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我不明白为什么这段代码能按我的预期工作：

package myPackage;

import java.util.InputMismatchException;
import java.util.Scanner;

public class ScanTest {

    public static void main(String[] args) 
    {
        Scanner scan1, scan2;
        double weight = 0.0;

        scan1 = new Scanner(System.in);
        System.out.print("Enter a rational: ");
        try {
            weight = scan1.nextDouble();
            System.out.println("You entered " + weight);
        }
        catch(InputMismatchException ime) {
            System.out.println("Invalid data.");
        }
        scan1.close();

        /*scan2 = new Scanner(System.in);
        System.out.print("Enter a rational: ");
        try {
            weight = scan2.nextDouble();
            System.out.println("You entered " + weight);
        }
        catch(InputMismatchException ime) {
            System.out.println("Invalid data.");
        }
        scan2.close();*/
    }
}

这段代码失败了：

package myPackage;

import java.util.InputMismatchException;
import java.util.Scanner;

public class ScanTest {

    public static void main(String[] args) 
    {
        Scanner scan1, scan2;
        double weight = 0.0;

        scan1 = new Scanner(System.in);
        System.out.print("Enter a rational: ");
        try {
            weight = scan1.nextDouble();
            System.out.println("You entered " + weight);
        }
        catch(InputMismatchException ime) {
            System.out.println("Invalid data.");
        }
        scan1.close();

        scan2 = new Scanner(System.in);
        System.out.print("Enter a rational: ");
        try {
            weight = scan2.nextDouble();
            System.out.println("You entered " + weight);
        }
        catch(InputMismatchException ime) {
            System.out.println("Invalid data.");
        }
        scan2.close();
    }
}

（请注意，这两个版本几乎相同。第二个版本中只有一个未注释的块。）

我从第二个版本得到的输出是：

Enter a rational: .97
You entered 0.97
Enter a rational: Exception in thread "main" java.util.NoSuchElementException
    at java.util.Scanner.throwFor(Unknown Source)
    at java.util.Scanner.next(Unknown Source)
    at java.util.Scanner.nextDouble(Unknown Source)
    at myPackage.ScanTest.main(ScanTest.java:28)

我不明白为什么 Scanner 对象在第一个块中等待输入，但在第二个块中抛出异常。我期望的行为是：

Enter a rational: .97
You entered 0.97
Enter a rational: .43
You entered 0.43

有问题的第 28 行是：

重量 = scan2.nextDouble();

我将不胜感激任何人对这种行为的洞察力！

4

1 回答 1

2

我相信对的调用scan1.close()也会关闭System.in，因此无法从流中读取更多数据。在这里查看close()方法描述。

所以......当您开始阅读时scan2，System.in将不再返回任何数据。

于 2013-01-21T20:29:13.093 回答