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Slick2D NullPointerException

I am making a game in Slick2D for Java, but when I tried to display an Image(the Slick2D type) it gives me this error:
java.lang.NullPointerException
at org.newdawn.slick.Graphics.drawImage(Graphics.java:1384)
at org.newdawn.slick.Graphics.drawImage(Graphics.java:1433)
at luke_r.games.java.broadway.states.M.render(M.java:21)
at org.newdawn.slick.state.StateBasedGame.render(StateBasedGame.java:199)
at org.newdawn.slick.GameContainer.updateAndRender(GameContainer.java:688)
at org.newdawn.slick.AppGameContainer.gameLoop(AppGameContainer.java:411)
at org.newdawn.slick.AppGameContainer.start(AppGameContainer.java:321)
at luke_r.games.java.broadway.C.main(C.java:21)
Here is the for the M class (excluding imports and class declaration, they are unneeded):
#Override
public void init(GameContainer gc, StateBasedGame sbg) throws SlickException
{
PL.x = 100;
PL.y = 100;
}
#Override
public void render(GameContainer gc, StateBasedGame sbg, Graphics g) throws SlickException
{
g.drawImage(I.title, 0, 0); //Line 21, the error
PL.render(g);
}
#Override
public void update(GameContainer gc, StateBasedGame sbg, int delta) throws SlickException
{}
#Override
public int getID()
{
return 0;
}

The problem is that I.title == null. Set it to a correct value.

In init function set the url to the image into I.title, i mean => I.title = new Image("-your URL-");
Slick2D works in this form: first of all he is init all params in method init , then until somthing happend that change the current state "he" render => update => render => update =>render => update ... You can set how much the update will occurs in the SetUpClass by using SetTargetFrameRate method.

How to update and move object with pathfinding?

I've implemented some pathfinding in my 2d game, but I don't know how to move my 'Zombie' character.
I'm using slick2d and everything gets put on the screen in the render method, however I want to update the zombies movement in the Zombie class.
Here is the method in the zombie class:
public void findPrey(){
Pathfinder pf = new Pathfinder();
pf.setStartNode(xPosition,yPosition);
for(PathCoordinates p: pf.calculatePath()){
if(p.getXPosition() > xPosition){
moveRight(2);
}if(p.getXPosition() < xPosition){
moveLeft(2);
}if(p.getYPosition() > yPosition){
moveUp(2);
}if(p.getYPosition() < yPosition){
moveDown(2);
}
}
}
So I want to loop through the List of x and y coordinates and move the zombie.
However nothings happens- the zombie doesnt move at all.
In my 'world' class I have the render method, which loops through the zombies and renders them, the code has been edited slightly:
public void render(GameContainer gc, StateBasedGame sbg, Graphics g)
throws SlickException {
for (Zombie z : zombies) {
if (!z.isDead()) {
z.render(gc, sbg, g);
z.findPrey();
}
}
}
So, basically my question is how - do I make my zombie character move?! Thanks for any help or guidance

You should first move the zombie, then render it.
There's another problem: you allow to move the zombie all the way it should with one findPrey() (using updatePosition() in code below) method call.
Zombie class:
public void updatePosition() {
Pathfinder pf = new Pathfinder();
pf.setStartNode(xPosition,yPosition);
PathCoordinates p = pf.calculatePath()[0];
if(p.getXPosition() > xPosition) {
moveRight(2);
} if(p.getXPosition() < xPosition) {
moveLeft(2);
} if(p.getYPosition() > yPosition) {
moveUp(2);
} if(p.getYPosition() < yPosition) {
moveDown(2);
}
}
World class:
public void render(GameContainer gc, StateBasedGame sbg, Graphics g) throws SlickException {
for(Zombie z : zombies) {
if(!(z.isDead())) {
z.updatePosition();
z.render(gc, sbg, g);
}
}
}

How to load an image using LWJGL

I've Googled a lot and could not find the answer to this. I know how to load an image, I need to load images that aren't 256 by 256 or a direct power of 2.
Like how could I load an image that's 128 by 384 or something like that.
What I'm using now:
Load an image:
public static Texture cow = loadTexture("res/cow.png");
private static Texture loadTexture(String file){
try {
return TextureLoader.getTexture("JPG", new FileInputStream(new File(file)));
} catch (IOException e) {
e.printStackTrace();
}
return null;
}
Now to draw it on a 3D poly face:
txt.bind();
GL11.glBegin(GL11.GL_QUADS);
{
//GL11.glColor3d(lightLevel, lightLevel, lightLevel);
GL11.glColor3d(l, l, l);
GL11.glTexCoord2f(0,0); GL11.glVertex3f(x1,y1, z1);
GL11.glTexCoord2f(1,0); GL11.glVertex3f(x1+(x2-x1), y1,z1);
GL11.glTexCoord2f(1,1); GL11.glVertex3f(x1+(x2-x1), y1+(y2-y1), z1+(z2-z1));
GL11.glTexCoord2f(0,1); GL11.glVertex3f(x1,y1+(y2-y1), z1+(z2-z1));
}
GL11.glEnd();
Now that works perfectly, I just need to load images that aren't a power of 2.

