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Get java to wait for user input

Barbara Jones 3207 19 May 2015

I’m attempt to make the main loop wait for the user to input something I added the loop:

while(!userInput.hasNext());

this is not work correctly. I have heard BufferedReader but I have never used it. So please solve my problem.

while(true) {
        System.out.println("Ready for a new command sir.");
        Scanner userInput = new Scanner(System.in);
 
        while(!userInput.hasNext());
 
        String input = "";
        if (userInput.hasNext()) input = userInput.nextLine();
 
        System.out.println("input is '" + input + "'");
 
        if (!input.equals("")) {
            //main code
        }
        userInput.close();
        Thread.sleep(1000);
    }
java java
Updated 19 May 2015
Barbara Jones
Barbara Jones

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1 Answers

Lillian Martin
Lillian Martin 19 May 2015
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There is no need for you to check for available input waiting and sleeping until there is since Scanner.nextLine() will block until a line is available.

Have a look at this example I wrote to demonstrate it:

public class ScannerTest {
 
        public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        try {
            while (true) {
                System.out.println("Please input a line");
                long then =System.currentTimeMillis();
                String line =scanner.nextLine();
                long now =System.currentTimeMillis();
                System.out.printf("Waited %.3fs for user input%n", (now - then) /
1000d);
                System.out.printf("User input was: %s%n", line);
            }
        }
catch(IllegalStateException |
NoSuchElementException e) {
            // System.in has been closed
            System.out.println("System.in was closed; exiting");
        }
    }
   
}

Please input a line

hello

Waited 1.892s for user input

User input was: hello

Please input a line

^D

System.in was closed; exiting 

So all you have to do is to use Scanner.nextLine() and your app will wait until the user has entered a newline. You also don't want to define your Scanner inside the loop and close it since you're going to use it again in the next iteration:

Scanner userInput = new Scanner(System.in);
    while(true) {
        System.out.println("Ready for a new command sir.");
 
        String input = userInput.nextLine();
        System.out.println("input is '" + input + "'");
 
        if (!input.isEmpty()) {
            // Handle input
        }
    }
   
}

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