We Keep Coding

NullpointerException when I use my own multiple method to paint an image

I am trying to paint multiple images depending on whether a Boolean is true or not. But instead when the boolean becomes true or not, I get a NullPointerException involving anything with my methods. (comment in code specifically pointing to where), and I'm 99% sure its because of the graphics and it's null.
Basically I'm also asking how to fix this and how to properly paint images using my own methods in one class. I know how to do it with each image in one class, but I have WAY more than just 2 images, I think I have almost 100, so i don't want to make 100 classes (:|). Here's my code:
List of images class:
public class Images{
public static Toolkit tk = Toolkit.getDefaultToolkit();
public static final Image Image1 = tk.getImage("src/images/image1.png"), Image2 = tk.getImage("src/images/image2.png");
public ImageObserver observer = null; //i just did this for no reason
public static Graphics g = Main.graphics;
public void paintImage1(Graphics g){
Images.g = g;
g.drawImage(Image1, 10, 10, observer); //NullPointerException points here, even if I replace 'pbserver' with null
}
public void paintImage2(Graphics g){
Images.g = g;
g.drawImage(Image2, 10, 10, observer); //strangely, it doesn't point here
}
}
Then, I cite and use it in my class that paints the image with my boolean like so:
public class PaintHandler{
public static Graphics graphics = Images.g;
public void PaintImages(boolean upheld){
if (upheld){
Images.paintImage1(graphics);//NullPionterException points here
}
else if (!upheld){
Images.paintImage2(graphics);//doesn't point here for some reason
}
}
}
The exception also points to the keybindings methods I use to make upheld true or not, another thing that is strange.
Like before, I don't want to make a class for every single image, I would prefer if they were all in one class. Additionally, when I try to use getGraphics() on the JFrame I'm using, I do through this GIGANTIC loop between making thing statics and not being able to apply static terms to non-static context, such as making a variable static, but then it says can't be used in static context, but then when you don't make it static a different variable says change it back to static, you don't make that static, and you go through this huge loop between making it static and changing to not be static (sorry for long explanation, was trying to be specific).

" when I try to use getGraphics() on the JFrame im using" - DON'T, this is not how custom is painting is done in Swing. getGraphics will, at best, return a snap shot of the last paint cycle and anything painted to it will be painted over on the next paint cycle and at worst, will return null.
Take a look at Painting in AWT and Swing and Performing Custom Painting for dore details about how painting works in Swing and how you should work with it...
Instead, create your self a custom component, extending from something like JPanel and use it's paintComponent to paint your images.
Normally, content stored in the src will be made available your application as embedded resources (this will depend on your IDE and build system), but generally speaking, you should not access any resource with the path containing src, this should be an immediate read flag for potential issues.
In this case you should be using Class#getResource to load the resource...
public class Images{
public static final Image Image1 = tk.getImage(Images.class.getResource("/images/image1.png"));
for example.
I would discourage you from using Toolkit.getImage as it will use a background thread to load the image and doesn't throw any kind of exception if the image wasn't loaded
Instead considering using ImageIO.read, see Reading/Loading an Image for more details
Like before, I don't want to make a class for every single image, I
would prefer if they were all in one class. Additionally, when I try
to use getGraphics() on the JFrame I'm using, I do through this
GIGANTIC loop between making thing statics and not being able to apply
static terms to non-static context, such as making a variable static,
but then it says can't be used in static context, but then when you
don't make it static a different variable says change it back to
static, you don't make that static, and you go through this huge loop
between making it static and changing to not be static (sorry for long
explanation, was trying to be specific).
Your images can be stored wherever you like, maybe in a Map of some kind. But they must be painted within a valid context, ie a component's paint method. You can paint as many images you want within in single component or spread them out if your prefer, that's tp up...
The issue with static is probably because you're ...
Still in the main method, which is static and/or
Trying to references (non-static) variables in other classes to which you've not created an instance to.
Only a runnable example will highlight which (if not both)