you could use slick2d, it is way easier to use and works with lwjgl! All you have to do is:
Image title = null;
public static void main(String[] args) {
}
#Override
public void init(GameContainer Gc, StateBasedGame Sbg)
throws SlickException {
/**
* Images
*/
title = new Image("gfx/main_menu/title/new_title.png");
}
#Override
public void render(GameContainer Gc, StateBasedGame Sbg, Graphics G)
throws SlickException {
/**
* Background
*/
G.setColor(Color.white);
G.fillRect(0, 0, w*s, h*s);
/**
* Images
*/
title.draw(titleY*s,titleX*s);
}

Slick2D smooth movement of object with cursor keys

I'm playing around with Slick2D and I'm running into a problem when trying to move a simple rectangle with the cursor keys.
Functionality wise, the code works. However, if you keep one of the cursor keys pressed, every now and then there is a little hick-up in the movement wen looking at the screen.
Can anyone suggest a code improvement to make the rectangle move smoothly?
Here is the test code I used:
public class SlickGame extends BasicGame {
// ==================================================================================
// Fields
// ==================================================================================
private Rectangle mPlayer;
// ==================================================================================
// Constructor
// ==================================================================================
public SlickGame() {
super("SlickGame");
}
// ===========================================================
// Methods for/from SuperClass/Interfaces
// ===========================================================
#Override
public void init(GameContainer pGameContainer) throws SlickException {
// create the player centered on the screen
mPlayer = new Rectangle(pGameContainer.getWidth()/2 - 20, pGameContainer.getHeight()/2 - 20, 40, 40);
}
#Override
public void update(GameContainer pGameContainer, int pDelta) throws SlickException {
Input input = pGameContainer.getInput();
int speed = 200;
float distance = speed * ((float)pDelta/1000);
if (input.isKeyDown(Input.KEY_LEFT)) {
mPlayer.setX(mPlayer.getX() - distance);
}
if (input.isKeyDown(Input.KEY_RIGHT)) {
mPlayer.setX(mPlayer.getX() + distance);
}
if (input.isKeyDown(Input.KEY_UP)) {
mPlayer.setY(mPlayer.getY() - distance);
}
if (input.isKeyDown(Input.KEY_DOWN)) {
mPlayer.setY(mPlayer.getY() + distance);
}
}
#Override
public void render(GameContainer pGameContainer, Graphics pGraphics) throws SlickException {
pGraphics.fill(mPlayer);
}
// ==================================================================================
// Methods
// ==================================================================================
public static void main(String[] args) {
try {
// create the game's container app
AppGameContainer container = new AppGameContainer(new SlickGame());
// adjust the resolution and disable fullscreen
container.setDisplayMode(800, 600, false);
// specify desired FPS
container.setTargetFrameRate(60);
// start the game
container.start();
}
catch (SlickException e ) {
e.printStackTrace();
}
}
}

Thanks, naughty_hacker. Adding container.setVSync(true); did the trick.

Game window flickers

Every couple seconds, the window in which my game is playing will briefly disappear and then reappear. I'm on Windows 7 with the latest version of Slick (a game library for Java). Here's the code I'm using:
package Main;
import org.newdawn.slick.*;
public class Main extends BasicGame{
public Main() {
super("Flashing window issue");
}
#Override
public void init(GameContainer gc) throws SlickException {
}
#Override
public void update(GameContainer gc, int delta) throws SlickException {
}
#Override
public void render(GameContainer gc, Graphics g) throws SlickException {
}
public static void main(String[] args) throws SlickException {
AppGameContainer app = new AppGameContainer(new Main());
app.setDisplayMode(800, 600, false);
app.start();
}
}
How can I fix this issue?
Progress so far:
Update: No solution found yet, but playing the game in fullscreen mode eliminates the flicker. Perhaps this will lead to a solution...
Update 2: Monitering the task manager shows that while the game is flickering its status in the task manager is 'Not Responding'.
Update 3: It seems to only happen when the mouse leaves the game area (regardless of whether the game window loses focus).
Update 4 - Current workaround:
app.setMouseGrabbed(true); // force the mouse to stay in the game area
then in update(...):
// exit when escape is pressed:
if (gc.getInput().isKeyDown(Input.KEY_ESCAPE)) {
gc.exit();
}

I'm not familiar with slick2d but does it have a concept of double buffering? that would be something you would want to turn on if you get flickering.