How to detect a key press in Java

Question

My son (aged 11) who is learning Java is going to enter a question and some code. He tells me he has not been able to find the answer to this question on the site. You will

Answer 1

I don't see anything in your code which actually listens for key presses or which modifies the position of x and y. The circle will only move if you change x and y and then repaint the screen.

You have set up the panel, etc here but not actually added anything to respond to user action. You will probably need to add a KeyListener somewhere to respond to key presses. This is probably either in the tutorial you are doing or in a previous or later tutorial.

Answer 2

KeyListener

You should be putting his KeyListener Event within the class you need it not in the Main Method!

addKeyListener(new KeyListener() {
    public void keyPressed(KeyEvent e) {
        //This is where you want to implement your key!
        if(e.getKeyCode() == KeyEvent.VK_SPACE){
             //Fire Gun!
        }
        ....
    }

    public void keyReleased(KeyEvent e) { //I dont really use these!
    }

    public void keyTyped(KeyEvent e) { //I dont really use these!
    }
});

More on Keys here: KeyListeners

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Public and Private:

Now for Public and Private, there is already an answer on StackOVerflow which I think does the job pretty well! Here

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Functions/Methods and Void:

As for Void, It is used in functions where you wish to return nothing! I do not know how well you know about functions but let me give you an example:

int sum;
public int addReturn(int x, int y){
    sum = x +y;
    return sum;
}

What this does is it adds the two ints given as x and y and then returns the sum as an integer!

With void what you can do is:

int sum;
public void add(int x, int y){
    sum = x+ y;
}

Here you return nothing but it still stores x+y into sum!

So in your class:

int Summation, x = 10, y = 10;
summation = addReturn(x,y); //<-- This will place summation = 10+10 = 20

For the void function you do not need to make a variable since it doesnt return a value, rather it sets the value within the function. All you need to do is call it

add(x,y);

If you need more help just comment! :) Also I find it awesome that your son is so interested in Java at such a young age. Great Parenting :D

Answer 3

Read about java.awt.event.KeyListener

A code should look like this:

    f.addKeyListener(new KeyListener() {
        @Override
        public void keyTyped(KeyEvent e) {
        }

        @Override
        public void keyPressed(KeyEvent e) {
            System.out.println("Key pressed code=" + e.getKeyCode() + ", char=" + e.getKeyChar());
        }

        @Override
        public void keyReleased(KeyEvent e) {
        }
    });

Answer 4

Suppose you need to detect the key f, use this:

//packageStatement
import javax.swing.*;
import java.awt.event.*;
//more imports
public class window {
 //code
 f.addKeyListener(new KeyListener() {
    @Override
    public void KeyTyped(KeyEvent e) {
        if(e.getKeyChar == 'f'){
           //code
        }
    }
    //more code
 });
 //more code
}

Remember to use the string literal, because the getKeyChar() method returns a char.

Answer 5

There's a great library which works smoothly and it listens to global events. It's the only thing which worked without any issues for me: https://github.com/kwhat/jnativehook

It grabs every event in the system, pretty cool for any development.

You can use it like this:

package main;

import org.jnativehook.GlobalScreen;
import org.jnativehook.NativeHookException;
import org.jnativehook.keyboard.NativeKeyEvent;
import org.jnativehook.keyboard.NativeKeyListener;

class MyKeyboard implements NativeKeyListener {

    public static void main(String[] args) {
        try {
            GlobalScreen.registerNativeHook();
        } catch (NativeHookException ex) {
            System.err.println("There was a problem registering the native hook.");
            System.err.println(ex.getMessage());

            System.exit(1);
        }

        GlobalScreen.addNativeKeyListener(new MyKeyboard());

        // Now press any keys and look at the output
    }

    @Override
    public void nativeKeyPressed(NativeKeyEvent e) {
    }

    @Override
    public void nativeKeyTyped(NativeKeyEvent nke) {
    }

    @Override
    public void nativeKeyReleased(NativeKeyEvent nke) {
    }

}

Answer 6

So GUI are event driven. Everything that occurs is driven by an event. A key press is an event. A component needs to listen for events, and do something when the event is fired. You can read more about how to write different listeners here. Some of the basic listeners are fairly easy to recognize (by name) like KeyListener or MouseListener. ActionListener is also pretty common for button presses. You'll want to go through the tutorials and learn the different listeners.

Note though that with Swing there are focus issues with using a KeyListener so it is preferred to use key bindings. As stated in the KeyListener tutorial:

To define special reactions to particular keys, use key bindings instead of a key listener

A tutorial for key bindings can be found here and an example can been seen here. Here's the gif from the example, showing the movement of rectangle with a key press using key bindings

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UPDATE just notice.

1. Don't override the paint method of JPanel

2. Never call repaint() from inside a paint/paintComponent method

   public void paint() {//This is where the graphic is painted to the screen
       repaint();
   }
   

Get completely rid of the above code.