一、Scanner对象

1、next注意

1) 一定要读到有效字符后才可以结束

2) 对输入之前遇到的空白字符，next()方法自动将其去掉

3) 只有输入有效字符之后才将其后面的空白作为分隔符或者结束符

4) next() 不能得到带有空格的字符串

2、nextline()注意

1) 以Enter作为结束符，也就是说nextline()方法返回的是输入回车之前的所有字符

2) 可以获得空格

不能将类名定为Scanner，会发生冲突

用法

import java.util.Scanner; // 导入包

public class Scanner1 {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        System.out.print("请输入:");

        if(scanner.hasNextDouble()){
            Double s = scanner.nextDouble(); // 类型改变
            System.out.print(s);
        } else {
            System.out.print("Don't Double!!!"); // 捕捉类型错误， try catch
        }

        scanner.close();
    }
}

顺序结构

判断结构

import java.util.Scanner;

public class Scanner1 {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        System.out.print("请输入成绩：");

        double score;
        while (scanner.hasNextDouble()) {
            score = scanner.nextDouble();

            if (score <= 100 && score >= 90) {
                System.out.println("优秀");
            } else if (score < 90 && score >= 80) {
                System.out.println("良");
            } else if (score >= 70 && score < 80) {
                System.out.println("一般");
            } else if (score >= 0 && score < 60) {
                System.out.println("不及格");
            } else if (score >= 60 && score < 70) {
                System.out.println("及格");
            } else {
                System.out.println("输入错误！");
            }
        }

        scanner.close();
    }
}

case分支语句

import java.util.Scanner;

public class Scanner1 {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        System.out.print("请输入成绩：");

        String op;

        op = scanner.next();

        switch(op) {
            case "A":
                System.out.print("优秀");
                break;
            case "B":
                System.out.print("良好");
                break;
            case "C":
                System.out.print("一般");
                break;
            case "D":
                System.out.print("及格");
                break;
            case "E":
                System.out.print("不及格");
                break;
            default:
                System.out.print("输入不合法！"); 
        }

        scanner.close();
    }
}

看源码 反编译

循环结构

while 循环 do while 循环 for循环

import java.util.Scanner;

public class Scanner1 {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        int odd = 0;
        int even = 0;
        for(int i = 1; i <= 100; i ++)
            if(i % 2 != 0) odd += i;
            else even += i;

        System.out.println("odd's sum:" + odd);
        System.out.println("even's sum:" + even);

        scanner.close();
    }
}

打印1000以内被5整除的数，每行输出3个

import java.util.Scanner;

public class Scanner1 {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        for(int i = 1; i <= 1000; i ++) {
            if(i % 5 == 0) {
                System.out.print(i + "\t");
                if(i % 15 == 0) {
                    System.out.println();
                }
            }
        }

        scanner.close();
    }
}

九九乘法表

import java.util.Scanner;

public class Scanner1 {
    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 + " = " + i * j + "\t");
            }

            System.out.print("\n");
        }
    }
}

for遍历值

import java.util.Scanner;

public class Scanner1 {
    public static void main(String[] args) {
        int[] a = new int[100000 + 10]; // 数组定义方式

        for(int i = 1; i <= 1e5; i ++) a[i] = i;

        for(int x : a) System.out.print(x + "\n"); 
    }
}

continue break goto

打印三角形

import java.util.Scanner;

public class Scanner1 {
    public static void main(String[] args) {

        Scanner scanner = new Scanner(System.in);

        System.out.print("请输入层数（大于0的整数）：");
        int n = scanner.nextInt();

        for(int i = 1; i <= n; i ++) {
            int t = i * 2 - 1; // *个数
            int cnt = n - i; //空格个数
            for(int j = 0; j < cnt; j ++) System.out.print(" ");
            for(int j = 0; j < t; j ++) System.out.print("*");
            System.out.print("\n");
        }

        scanner.close();
    }
}