Java Snake Game -- Redrawing a specifc drawString() method

The problem:
I've been creating a Snake game in Java and everything is working smoothly. The only problem I've having is displaying the score. Each time the snake "eats" an apple, the score increases by 10. Instead of displaying the current score, it simply writes over the present score without actually erasing it. When I attempt to erase the content before re-displaying, I get an error and nothing appears, other than my background colour.
I have other Graphics components, including a paint() that I do NOT want affected.
My attempt (theoretical):
I display the score using a Graphics object that calls
drawString("Current score: " + currScore, 0, (Constants.TOTAL_HEIGHT + 15));
I figured I have to call a clearRect() method, which makes sense to me. Unfortunately, I get a NullPointerException at the line specifying clearRect().
I'm not using Graphics2D just so I can first be familiar with Graphics. If resolving this issue involves Graphics2D, I have no problem using it.
My attempt (the method in question):
public void displayScore(Graphics g) {
//clearScore.clearRect(0, getY(), getWidth(), getHeight());
//g.clearRect(0, getY(), getWidth(), getHeight());
g.drawString("Current score: " + currHighScore, 0, (Constants.TOTAL_HEIGHT + 15));
}
In the code, clearScore is a Graphics object declared in the class outside of any methods. My reason for using this was to avoid affecting the other graphics. I attempted each of the clearRect() lines and both produced the same NullPointerException, which is why I have them commented out.
(Constants.TOTAL_HEIGHT + 15) is responsible for assigning the Y-coordinate.
The Error Message:
Exception in thread "Thread-3" java.lang.NullPointerException
at com.nmw.view.CanvasFrame.displayScore(CanvasFrame.java:149)
at com.nmw.view.CanvasFrame.drawAll(CanvasFrame.java:43)
at com.nmw.view.CanvasFrame.run(CanvasFrame.java:64)
at java.lang.Thread.run(Thread.java:722)
drawAll is where I call all of my methods that take in Graphics to draw a segment of the game.
Apologies for the lengthy question =/

Your problem is that you are likely trying to draw with a null Graphics object. This usually occurs if you are not drawing within the paint(...) method if AWT or paintComponent(...) if Swing. The solution,
draw within one of those two methods (depending on the library you're using).
Always call the super's method within your painting method.
Or use a Label (AWT) or JLabel (Swing) to display the data.
To be blunt, your statement: "I have other Graphics components, including a paint() that I do NOT want affected." -- is an unrealistic requirement if this is an AWT project and if you don't want to use a Label.
And this statement:
In the code, clearScore is a Graphics object declared in the class outside of any methods. My reason for using this was to avoid affecting the other graphics.
Is not how you should do drawing, ever. You're finding out now why. If you happen to get a Graphics context during the running of the program, use it to assign Graphics to your class field, and then try to use that class field, you'll often get the NPE like you're seeing because a Graphics object thus obtained is not long-lasting.
If you need more in-depth help, please post your actual assignment requirements and more code. In particular your drawing methods. Best would be to create and post an sscce.
Edit
You state in comment:
My Snake game is an independent project I've been doing to learn more about Graphics. I've only been using the AWT library, however, I'll try using a Label. Thank you for your response and explanation of why I got the NullPointerException
If this is for your own project and not a school assignment, then I think that you will want to avoid using AWT and instead use Swing. It is much more powerful and flexible than AWT. There really is no reason why you'd want to use AWT now adays.

