Thread: A beginner needs help in oop

Hello everyone!

I've tried to create objects from classes and it worked until I wanted to draw the new objects I created.

package objectclasstrain;
import javax.swing.SwingUtilities;
import javax.swing.JFrame;
import javax.swing.JPanel;
import java.awt.Dimension;
import java.awt.Color;
import java.awt.Graphics;
 
 
 
 
public class ObjectClassTrain {
 
 
 
 
 
 
    public static void main(String[] args) {
 
        square squareRed = new square();
        System.out.println(squareRed.returnXposition());
        System.out.println(squareRed.returnYposition());
 
        SwingUtilities.invokeLater(new Runnable(){
            @Override
            public void run(){
                createAndShowGUI();
            }
        });
 
    }
 
    public static void createAndShowGUI(){
 
        JFrame f = new JFrame("JFrame test");
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.setResizable(false);
        f.add(new myPanel());
        f.pack();
        f.setVisible(true);
 
    }
 
}
 
class myPanel extends JPanel{
 
 
     int room_width = 640;
     int room_height = 480;
 
 
 
 
    public myPanel(){
 
 
 
    }
 
 
 
 
 
 
    @Override
    public Dimension getPreferredSize(){
 
        return new Dimension(room_width, room_height);
 
    }
 
    @Override
    public void paintComponent(Graphics g){
        super.paintComponent(g);        
        g.setColor(Color.BLACK);
        g.fillRect(squareRed.returnXposition(), 0, room_width, room_height);
 
    }
 
 
}
 
 
class square{
 
    int room_width = 640;
     int room_height = 480;
 
    double Xposition;
    double Yposition;
 
         public square(){
 
             Xposition = (int) (Math.random() * room_width);
             Yposition = (int) (Math.random() * room_height);
             }
 
         public double returnXposition(){
             return Xposition;
         }
 
         public double returnYposition(){
             return Yposition;
         }
 
 
}

I don't really know how I could get access to the squareRed's Xposition variable.

oh, and another problem is that there is only one line of code which will paint the objects, and I want to be able, for example,
to click on the screen to create an object which is automatically be painted on the screen. And to do that I can't just create new lines
of code for every object that could be created.

What should I do?

Small point, but classes should begin with a capital letter like Square.

You can't access local variables in main() - or other methods - from different places in the code. So make what you want an instance variable. You can initialise it within the createAndShowGui() method and access it elsewhere.

If your intention is to have a whole bunch of squares the instance variable would most naturally be a list of squares rather than a single square. The painting code can run through the list and paint each one and other code can add/remove individual squares to taste. This is a separate question: get a single square created and accessible first.

Originally Posted by pbrockway2

You can't access local variables in main() - or other methods - from different places in the code. So make what you want an instance variable. You can initialise it within the createAndShowGui() method and access it elsewhere.

Like this?

public static void createAndShowGUI(){
 
 
        Square squareRed = new Square();
 
 
 
        JFrame f = new JFrame("JFrame test");
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.setResizable(false);
        f.add(new myPanel());
        f.pack();
        f.setVisible(true);
 
    }

That works but how can I get access to Xposition in the paintComponent?
This gives me an error:

    public void paintComponent(Graphics g){
        super.paintComponent(g);        
        g.setColor(Color.BLACK);
        g.fillRect(squareRed.returnXposition(), 0, room_width, room_height);
 
    }

Could you explain a small point(I only find floating-point in google)

That is a lot of classes. Usually you wouldn't want that many classes that didn't do too much, I'm not sure if it is an assignment where you are supposed to have that many classes or not, but it can make it a bit difficult. One way of looking at it is that your myPanel class is dependent upon information from the Square class. Usually you take care of the dependencies while you initialize it, or construct it, as it is a necessary part of the construction process.

Originally Posted by theoriginalanomaly

That is a lot of classes. Usually you wouldn't want that many classes that didn't do too much, I'm not sure if it is an assignment where you are supposed to have that many classes or not, but it can make it a bit difficult. One way of looking at it is that your myPanel class is dependent upon information from the Square class. Usually you take care of the dependencies while you initialize it, or construct it, as it is a necessary part of the construction process.

I've only been using java for a short period of time. I program stuff, like creating a window, the way the tutorials says I should do it. So I dont know how to create, for example the code in the first post, but with less classes. Could you explain please?

Originally Posted by theoriginalanomaly

I'm not sure if it is an assignment where you are supposed to have that many classes or not, but it can make it a bit difficult.

