I have an arraylist of entites that I want to move randomly. No matter what I do, they won't move. Here is my female class:

import java.awt.Graphics2D;
import java.awt.Image;
import java.awt.Rectangle;
import java.util.Random;

import javax.swing.ImageIcon;

public class Female extends Entity {
static int femaleX = 0;
static int femaleY = 0;
double walkSpeed = .1f;
Random rand = new Random();
int random;
int dir;

Player player;

public Female(int posX, int posY) {
    super(posX, posY);


public void update() {
    posX += femaleX;
    posY += femaleY;


public void draw(Graphics2D g2d) {
    g2d.drawImage(getFemaleImg(), posX, posY, null);
    if (Player.showBounds == true) {

public Image getFemaleImg() {
    ImageIcon ic = new ImageIcon("res/female.png");
    return ic.getImage();

public Rectangle getBounds() {
    return new Rectangle(posX, posY, getFemaleImg().getHeight(null),

public void moveFemale() {
    random = rand.nextInt(3);

    if (random == 0) {
        dir = 0;
        posX -= (int) walkSpeed;


And here is how I update the female class in the main class:

public void actionPerformed(ActionEvent ae) {

    for(int i = 0; i < females.size(); i++){
        Female tempFemale = females.get(i);

If I do something like this(in the female update method):

public void update() {
    posX += femaleX;
    posY += femaleY;

    posX -= walkSpeed;


The characters move with no problem. Why is this?

  • 3
    \$\begingroup\$ Dude, what's with the females? Are you objectifying them? That's a danger zone! \$\endgroup\$ – Engineer Oct 28 '12 at 18:01
  • \$\begingroup\$ -__- thanks for your help.. they were just part of a game! \$\endgroup\$ – opiop65 Oct 28 '12 at 18:10

Because (prior to your fix) that's exactly what you've asked it to do. Nowhere do I see femaleX or femaleY being updated; thus, they remain zero, and thus you add zero to the current position, so nothing moves.

I think you are not clear on what your code is supposed to be doing, here is how this is always meant to be approached:

float ax = 0;
float ay = 0;
float sx = 0;
float sy = 0;
//displacment AKA position
float x = 0;
float y = 0;


void update(...)
    //after all input and AI, etc. etc., finalise your physics
    //using the basic Equations of Motion a la Isaac Newton:
    //new velocity is old velocity plus current acceleration (usually derived from inputs)
    sx += ax;
    sy += ay;
    //new position is old position plus current speed
    x += sx;
    y += sy;

Whereas in your code, it almost seems as if you are saying position = position + someOtherPosition, which cannot really work. Speed (or as a vector, velocity) is the diff you apply to displacement, just as acceleration is the diff you apply to speed.

PS. Using acceleration is completely up to you -- you can just set velocities in your input handler code if you like. Many games do not need acceleration.

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You cast walkSpeed to an int before adding it to the position in moveFemale(), its value will be 0 so nothing changes.

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You have to call moveFemale(); in the update-method.

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  • 2
    \$\begingroup\$ Information about why this is a problem and how this solves it makes for a better answer. \$\endgroup\$ – MichaelHouse May 24 '14 at 14:48

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