使用 Java 扫描器顺序读取标准输入时抛出异常

Question

我正在学习如何在 Java 中使用 Scanner，我想从 stdin 中读取一个整数和一个字符串，所以我编写了以下代码：

import java.util.Scanner;

public class TryScanner {
    public void readInt() {
        Scanner in = new Scanner(System.in);
        System.out.println("Please type in an integer");
        int length = in.nextInt();
        System.out.println("Integer you type is: " + length);
        in.close();

    }
    public void readString() {
        Scanner in = new Scanner(System.in);
        System.out.println("Please type in a string");
        String s = in.nextLine();
        System.out.println("String you type is: " + s);
        in.close();

    }
    public static void main(String[] args) {
        TryScanner instance = new TryScanner();

        instance.readInt();
        // throw exception when called after readInt()
        instance.readString();
    }
}

我的 readInt 和 readString 函数分别运行良好，但是，当我顺序调用它们时，只有第一个函数运行，第二个函数抛出异常：

Please type in an integer
3
Integer you type is: 3
Please type in a string
Exception in thread "main" java.util.NoSuchElementException: No line found
    at java.util.Scanner.nextLine(Scanner.java:1597)
    at helloworld.Sort.readString(Sort.java:17)
    at helloworld.Sort.main(Sort.java:27)

谁能解释一下我哪里做错了？

Answer 1

删除对

的调用

in.close();

当您关闭 in 时，您还 close System.in 全局（并且它不会重新打开）。

public void readInt() {
    Scanner in = new Scanner(System.in);
    System.out.println("Please type in an integer");
    int length = in.nextInt();
    System.out.println("Integer you type is: " + length);
    // in.close();
}
public void readString() {
    Scanner in = new Scanner(System.in);
    System.out.println("Please type in a string");
    String s = in.nextLine();
    System.out.println("String you type is: " + s);
    // in.close();
}

此外，您可以考虑将 Scanner 传递给方法。

public static int readInt(Scanner in) {
    System.out.println("Please type in an integer");
    return in.nextInt();
}
public static String readString(Scanner in) {
    System.out.println("Please type in a string");
    return in.nextLine();
}
public static void main(String[] args) {
    Scanner in = new Scanner(System.in);
    System.out.println("Integer you type is: " + readInt(in));
    System.out.println("String you type is: " + readString(in));
}

Answer 2

您也可以使用下面的代码

import java.util.Scanner;

public class TryScanner {
    Scanner in = new Scanner(System.in);

    public void readInt() {

        System.out.println("Please type in an integer");
        int length = in.nextInt();
        System.out.println("Integer you type is: " + length);

    }

    public void readString() {
        System.out.println("Please type in a string");
        String s = in.nextLine();
        System.out.println("String you type is: " + s);

    }

    public static void main(String[] args) {
        TryScanner instance = new TryScanner();

        instance.readInt();
        instance.readString();
        instance.in.close();
    }
}

使用 Java 扫描器顺序读取标准输入时抛出异常

Exception thrown when use Java Scanner to read from stdin sequentially

java

stdin

java.util.scanner