How to use keyListener properly in Java

Question

I am running into problems trying to use KeyListener in java.
I am trying to write a program that runs until a key is pressed, and then outputs that key.
This is a stepping stone to more elaborate code, so the use of a method to print the key pressed is just being used as a kind of prototype.

Here is the code:

import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;

public class keylistener implements KeyListener{
    public keylistener(){
        addKeyListener(this);
    }

    public void keyPressed(KeyEvent e){
        int key = e.getKeyCode();
        keylistener output = new keylistener();
        output.print(key);
    }

    public void keyReleased(KeyEvent e){}
    public void keyTyped(KeyEvent e){}

    public void print(int key){
        System.out.println(key);
    }

    public static void main(String[] args){
    }
}

The program runs and then ends directly after.
I have never used KeyListener before, and I cannot figure out how to make the program wait for the key to be pressed.

Your main method is empty.... Typically you use a keylistener with a GUI, though — OneCricketeer, Oct 10 '16 at 17:50

Reign · Answer 1 · 2016-10-10T18:48:11.140

The KeyListener is intended to be used in GUI apps, not in console apps as KeyListener requires GUI components to run. Usually in a console app you would use a BufferedReader and InputStreamReader.

Here is a console app that demonstrates the idea of using a while loop with the BufferedReader and InputStreamReader to keep the app running waiting for input. Although there are other ways to do this. I made this very explicit so you can grasp the concept.

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;


public class StackStuff {

public static void main(String[] args) throws IOException {
     BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
     boolean runs = true ;
     System.out.println("Press any key to kill this app...");
     while(runs){
          input =br.readLine();
         if (!(input==null)){
             System.out.println(input);
             runs=false;
          }
         }
     System.out.println("I am dead!!");
     System.exit(0); 

}

score 0 · Answer 2 · answered Oct 10 '16 at 17:58

I don't think I've ever added a KeyListener to a console app, (probably because KeyListener is part of the AWT package, which mainly has GUI components), but here is a sample GUI app to demonstrate.

Your first problem, though, is that you should have added new keylistener() into your main method, though that has to be added to a component itself in order to be ran.

public class GuiMain extends JFrame { // A GUI Frame

    public GuiMain() {
        // Add the listener
        this.addKeyListener(new KeyAdapter() { 
            @Override
            public void keyPressed(KeyEvent e) {
                super.keyPressed(e);
                // Print out the code
                System.out.println(e.getKeyCode());
            }
        });

        // Show something
        add(new JLabel("Hello World!"));
        pack();
    }

    public static void main(String[] args) {
        // Run the GUI
        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                GuiMain gui = new GuiMain();
                gui.setVisible(true);
            }
        });
    }

}

How to use keyListener properly in Java

2 Answers2

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