Java Scanner类因多次使用而失败
我不明白为什么这段代码能像我期望的那样工作:Java Scanner类因多次使用而失败,java,java.util.scanner,Java,Java.util.scanner,我不明白为什么这段代码能像我期望的那样工作: 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;
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();*/
}
}
此代码失败:
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。)
我从第二个版本获得的输出是:
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
第28行有问题:
重量=扫描2.nextDouble()
我将感谢任何人对这种行为的洞察 我相信对
scan1.close()
的调用也会关闭System.in
,这样就不能从流中读取更多数据了。查看close()
方法说明
所以。。。当您开始从
scan2
读取数据时,System.in
将不再返回任何数据。因此,OP最好在输入采集/显示结束时关闭两个扫描仪。啊,一定是这样!正在通过调用scan1.close()关闭System.in。谢谢你,劳里先生,还有平克勒芬森游行!