How to exit the program when Ctrl+D is entered in Java?

Question

Below is a section of my Reverse Polish Calculator.

If an integer is entered, push it to the stack and, if = is pressed, peek the result. However, I want to add another condition: if CTRL + D is pressed by the user, the program exits.

I've had a look online but can't seem to find any solutions. Any ideas? Thanks.

Scanner mySc = new Scanner(System.in);
//If input is an integer, push onto stack.
 if (mySc.hasNextInt()) {       
    myStack.push(mySc.nextInt());
} 
//Else if the input is an operator or an undefined input. 
else if (mySc.hasNext()) {
    //Convert input into a string.
    String input = mySc.nextLine(); 
    //Read in the char at the start of the string to operator.
    char operator = input.charAt(0); 
    if (operator == '=') {
        //Display result if the user has entered =.
    }
**else if ("CTRL-D entered") {
    System.exit(0);
    }**

wait: do you want the user to WRITE CTRL-D or press the buttons? — Philipp Sander, Nov 26 '15 at 14:05
Ok I will have to look that up. What about using bufferedreader? Is there a way using that? — George Burslem, Nov 26 '15 at 14:06
bufferedreader wont help either. you need the program to listen to keyevents. this has nothing to do with reading the console input. — Philipp Sander, Nov 26 '15 at 14:08
If the scanner hasNext methods return false, you can almost be sure the user pressed ctrl + d (for the unknowing, ctrl + d) closes the input stream to the program, atleast on linux — Ferrybig, Nov 26 '15 at 14:14
`CTRL+D` is a key combination native to Unix systems. You should not base a Java program on native features. Why noy declaring your own universal end-of-stream mark (for example a single period `.` )? — Little Santi, Nov 26 '15 at 14:40

Titus · Answer 1 · 2015-11-26T14:34:16.830

Try this:

public static void main(String[] args) {
    try {
        byte[] b = new byte[1024];
        for (int r; (r = System.in.read(b)) != -1;) {
            String buffer = new String(b, 0, r);
            System.out.println("read: " + buffer);
        }
    } catch (Exception e) {
        e.printStackTrace();
    }
}

In this case the loop will stop when you press CTRL+D that is because CTRL+D sends an EOF signal to the System.in InputStream which is -1. That is the case on *nix systems, for Windows system, the EOF signal is sent using the CTRL+Z key combination

score 0 · Answer 2 · edited Sep 16 '17 at 20:10

I assume you are talking about a console application?

On unix systems, pressing Ctrl+D closes the stdin for an application. This will result in the input stream underneath Scanner closing, which will cause mySc.hasNext() to return false.

On windows, users need to press Ctrl+Z enter for the same effect.

If the input stream has not been closed, then mySc.hasNext() will block until it can return true.

So you need to wrap your program in a while(mySc.hasNext()) loop, e.g.

    public static void main(String[] args) {

        Scanner mySc = new Scanner(System.in);

        while(mySc.hasNext()) {

//If input is an integer, push onto stack.
            if (mySc.hasNextInt()) {
                System.out.println(String.format("myStack.push(%s);", mySc.nextInt()));
            }
//Else if the input is an operator or an undefined input.
            else {
                //Convert input into a string.
                String input = mySc.nextLine();
                System.out.println(String.format("input = %s", input));

                //Read in the char at the start of the string to operator.
//                char operator = input.charAt(0);
//                if (operator == '=') {
//                    //Display result if the user has entered =.
//                }
//            **else if ("CTRL-D entered") {
//                System.exit(0);
//            }**
            }
        }
    }

score 0 · Answer 3 · answered Nov 19 '16 at 08:07

give this code a try. it actually worked for me

// press ctrl+Z(windows) and ctrl+D(mac, linux) for input termination
StringBuilder out = new StringBuilder();
String text = null;
Scanner scanner = new Scanner( System.in );
while( scanner.hasNextLine() )
{
    text = new String( scanner.nextLine() );
    out.append( text );
}
scanner.close();
System.out.println( out );
System.out.println( "program terminated" );

How to exit the program when Ctrl+D is entered in Java?

3 Answers3

Linked

Related