Bfrank Bfrank - 3 days ago 4
Java Question

How do I move a graphic across the screen with arrow keys?

I'm trying to create the beginning of a simple game. The first thing I am trying to do is import a graphic into my code and move it across the screen. I was able to draw a ball on the screen and move it around but when I import a graphic from a file I am unable to move it around. What am I missing or doing wrong?

import javax.swing.*;
import java.awt.Graphics;
import java.awt.*;
import java.awt.event.*;
import javax.swing.ImageIcon;
import java.awt.Color;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.KeyListener;
import javax.swing.JPanel;
import javax.swing.Timer;

public class Game extends JPanel implements ActionListener, KeyListener {

Timer t = new Timer(5, this);
double x = 0, y = 0, velX = 0, velY = 0;
private ImageIcon image;


public Game(){
setBackground(Color.WHITE);
t.start();
addKeyListener(this);
this.setFocusable(true);
setFocusTraversalKeysEnabled(false);
image = new ImageIcon ("ship.gif");

}

public void paintComponent(Graphics g){
super.paintComponent(g);
ImageIcon i = new ImageIcon("C:\\Users\\Bryan\\Pictures\\ship.gif");
i.paintIcon(this, g, 0, 0);

}

public void actionPerformed(ActionEvent e){
repaint();
x += velX;
y += velY;

if(x<0){
velX = 0;
x = 0;
}

if(x>750){
velX = 0;
x = 750;
}

if(y<0);{
velY = 0;
y = 0;
}

if(y>550){
velY = 0;
y = 550;
}
}

public void up(){
velY = -1.5;
velX = 0;
}

public void down(){
velY = 1.5;
velX = 0;
}

public void left(){
velX = -1.5;
velY = 0;
}

public void right(){
velX = 1.5;
velY = 0;
}

public void keyPressed(KeyEvent e){
int code = e.getKeyCode();

if (code == KeyEvent.VK_UP){
up();
}

if (code == KeyEvent.VK_DOWN){
down();
}

if (code == KeyEvent.VK_LEFT){
left();
}

if (code == KeyEvent.VK_RIGHT){
right();
}
}

public void keyTyped(KeyEvent e){}

public void keyReleased(KeyEvent e){

// velX = 0;
// velY = 0;
int code = e.getKeyCode();

if (code == KeyEvent.VK_UP){
velY = 0;
}
if (code == KeyEvent.VK_DOWN){
velY = 0;
}
if (code == KeyEvent.VK_LEFT){
velX = 0;
}
if (code == KeyEvent.VK_RIGHT){
velX = 0;
}
}

}


My driver is in another class as follows:

import java.awt.Color;

import javax.swing.JFrame;

public class GameDriver {

public static void main(String[] args) {

JFrame f = new JFrame();
Game g = new Game();
f.add(g);
f.setVisible(true);
f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
f.setSize(800,600);

}
}

Answer

Two big problems here:

public void paintComponent(Graphics g){
    super.paintComponent(g);
    ImageIcon i = new ImageIcon("C:\\Users\\Bryan\\Pictures\\ship.gif");
    i.paintIcon(this, g, 0, 0);
}
  1. You're reading from a file from within paintComponent(...). Never do this as this will slow your drawing unnecessarily. Read the image once, perhaps in a constructor, and then use the stored image variable in drawing. The paintComponent method should be for painting only, and it should be lean, mean and fast.
  2. You're drawing at 0, 0 always. If you want to move something, draw at a variable position, and then change the values held by the variable and repaint.

Also: You should use Key Bindings to accept key strokes in a Swing application as this will help solve focus issues.

For example, please have a look at my code in this answer.

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