Java基础之流程控制

作用：获取用户的输入

语法：

Scanner scanner = new Scanner(System.in);

Scanner中的方法：

next()：用于接收用户输入的数据，且一定要读取到有效字符，遇到空格则结束

import java.util.Scanner;

public class Demo01 {
    public static void main(String[] args) {
        //扫描器对象，用于接收键盘数据
        Scanner scanner = new Scanner(System.in);

        System.out.println("使用next()方式接收：");

        //判断用户有没有输入字符串
        if (scanner.hasNext()){
            String str = scanner.next();
            System.out.println("输出的内容为："+str);
        }
        //属于IO的类用完要关闭，否则会一直占用资源
        scanner.close();
    }
}

nextLine():用于接收用户输入的数据，结束条件为enter

import java.util.Scanner;

public class Demo02 {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        System.out.println("使用nextLine()接收:");

        if (scanner.hasNextLine()){
            String str = scanner.nextLine();
            System.out.println("接收的数据为："+str);
        }

        scanner.close();
    }

}

使用hasNext()方法和hasNextLine()方法判断接收的数据！

进阶使用：

import java.util.Scanner;

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

        double sum = 0;
        //计算输入了多少个数
        int m = 0;

        while (scanner.hasNextDouble()){
            //接收用户输入的数据
            double x = scanner.nextDouble();

            m = m + 1;  //m++
            sum = sum + x;
            System.out.println("你输入了第"+m+"个数据，当前结果为sum="+sum);
        }

        System.out.println("输入的数据的和为："+sum);
        System.out.println("输入的数据的平均值为："+(sum/m));


        scanner.close();
    }
}

概念：按照步骤一步步执行，它是所有算法的基础！

1. 单选择

   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方法，判断字符串是否相等
           if (s.equals("Hello")){
               System.out.println(s);
           }
   
           System.out.println("End");
           scanner.close();
       }
   }
   
   

2. 双选择

   import java.util.Scanner;
   
   public class IfDemo02 {
       public static void main(String[] args) {
           Scanner scanner = new Scanner(System.in);
   
           System.out.println("请输入成绩：");
           int score = scanner.nextInt();
           
           if (score>=60){
               System.out.println("及格");
           }else {
               System.out.println("不及格！");
           }
           
           
           scanner.close();
       }
   }
   
   

3. 多选择

   import java.util.Scanner;
   
   public class IfDemo03 {
       public static void main(String[] args) {
           Scanner scanner = new Scanner(System.in);
           System.out.println("请输入成绩：");
   
           int score = scanner.nextInt();
   
           if (score==100){
               System.out.println("满分");
           }else if (90<=score && score<100){
               System.out.println("A级");
           }else if (80<=score && score<90){
               System.out.println("B级");
           }else if (70<=score && score<80){
               System.out.println("C级");
           }else if (60<=score && score<70){
               System.out.println("D级");
           }else if (score<60){
               System.out.println("不及格");
           }else {
               System.out.println("成绩不合法");
           }
       }
   }
   
   

//字符串比较是jdk7新特性
        //字符的本质还是数字

public class SwitchDemo01 {
    public static void main(String[] args) {
        //case穿透，即没break就会把下面的所有case输出
        
        char grade = '4';

        switch (grade){
            case 'A':
                System.out.println("优秀");
                break;
            case 'B':
                System.out.println("良好");
                break;
            case 'C':
                System.out.println("中等");
                break;
            case 'D':
                System.out.println("及格");
                break;
            case 'E':
                System.out.println("挂科");
                break;
            default:
                System.out.println("未知等级");
        }
    }
}

都不终止代码的运行，只是跳出循环

break:用于强行退出循环（主要用于switch和一些需要跳出代码块的循环语句中）

public class BreakDemo {
    public static void main(String[] args) {
        int i = 0;
        while (i<100){
            i++;
            System.out.println(i);
            if (i==20){
                break;
            }
        }
    }
}

continue：只跳出某一次循环(即跳过尚未执行的语句，进入下一次循环，再接着判断是否满足)

public class ContinueDemo {
    public static void main(String[] args) {
        int i = 0;
        while (i<100){
            i++;
            if (i%5==0){
                System.out.println();
                continue;
            }
            System.out.println(i);
        }
    }
}