I'm not taking any classes of any sort, I'm just a person who likes to learn and enjoys Java

The error message:

Exception in thread "AWT-EventQueue-0" java.lang.RuntimeException: Uncompilable source code - Erroneous tree type: <any>
at objectclasstrain.myPanel.paintComponent(ObjectClas sTrain.java:85)
at javax.swing.JComponent.paint(JComponent.java:1037)
at javax.swing.JComponent.paintChildren(JComponent.ja va:870)
at javax.swing.JComponent.paint(JComponent.java:1046)
at javax.swing.JComponent.paintChildren(JComponent.ja va:870)
at javax.swing.JComponent.paint(JComponent.java:1046)
at javax.swing.JLayeredPane.paint(JLayeredPane.java:5 67)
at javax.swing.JComponent.paintChildren(JComponent.ja va:870)
at javax.swing.JComponent.paintToOffscreen(JComponent .java:5139)
at javax.swing.RepaintManager$PaintManager.paintDoubl eBuffered(RepaintManager.java:1523)
at javax.swing.RepaintManager$PaintManager.paint(Repa intManager.java:1454)
at javax.swing.RepaintManager.paint(RepaintManager.ja va:1257)
at javax.swing.JComponent.paint(JComponent.java:1023)
at java.awt.GraphicsCallback$PaintCallback.run(Graphi csCallback.java:21)
at sun.awt.SunGraphicsCallback.runOneComponent(SunGra phicsCallback.java:60)
at sun.awt.SunGraphicsCallback.runComponents(SunGraph icsCallback.java:97)
at java.awt.Container.paint(Container.java:1778)
at java.awt.Window.paint(Window.java:3390)
at javax.swing.RepaintManager$3.run(RepaintManager.ja va:825)
at javax.swing.RepaintManager$3.run(RepaintManager.ja va:802)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.AccessControlContext$1.doIntersectio nPrivilege(AccessControlContext.java:87)
at javax.swing.RepaintManager.paintDirtyRegions(Repai ntManager.java:802)
at javax.swing.RepaintManager.paintDirtyRegions(Repai ntManager.java:745)
at javax.swing.RepaintManager.prePaintDirtyRegions(Re paintManager.java:725)
at javax.swing.RepaintManager.access$1000(RepaintMana ger.java:46)
at javax.swing.RepaintManager$ProcessingRunnable.run( RepaintManager.java:1668)
at java.awt.event.InvocationEvent.dispatch(Invocation Event.java:209)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.j ava:672)
at java.awt.EventQueue.access$400(EventQueue.java:81)
at java.awt.EventQueue$2.run(EventQueue.java:633)
at java.awt.EventQueue$2.run(EventQueue.java:631)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.AccessControlContext$1.doIntersectio nPrivilege(AccessControlContext.java:87)
at java.awt.EventQueue.dispatchEvent(EventQueue.java: 642)
at java.awt.EventDispatchThread.pumpOneEventForFilter s(EventDispatchThread.java:269)
at java.awt.EventDispatchThread.pumpEventsForFilter(E ventDispatchThread.java:184)
at java.awt.EventDispatchThread.pumpEventsForHierarch y(EventDispatchThread.java:174)
at java.awt.EventDispatchThread.pumpEvents(EventDispa tchThread.java:169)
at java.awt.EventDispatchThread.pumpEvents(EventDispa tchThread.java:161)
at java.awt.EventDispatchThread.run(EventDispatchThre ad.java:122)

Did you mean this?

C:\Users\BLANK\Documents\NetBeansProjects\ObjectCl assTrain\src\objectclasstrain\ObjectClassTrain.jav a:85: cannot find symbol
symbol : variable squareRed
location: class objectclasstrain.myPanel
g.fillRect(squareRed.returnXposition(), 0, room_width, room_height);
1 error
C:\Users\BLANK\Documents\NetBeansProjects\ObjectCl assTrain\nbproject\build-impl.xml:949: The following error occurred while executing this line:
C:\Users\BLANK\Documents\NetBeansProjects\ObjectCl assTrain\nbproject\build-impl.xml:268: Compile failed; see the compiler error output for details.
BUILD FAILED (total time: 1 second)

I'm using netbeans. If that is not the right error message then I do not know what I should do.

squareRed is defined in (Probably not a very good choice now when I think of it.)"createAndShowGUI()":

public static void createAndShowGUI(){


Square squareRed = new Square();



JFrame f = new JFrame("JFrame test");
f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
f.setResizable(false);
f.add(new myPanel());
f.pack();
f.setVisible(true);

}

So which way is the most efficient way do you think? (I'm sorry if you find me to lazy to google it) I moved "Square squareRed = new Square();" to inside the "class Square" place but after that? should I move the whole paintComponent inside class square

Originally Posted by Norm

the other class could be an inner class of the class with the variable's definition

but that did not work either :/

Please use Code tags, not quote tags, when posting code.


