Java Scanner does not wait for input

问题

I have two blocks of code here. One scanner properly waits user input, and the other just blows right through it and calls nextInt() which returns a NoSuchElementException. Here is the block that works:

public void startGame() {
   out.println("Player1: 1 for dumb player, 2 for smart player, 3 for human player.");
   Scanner scan = new Scanner(System.in);
   p = scan.nextInt();

   if (p == 1)
        p1 = new DumbPlayer("ONE");
   if (p == 2)
        p1 = new SmartPlayer("ONE");
   else 
       p1 = new HumanPlayer("ONE");

   out.println("Player2: 1 for dumb player, 2 for smart player, 3 for human player.");
   p = scan.nextInt();

   if (p == 1)
        p2 = new DumbPlayer("TWO");
   if (p == 2)
        p2 = new SmartPlayer("TWO");
   else 
        p2 = new HumanPlayer("TWO");

   scan.close();

And here is the block that does not:

public int findBestMove(Set<Integer> moves, Board b) {

    Set<Integer> set = new HashSet<Integer>();

    out.println("Player " +name+ ", select a column from 1-7: ");
    Scanner scan = new Scanner(System.in);  <--here it should wait for input, but does not!
    int move = scan.nextInt();   <-- NoSuchElementException
    scan.close();

    for (int x = 1; x <= 7; x++) {
        set.add(move);
        move += 7;
    }
    ....etc

Both of these are separate classes, and are called from a main method in yet another class. Basically main() calls startGame(), which in turn calls the findBestMove() method of some Player class...which is where the non-working code resides. Are there times in the program where it is not appropriate to take input? I was under the impression that anytime I wanted user input, I could use this approach. Thanks!

回答1:

According to the java.util.Scanner javadoc, Scanner.close() closes the associated stream if this stream implements the Closeable interface. java.lang.System.in is an InputStream, which implements the Closeable interface. Hence, after calling Scanner.close() on a Scanner which is associated with System.in, the System.in stream is closed and not available anymore.

The following SSCCE works for me. I have removed some code from the question which is not relevant to the actual issue. Note that with this approach, while it works, Eclipse gives me the warning "Resource leak: 'scan' is never closed", so a better solution would be to use one Scanner instance only.

package com.example;

import java.util.Scanner;
import java.util.Set;
import static java.lang.System.out;

public class ScannerTest {
    int p = 0;
    String name = "Test";

    public void startGame() {
        out.println("Player1: 1 for dumb player, 2 for smart player, 3 for human player.");
        Scanner scan = new Scanner(System.in);
        p = scan.nextInt();

        out.println("Result1: " + p);

        out.println("Player2: 1 for dumb player, 2 for smart player, 3 for human player.");
        p = scan.nextInt();

        out.println("Result2: " + p);

        // scan.close();   // Do not close the Scanner to leave System.in open
    }

    public int findBestMove(Set<Integer> moves, Object /*Board*/ b) {
        out.println("Player " +name+ ", select a column from 1-7: ");
        Scanner scan = new Scanner(System.in);
        int move = scan.nextInt();
        // scan.close();   // Do not close the Scanner to leave System.in open

        out.println("Move: " + move);

        return 0;
    }

    public void run() {
        startGame();
        findBestMove(null, null);
    }

    public static void main(String[] args) {
        ScannerTest st = new ScannerTest();
        st.run();
    }
}

回答2:

nextInt() does not discard \n in the stream, so the next call finds nothing. You have to skip it manually with Scanner.skip("\n") or Scanner.nextLine()

EDIT After figuring out that "skipping readInt()" meant that the code threw an exception. and with @Andreas help, here's an alternative solution. In java6 a new class Console has been added for providing a better interface to stdin. And it returns a Reader that doesn't care how much you close it. So the following snippet works just fine:

    Scanner fi = new Scanner(System.console().reader());
    System.out.println(fi.nextInt());
    fi.close();

    fi = new Scanner(System.console().reader());
    System.out.println(fi.nextInt());
    fi.close();

回答3:

Try using scan.reset(); instead of scan.close(). scan.close() will throw NoSuchElementException as Denis Tulskiy said.

回答4:

I just spent about 15 minutes troubleshooting this issue and my problem was different than all of the above.

The code is fine, but I am using Sublime Text and running it in the terminal emulator inside sublime text, on windows. This prevented the scanner from actually allowing me to try to input something.

I ran the app using cmd and it worked fine. I hope this helps someone.

来源：https://stackoverflow.com/questions/13374826/java-scanner-does-not-wait-for-input

标签

java

user-input

java.util.scanner