Design decision : Adding new arguments to a method V/S creating an instance variable

Lets suppose we have a class Shape which has a method rotate(int velocity). This method makes a shape rotate with a speed of velocity(the parameter passed to rotate). This method has been called in a project, say at 100 places.
But now a new requirement comes, that the rotate functionality will also depend on the color of the shape, i.e. if the color is blue then the velocity should be decreased by 1, else no change should be made.
One solution to this problem would be to change the rotate(int velocity) method to rotate(int velocity, Color color), then add an if statement inside rotate method to check for the color, and make a change in 100 calls of rotate.
E.g.
shape.rotate(50, blue) ;
Inside the rotate method,
void rotate(int velocity, Color color) {
if(color == blue)
--velocity ;
}
Another solution would be to make color as an instance variable of the shape object, and then without adding a new argument to the rotate method, simply set the color before calling it, and squeeze the if check inside the rotate method.
E.g.
shape.setColor(blue) ;
shape.rotate(50) ;
Inside the rotate method,
void rotate(int velocity) {
if(this.color == blue)
--velocity ;
}
Yet another solution would be to overload the rotate method and create a new method named rotate(int velocity, Color color) and use it in the new calls. This would leave the existing code which uses rotate(int velocity) unchanged.
Which of these would be the best possible solution? Or, does there exist a better solution? If yes, then what could it be?
Regards

I'd say there are a couple of questions you need to ask yourself.
Do you care about the color outside of the rotate method? If yes, make it an instance variable; if no, pass it to the rotate method.
Are you likely to care about the color outside of the rotate method further down the line? If yes, make it an instance variable; if no, pass it to the rotate method.
Are you always going to care about the color when calling the rotate method? If yes, make it an argument (to force them to set the color when rotating the shape).

A good principle of OO is to co-locate related behavior and state. In this case the rotate behavior of shape depends on the colour state of shape, so it makes sense to co-locate both in the Shape class, c.q. create a field 'colour' and use it within the rotate method to customize the rotation behavior.
Apart from this design decision, you are really also asking about a refactoring decision: how do I handle the application code that depends on shape? My approach in cases like this is to think ahead: how many changes like these to the Shape class can we expect? If this is a rare change then you could just go ahead and change all the code locations that initialize the shape class so a colour is set. If shape changes more often, then you should be more rigorous and make your code less tightly coupled. A way to do that in this case is to create an abstract factory (or use the factory offered by a D.I. framework like Spring) so that the application code does not need to know the creation details of shape.
BTW your third option seems sub-optimal to me: part of the code is not made aware of the addition of the colour state to shape, and keeps calling the old 'deprecated' rotate method. This means that setting a shape's colour to blue will not universally affect the rotation behavior, but only in 'special cases'. This weakens the design and makes it harder for the developers after you to understand it.

I think the first option is is tedious to implement. What if you miss at one place, what if later u realize that you need rotate(single parameter) again.
The second option is irrelevant as many have already pointed out.
3rd I think is the best solution, as it will not break your code. You can have both the overloaded method, can use any of them as per requirement.

As for me, I see classic example of inheritance usage here.
class Shape {
public void rotate(int v) {}
}
class GreenShape extends Shape {
public void rotate(int v){
super.rotate(v + 10);
}
}

Optimizing Java Graphics

I have a custom UI drawn for my java application. Right now I draw the entire UI from scratch. I know for a fact some parts of the UI are static. I know I could save these static parts to an image, but will this yield a improvement in performance (Since even an image must be drawn every frame)?
Is it plausible to save a reference to the Graphics2D object after the UI has been drawn and assign that to the new graphics object every frame (starting from a point where all the static components are drawn)?
Thanks in advance,
Alan

You don't need to redraw everything in every frame. So if you have static parts of your UI (or even dynamic parts that you know haven't changed since last frame) then you simply don't need to repaint them.
In my code (Swing games and simulations mostly) I usually try to follow the following rules:
Pre-prepare static images (e.g. BufferedImage textures for UI elements)
Override the paintComponent() method for each UI element individually to do the painting
Only call the repaint() method of any given UI element if I know that something has changed
Call repaint() in a timer-based loop for animation, but only call it on the portion of the UI that is being animated (e.g. a nested JPanel)
This approach seems to work and perform pretty well (though I'd welcome comments if there are ways to improve it!!)

