System.in 获取键盘输入

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此处说明两种使用 System.in 获取键盘输入的两种方法，分别是Scanner 和 InputStreamReader.

其中 System.in 在System类中的定义如下：

package com.study;

import java.io.BufferedReader;

import java.io.IOException;

import java.io.InputStreamReader;

import java.util.Scanner;

public class InputFromCommandLine {

    public static void main(String[] args) {

        input2();

    }

    private static void input1() {

        try (Scanner sc = new Scanner(System.in)) {

            System.out.println("Input a num:");

            int value = sc.nextInt();

            System.out.println("Value Input:" + value);

        } catch (Exception e) {

            e.printStackTrace();

        }

    }

    private static void input2() {

        try (BufferedReader br = new BufferedReader(new InputStreamReader(System.in))) {

            System.out.println("Input a num:");

            String value = br.readLine();

            System.out.println("Value Input:" + value);

        } catch (IOException e) {

            e.printStackTrace();

        }

    }

}

使用Scanner类时，需要注意，Scanner获取的是一个字符流。nextInt方法会不断读取字符，直到遇到非数字字符；nextLine会不断读取字符，直到遇到换行符。

package com.thinkjava.book;

import java.util.Scanner;

/**

 * Demonstrates a common problem using Scanner.

 * Scanner 获取的是字符流

 */

public class ScannerBug {

    public static void main(String[] args) {

        String name;

        int age;

        Scanner in = new Scanner(System.in);

        System.out.print("What is your name? ");

        name = in.nextLine();

        System.out.print("What is your age? ");

        age = in.nextInt();

        System.out.printf("Hello %s, age %d\n", name, age);

        System.out.print("What is your age? ");

        age = in.nextInt();

        in.nextLine(); //读取换行符，加上这行以避开bug

        System.out.print("What is your name? ");

        name = in.nextLine();

        System.out.printf("Hello %s, age %d\n", name, age);

    }

}

如下是一个通过System.in 方法获取键盘输入的方法,代码以及运行后的效果如下：

package com.thinkjava.book;

import java.util.Scanner;

/**

 * Converts centimeters to feet and inches.

 */

public class Convert {

    public static void main(String[] args) {

        double cm;

        int feet, inches, remainder;

        final double CM_PER_INCH = 2.54;

        final int IN_PER_FOOT = 12;

        Scanner in = new Scanner(System.in);

        // prompt the user and get the value

        System.out.print("Exactly how many cm? ");

        cm = in.nextDouble();

        // convert and output the result

        inches = (int) (cm / CM_PER_INCH);

        feet = inches / IN_PER_FOOT;

        remainder = inches % IN_PER_FOOT;

        System.out.printf("%.2f cm = %d ft, %d in\n",

                          cm, feet, remainder);

    }

}