Question

I’m coding a game and I’m using Java’s Swing. And right now i’m trying to get the KeyListeners and Action listeners to work.

What I’m trying to do is to make my object to move according to what key i’m pressing. (Left,Right,Up,Down), But for some reason nothing happens when i press either of these keys, but when i press 3 of them at the same time. the object is strangely moving to the left..

So here’s my code for the class to create the Runner-object:

import java.awt.*;


public class Runner{
    private int xpos, ypos, base, side;

    public Runner(int b, int h ) {
        base = b;
        side = h;
    }
    public void setPosition(int x, int y){
        xpos = x;
        ypos = y;
    }
    public void view(Graphics g) {
        int x[] = { xpos, xpos-base/2, xpos + base/2};
        int y[] = { ypos, ypos + side, ypos + side };
        g.setColor(Color.lightGray);
        g.fillPolygon( x, y, 3 );
        g.setColor(Color.darkGray);
        g.drawLine(xpos, ypos, xpos, ypos + side);
    }
    public void shoot(Graphics g){
        g.setColor(Color.red);
        g.drawLine(xpos,ypos, xpos, 0);
    }
}

And here’s the code thats suppose to run the damn thing:

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;


public class RunningGame extends JPanel implements KeyListener, ActionListener{
    Runner rs;
    int x,y;
    Timer t;
    boolean shot = false;
    boolean left = false, right = false, up = false, down = false;

    public RunningGame() {
        x = 100;
        y = 150;
        rs = new Runner(40,60);
        rs.setPosition(x,y);
        this.addKeyListener(this);
        this.setBackground(Color.black);
        t = new Timer(40, this);
        t.start();
    }
    public void paintComponent(Graphics g){
        super.paintComponent(g);
        rs.view(g);
        if(shot) rs.shoot(g);
    }
    public void keyPressed(KeyEvent e) {
        if (e.getKeyCode() == 37) {left = true;}
        if (e.getKeyCode() == 39) {right = true;}
        if (e.getKeyCode() == 38) {up = true;}
        if (e.getKeyCode() == 40) {down = true;}
        if (e.getKeyCode() == 32) {shot = true;}

        rs.setPosition(x,y);
        this.repaint();
}
    public void keyReleased(KeyEvent e){
        if (e.getKeyCode() == 37) left = false;
        if (e.getKeyCode() == 39) right = false;
        if (e.getKeyCode() == 38) up = false;
        if (e.getKeyCode() == 40) down = false;
        if (e.getKeyCode() == 32) shot = false;
        this.repaint();
    }
public void keyTyped(KeyEvent e){}
    public void actionPerformed(ActionEvent e) {
        if (left) {
            if(right){
                right = false;
                x = x - 10; shot = false;
            }
        }
        if (right) {
            if(left){
            left = false;
            x = x + 10; shot = false;
            }
        }
        if (up) {
            if(down){
                down = false;
                y = y - 10; shot = false;
            }
        }
        if (down) {
            if(up){
                up = false;
                y = y + 10; shot = false;
            }
        }
        rs.setPosition(x,y);
        this.repaint();
}

    public static void main(String[] args) {
        JFrame f = new JFrame();
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.setSize(300, 300); f.setLocation(100,100);
        f.setTitle("Running");
        RunningGame p = new RunningGame();
        f.add(p); f.setVisible(true);
        p.requestFocus();

    }
}

(This is not the final code it’s just using an example with a spaceship, later i will use a different object, just wanna test the KeyListener and ActionListener so it works before proceeding.)

Anyways can anyone help me make the space ship move smoothly? and without having to release all keys to activate another? i.e If i hold left i want it to be able to press another button. so that if i press right, the space ship will start to move in that direction instead.

//MrElephants

Answer 1

In the blocks that look like:

if (left) {
    if(right){
        right = false;
        x = x - 10; shot = false;
    }
}

I think you should have x = x - 10; outside the second if:

if (left) {
    if(right){
        right = false;
        shot = false;
    }
    x = x - 10;
}

although I’m not really sure what that inner if is for, maybe you should remove it completely (but keep the x -= 10 etc.). This should be sufficient to make the movement seem natural.