Java：您实际上如何使用KeyListener？

Question

I've been searching everywhere for how to take input from the keyboard. I can find a ton of resources for how to modify what keylistener does on key press, I can find how to add it to a swing text line, and plenty of other miscellaneous things about it. But I can't find out how to actually put it in a file and use it.

For now I just want to have a main class whose main function just has an endless loop, and everytime you press a key it prints "Key _ has been pressed". How do I do this?

I've made a MyKeyListener that extends KeyAdapter that includes the system.out.print of the string under keyPressed (I assume this is workable). But what do I put in the main class to actually use it? Including it and initializing isn't enough.

Here is my key listener file : `

import java.awt.event.KeyAdapter;
import java.awt.event.KeyEvent;
public class GKeyListener extends KeyAdapter {

public void keyPressed(KeyEvent e){
    System.out.println("Key Pressed: " + e.getKeyChar());
}
public void keyReleased(KeyEvent e){
    System.out.println("Key Released: " + e.getKeyChar());
}
public void keyTyped(KeyEvent e){
    System.out.println("Key Typed: " + e.getKeyChar());
}

Here is a main file im working with and don't know where to go from: import java.awt.KeyEventDispatcher; import java.awt.KeyboardFocusManager; import java.awt.event.KeyEvent;

public class tempMain {
public static void main(String[]  args){
    while(true){
        GKeyListener listen =new GKeyListener();
        //addKeyListener(listen);
    }
}

Answer 1

The KeyListener is a class of the Java AWT GUI framework. It is part of the AWT event handling system, eg used to capture key events of text input components (such as TextArea, TextField etc.).

Without a component, it doesn't make much sense to use the KeyListener.

If you want to do some experiments, you can capture key input without a text input component, using the KeyboardFocusManager . This example shows both, binding a KeyListener to a text component and using the global KeyboardFocusManager .

public class KeyExample extends JFrame {
    public KeyExample() {
        JTextField jTextField = new JTextField(20);
        jTextField.addKeyListener(new KeyAdapter() {
            @Override
            public void keyTyped(KeyEvent e) {
                System.out.println("KeyListener: " + e.getKeyChar());
            }
        });
        add(jTextField);
        pack();
        setDefaultCloseOperation(EXIT_ON_CLOSE);
    }

    public static void main(String[] args) {
        KeyboardFocusManager.getCurrentKeyboardFocusManager().addKeyEventDispatcher(new KeyEventDispatcher() {
            @Override
            public boolean dispatchKeyEvent(KeyEvent e) {
                System.out.println("KeyEventDispatcher: " + e.getKeyChar());
                return false;
            }
        });
        new KeyExample().setVisible(true);
    }
}

You should use the KeyboardFocusManager only in rare situations. Normally, handling events using the KeyListener should be enough.

Answer 2

What this code does:

public class tempMain {
public static void main(String[]  args){
    while(true){
        GKeyListener listen =new GKeyListener();
        //addKeyListener(listen);
    }
}

is repeatedly create a KeyListener, thousands of times ! It does nothing with the KeyListener of use, and again creates thousands of KeyListeners when only one is necessary. Instead you should consider the following:

• Create a GUI first. KeyListeners don't work and don't make sense without a Swing or AWT GUI. Prefer use of Swing over AWT.
• Add the Single KeyListener to a component of the GUI that has focus.
• Seriously consider using Key Bindings in place of a KeyListener for greater flexibility and code reusability.

---

Edit
You state in comment,

but without adding a text field I'm not sure how to actually call the listener to actually exist and be working

• For the record, you should never use a KeyListener with a JTextComponent such as a JTextField as there are much better and safer ways to trap input in these components.
• It sounds as if you want a general OS-wide key listener. If so, you're barking up the wrong tree as Java does not easily support this.
• For this sort of functionality you need to tie into the OS with either JNA or JNI.
• Or use a 3rd party library that gives your app this functionality
• Or tie into a utility program such as AutoIt to give your app this functionality.