There are two main optimizations you can do here. The first is to make sure that when you cause your UI to be repainted, usually done by calling repaint, make sure you call the version of repaint where you specify a rectangle that has changed. Only make the rectangle big enough to encompass the parts that actually have changed, not the static parts. For this to be effective you also have to pay attention to the clipRect in the Graphics2D object you are passed in paint(). That is used by the system to tell you exactly what needs to be repainted, in the above case usually the rectangle that you passed to repaint. Don't paint anything that lies entirely outside that rectangle.
You can also get significant performance improvements by caching the static parts of your interface in an image. Writing an image is by far the fastest way of getting things onto a screen. My measurements indicate that small images are faster than even a few simple drawing primitives. However you need to make sure the image characteristics match the screen, by using createCompatibleImage().
Of course you may be using a lot of memory to get this speedup. I would recommend testing to see if you need to do image caching before implementing it.

if some parts of the screen is completely static, then never redraw that part. Don't do a full-screen/window clear, just clear the part of the screen/window that changes all the time.
This way, you don't unnecessarily redraw the static image.

Java Classes in game programming?

I'm doing a little strategy game to help me learn Java in a fun way. The thing is I visioned the units as objects that would self draw on the game map (using images and buffering) and would react to the mouse actions with listeners attached to them.
Now, based on some tutorials I've been reading regarding basic game programming, all seems to be drawn in the Graphics method of my Map class. If a new unit emerges, i just update the Map.Graphics method, it's not as easy as making a new Unit object which would self draw... In this case, I'd be stuck with a whole bunch of Map methods instead of using classes for rendering new things.
So my question is, is it possible to use classes for rendering units, interface objects, etc, or i'll have to create methods and just do some kind of structural programming instead of object oriented? I'm a little bit confused and I'd like to have a mental blueprint of how things would be organized.
Thanks!

Sounds like your objects need to have a common interface that they can all be down-casted to for rendering.
E.g. have a generic list of your interface type, populate it with the game objects, then enumerate the list and call the common method to do the rendering.
This is C# code, but it should be similar for Java
public interface IRenderable
{
void RenderMe(Graphics graphics)
}
List<IRenderable> myGameObjects;
foreach (IRenderable myGameObject in myGameObjects)
{
myGameObject.RenderMe(myMap);
}
While the objects are held in the list they can respond to events and update their internal state ready for the next render cycle.

As ck said, you could do this with a drawable interface, something like:
public interface GameElement {
public void draw(Graphics g);
}
public class Sprite implements GameElement{
private Image image;
private int x;
private int y;
public void draw(Graphics g) {
g.drawImage(image,x,y,null);
}
}
public class GameLoop {
public void drawElements(final Graphics2D g, final List<GameElement> elements) {
for (final GameElement gameElement : elements) {
gameElement.draw(g);
}
}
}
but the way you said you want to do it, with a listener, may give you problems in the future.
As dash-tom-bang already pointed, all elements are rendered in a single game loop, after all of them were already updated. If you do this with a listener and you have a game loop or any other thread modifying the unit list, you can end up with a concurrent access modification.
Besides, this way you will draw all elements in a single point, that will avoid some weird flickering on your game.

While "real" game projects come in many forms, ultimately you have something like this:
while True:
ProcessUserInput()
for o in objects:
o.Update()
for o in objects:
o.Render()
Use whatever mechanism you find dear to implement any and all of these functions. :) Polymorphism is fine, so you should feel free to use that if you're comfortable with that. The "world map" likely should be the first item in the list, unless your rendering system is sufficiently advanced to handle out-of-order drawing (depth sorting, basically).
From your question, it may be that the units are owned by the map? In that case, the Map.Render function would iterate over all of the units that it owns, calling Render on each one of them in turn. Regardless, it's probably cleanest if the code to draw a unit is in the class for that unit.
Good luck!