Java: Player movement bug

Question

Apologies in advance. I'm self taught and have only been learning for a month.

This is for a simple rectangle in a window that I move with WASD keys.

public LinkedList<Object> object;

public KeyInput(LinkedList<Object> object){
    this.object = object;
}

public void keyPressed(KeyEvent e){

    int key = e.getKeyCode();

        Object myPlayer = Main.object.get(0);

        if(myPlayer.getName() == "PlayerOne"){
            if(key == KeyEvent.VK_W) 
                myPlayer.setVelocityY(myPlayer.getVelocityY() - 5);
            if(key == KeyEvent.VK_S)
                myPlayer.setVelocityY(myPlayer.getVelocityY() + 5);
            if(key == KeyEvent.VK_D) 
                myPlayer.setVelocityX(myPlayer.getVelocityX() + 5);
            if(key == KeyEvent.VK_A)
                myPlayer.setVelocityX(myPlayer.getVelocityX() - 5);
        }

    if(key == KeyEvent.VK_ESCAPE) System.exit(0);
}

public void keyReleased(KeyEvent e){
    int key = e.getKeyCode();

        Object tempObject = Main.object.get(0);

        if(tempObject.getName() == "PlayerOne"){
            if(key == KeyEvent.VK_W) 
                tempObject.setVelocityY(tempObject.getVelocityY() + 5);
            if(key == KeyEvent.VK_S)
                tempObject.setVelocityY(tempObject.getVelocityY() - 5);
            if(key == KeyEvent.VK_D) 
                tempObject.setVelocityX(tempObject.getVelocityX() - 5);
            if(key == KeyEvent.VK_A)
                tempObject.setVelocityX(tempObject.getVelocityX() + 5);
        }
}

/////////////////////////////////////////////////////////////

public void setVelocityX(int velocityX){        
    this.velocityX = velocityX;
    //Prevent a speed higher than 5.
    if(this.velocityX > 5) this.velocityX = 5;
    if(this.velocityX < -5) this.velocityX = -5;
}
public void setVelocityY(int velocityY){
    this.velocityY = velocityY;
    if(this.velocityY > 5) this.velocityY = 5;
    if(this.velocityY < -5) this.velocityY = -5;}

Movement works exactly as I intended. No delay between keypresses, movement is smooth, and diagonal movement works. Problem is.. If I Hold UP, the player moves up as intended. I then hold DOWN.. and the player will moves down, also as intended. But when I release DOWN player will not move up even though I never released up. Then when I finally release UP, the player moves all the way to the bottom. Thank you.

If I key is pressed does it not update getVelocity? According to the math, holding up would set velocityX to -5. Then holding down adds +5 to velocityX. Thus making it 0. But the player moves down anyway. I've been trying to fix this for 10 hours. Any help would be amazing.

Not sure if it is the problem, but the way you are comparing String objects is wrong : https://stackoverflow.com/questions/513832/how-do-i-compare-strings-in-java — Arnaud, Nov 02 '17 at 08:58
I know that's not correct, but it compares them correctly for some reason so I didn't fix it. — , Nov 02 '17 at 09:07
Setting keyPressed to 0 wasn’t working very well. For instance if I moved the player left, then switched to right, it would take about a second for the player to move right. Making it do the opposite makes left and right / up and down movement instant — , Nov 02 '17 at 09:30
Why does " I then hold DOWN.. and the player will moves down" is the expected behavior ? With your code, if you press both up and down, player shouldn't move. — Asoub, Nov 02 '17 at 09:40
@Asiub That’s what I’m saying. It’s want it to do. I don’t know why it works like that. it should be setting velocity to 0 — , Nov 02 '17 at 09:42
I'm not sure how KeyEvent really works. My only advice is that you should debug both methods by logging the key pressed/or released and the current state of the object X and Y, after and before setting, and with timestamp. — Asoub, Nov 02 '17 at 09:50

Arnaud · Answer 1 · 2017-11-02T09:48:01.423

The problem with keyPressed, is that many events will be sent for a given key, as long as it is maintained pressed.

What you could do is ignore the event if the last one was for the same pressed key .

In your class, you may declare some int to store the value of last key :

private int lastKey = 0;

Then use it this way (see the comments in the code) :

public void keyPressed(KeyEvent e){

    int key = e.getKeyCode();

    // if last key was the same one, ignore :
    if (key == lastKey) {
        return;
    }

    Object myPlayer = Main.object.get(0);

    if(myPlayer.getName().equals("PlayerOne")){
        if(key == KeyEvent.VK_W) 
            myPlayer.setVelocityY(myPlayer.getVelocityY() - 5);
        if(key == KeyEvent.VK_S)
            myPlayer.setVelocityY(myPlayer.getVelocityY() + 5);
        if(key == KeyEvent.VK_D) 
            myPlayer.setVelocityX(myPlayer.getVelocityX() + 5);
        if(key == KeyEvent.VK_A)
            myPlayer.setVelocityX(myPlayer.getVelocityX() - 5);
    }

    // set the current key as the last key
    lastKey = key;

    if(key == KeyEvent.VK_ESCAPE) System.exit(0);
}

Also, the String comparison has been fixed in this example .

Java: Player movement bug

1 Answers1