使用while循环时，需注意不要出现死循环！

public class WhileDemo02 {
    public static void main(String[] args) {
        //计算1~100的和
        int i = 0;
        int sum = 0;
        while (i<=100){
            sum = sum + i;
            i++;
        }
        System.out.println(sum);
    }
}

while循环是先判断后执行；do…while循环则是先执行后判断（保证最少执行一次）！

public class DoWhileDemo01 {
    public static void main(String[] args) {
        int i = 0;
        int sum = 0;

        do {
            sum = sum + i;
            i++;
        }while (i<=100);
        System.out.println(sum);
    }
}

//while和do...while区别
public class DoWhileDemo02 {
    public static void main(String[] args) {
        int a = 0;
        while (a<0){
            System.out.println(a);
            a++;
        }
        System.out.println("=============");
        do {
            System.out.println(a);
            a++;
        }while (a<0);
    }
}

public class ForDemo01 {
    public static void main(String[] args) {
        //for循环和while循环的区别
        int i = 0;
        int a;
        int sum = 0;
        int sum1 = 0;

        while (i<100){
            i++;
            sum += i;
        }
        System.out.println(sum);
        System.out.println("========");
        for (a=0;a<=100;a++){
            sum1 = sum1 + a;
        }
        System.out.println(sum1);
    }
}

public class ForDemo02 {
    public static void main(String[] args) {
        //计算0~100之间奇数和偶数的和

        int oddSum = 0;
        int evenSUm = 0;

        for (int i = 0; i < 101; i++) {
            if (i%2!=0){
                oddSum += i;
            }else {
                evenSUm += i;
            }
        }
        System.out.println("奇数的和为："+oddSum);
        System.out.println("偶数的和为："+evenSUm);
    }
}

public class ForDemo03 {
    public static void main(String[] args) {
        //输出1~1000之间能被5整除的数，每行输出3个
        for (int i = 0; i < 1001; i++) {
            if (i%5==0){
                System.out.println(i+"t");
            }
            if (i%(5*3)==0){//每行
                System.out.println();
                //System.out.print("n");
            }
            //print不换行输出
            //println换行输出
        }
    }
}


public class ForDemo05 {
    public static void main(String[] args) {
        for (int i=1;i<=9;i++) {
            for (int j = 1; j <= i; j++) {
                System.out.print(j + "*" + i + "=" + (j * i) + "t");
            }
            System.out.println();
        }
    }
}




public class ForDemo04 {
    public static void main(String[] args) {
        for (int i=1;i<=9;i++) {
            //将输出的结果居中
            for (int a=i;a<9;a++){
                System.out.print("t");
            }
            for (int j = 1; j <= i; j++) {
                System.out.print(j + "*" + i + "=" + (j * i) + "t");
            }
            System.out.println();
        }
    }
}

增强for循环

public class ForDemo06 {
    public static void main(String[] args) {
        //增强for循环
        int[] numbers = {1,2,3,4,5};

        //普通方法
        for (int i = 0;i<5;i++){
            System.out.println(numbers[i]);
        }
        System.out.println("========");

        //用于遍历数组
        for (int x:numbers){
            System.out.println(x);
        }
    }
}

public class Test01 {
    //打印1~100的所有质数
    public static void main(String[] args) {
        for (int i = 2; i <= 100; i++) {//1既不是质数也不是和数，所以从2开始
            boolean k = true;
            for (int n = 2; n < i; n++) {
                if (i % n == 0) {
                    k = false;
                    break;
                }
            }
            if (k) {
                System.out.print(i + " ");
            }
        }
    }
}

public class Test02 {
    public static void main(String[] args) {
        //打印5行三角形
        for (int i = 1; i <= 5; i++){
            for (int j = 5; j >= i; j--){
                System.out.print(" ");
            }
            for (int j = 1; j <= i; j++){
                System.out.print("*");
            }
            for (int j = 1; j < i; j++){
                System.out.print("*");
            }
            System.out.println();
        }
    }
}

作用：用于调试代码，可以检查代码中数据的走向！

流程控制的语句都可以嵌套！