //equals:判断字符串是否相等
        if(s.equals("hello")){
            System.out.println("答对了！");
        }
        System.out.println("结束");
        scanner.close();
    }
}

2.if双选择

package com.zhu.struct;

import java.util.Scanner;

public class IfDemo02 {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.println("请输入内容：");
        int score = scanner.nextInt();
        //equals:判断字符串是否相等
        if(score>=60){
            System.out.println("及格了！");
        }
        else {
            System.out.println("不及格");
        }
        scanner.close();
    }
}

3.if多选择

4.if嵌套

5.switch多选择

package com.zhu.struct;

public class SwitchDemo01 {
    public static void main(String[] args) {
        char grade = 'A';
        //switch匹配一个具体的值
        switch (grade){
            case 'A':
                System.out.println("优秀");
                break;  //case穿透  如果不加break，匹配到后还会继续往下执行
            case 'B':
                System.out.println("良好");
            case 'C':
                System.out.println("及格");
            case 'D':
                System.out.println("不及格");
            default:
                System.out.println("未知");
        }
    }
}

反编译：将class文件直接拷到项目中，即可查看

四、While循环结构

package com.zhu.struct;

public class WhileDemo01 {
    public static void main(String[] args) {
        int n = 0;
        while (n<100){
            n++;
            System.out.println(n);
        }
    }
}

五、Do While循环

六、For循环

package com.zhu.struct;

public class ForDemo01 {
    public static void main(String[] args) {
        int sum = 0;
        for(int i=1;i<=100;i++){
            sum += i;
        }
        System.out.println(sum);
    }
}

idea快捷键100.for

注意事项：

• 初始化时可申明一个或多个变量，也可以为空

练习1

package com.zhu.struct;

public class ForDemo02 {
    public static void main(String[] args) {
        //练习1：计算0到100之间奇数和偶数的和
        int sum_1 = 0;
        int sum_2 = 0;
        for (int i = 0; i <=100; i++) {
            if(i%2==0){
                sum_2 += i;
            }else{
                sum_1 += i;
            }
        }
        System.out.println("奇数和为："+sum_1);
        System.out.println("偶数和为："+sum_2);
    }
}

练习2

package com.zhu.struct;

public class ForDemo03 {
    public static void main(String[] args) {
        //练习2：输出1-1000中能被5整除的数，且每行输出3个
        for (int i = 1; i <= 1000; i++) {
            if(i%5==0){
                System.out.print(i+"\t");
            }
            if(i%15==0){
                System.out.println();
            }
        }
    }
}

print和println的区别
println输出完自动换行、print不会自动换行

练习3

package com.zhu.struct;

public class ForDemo04 {
    public static void main(String[] args) {
        //练习3：打印99乘法表
        for (int i = 1; i < 10; i++) {
            for (int j = 1; j <=i; j++) {
                System.out.print(i+"*"+j+"="+i*j+"\t");
            }
            System.out.println();
        }
    }
}

七、增强for循环

package com.zhu.struct;

public class ForDemo05 {
    public static void main(String[] args) {
        int [] numbers = {10,20,30,40,50};
        for(int x :numbers){
            System.out.println(x);
        }
    }
}

八、break，continue，goto

练习

package com.zhu.struct;

public class ForDemo06 {
    public static void main(String[] args) {
        //打印五行三角形
        for (int i = 1; i <= 5; i++) {
            for (int j = 0;j<(5-i);j++){
                System.out.print(" ");
            }
            for (int j = 0;j<i;j++){
                System.out.print("*");
            }
            for (int j = 1;j<i;j++){
                System.out.print("*");
            }
            System.out.println();
        }
    }
}