Square squareRed = new Square(); <--This is inside the square class. This makes it visible only within the square class. from public class Square to the closing brace. If you want to use this variable in the train class, define it in the train class instead of the square class.

java.awt.Graphics does not have a method that takes a double and 3 integers. You are using Xposition = (int) (Math.random() * room_width); to get an integer, but still storing it inside a variable of type double. So there is an automatic .0 added to the end so to speak. Make the variables hold integers, or change the value from double to integer when drawing it.

public class Square { ... } does not extend a class (specifically). So super() would refer to java.lang.Object, which does not have a method named paintComponent(int, int, int, int). So when you try to "override" this method in the Square class, it is not there to be overridden.

I think I got it to work now!

The problem was that g.fillRect(squareRed.Xposition, 0, 32, 32); required four ints but I had instead given it a double, an int, an int and another int. (I saw jps's comment right after this achievement)
So I converted the double Xposition into an integer, which worked perfectly!

package objectclasstrain;
import javax.swing.SwingUtilities;
import javax.swing.JFrame;
import javax.swing.JPanel;
import java.awt.Dimension;
import java.awt.Color;
import java.awt.Graphics;
 
public class ObjectClassTrain {
 
    public static void main(String[] args) {
 
        SwingUtilities.invokeLater(new Runnable(){
            @Override
            public void run(){
                createAndShowGUI();
            }
        });
    }
 
    public static void createAndShowGUI(){
        JFrame f = new JFrame("JFrame test");
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.setResizable(false);
        f.add(new myPanel());
        f.pack();
        f.setVisible(true);
    }
}
 
class myPanel extends JPanel{
 
     int room_width = 640;
     int room_height = 480;
 
    public myPanel(){
    }
 
    @Override
    public Dimension getPreferredSize(){
        return new Dimension(room_width, room_height);
    }
 
    Square squareRed = new Square();
 
    @Override
    public void paintComponent(Graphics g){
        super.paintComponent(g);        
        g.setColor(Color.BLACK);
        g.fillRect(squareRed.XpositionINT, squareRed.YpositionINT, 32, 32);
 
    }
}
 
 
class Square{
 
    int room_width = 640;
    int room_height = 480;
 
    double Xposition = 64;
    double Yposition = 64;
 
         public Square(){
             Xposition = (int) (Math.random() * room_width);
             Yposition = (int) (Math.random() * room_height);
             }
 
         int XpositionINT = (int) Xposition;
         int YpositionINT = (int) Yposition;
 
         public double returnXposition(){
             return XpositionINT;
         }
 
         public double returnYposition(){
             return YpositionINT;
         }
}

But what I'm wondering now is, how can I change a value inside the object when declaring an it.

something similar to this?:

Square squareRed = new Square();
squareRed.Xposition = 64;

getters and setters are what they are usually called. Instead of using a constructor, you have a setMethod() and a getMethod(). At least that is one way. squareRed.setXposition(64), or setXYPosition(x, y)

Do you mean something like this?:

The whole code:

package objectclasstrain;
import javax.swing.SwingUtilities;
import javax.swing.JFrame;
import javax.swing.JPanel;
import java.awt.Dimension;
import java.awt.Color;
import java.awt.Graphics;
 
 
 
 
public class ObjectClassTrain {
 
 
 
 
 
 
    public static void main(String[] args) {
 
 
 
        SwingUtilities.invokeLater(new Runnable(){
            @Override
            public void run(){
                createAndShowGUI();
            }
        });
 
    }
 
    public static void createAndShowGUI(){
 
 
 
 
 
 
        JFrame f = new JFrame("JFrame test");
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.setResizable(false);
        f.add(new myPanel());
        f.pack();
        f.setVisible(true);
 
    }
 
}
 
class myPanel extends JPanel{
 
 
     int room_width = 640;
     int room_height = 480;
 
 
 
 
    public myPanel(){
        double x = 64;
 
 
 
    }
 
 
 
 
 
 
    @Override
    public Dimension getPreferredSize(){
 
        return new Dimension(room_width, room_height);
 
    }
 
 
     Square squareRed = new Square();
     squareRed.Xposition(55);
 
 
    @Override
    public void paintComponent(Graphics g){
        super.paintComponent(g);        
        g.setColor(Color.BLACK);
        g.fillRect(squareRed.returnXposition(), squareRed.YpositionINT, 32, 32);
 
    }
 
 
 
 
}
 
 
class Square{
 
    int room_width = 640;
     int room_height = 480;
 
    double Xposition = 333;
    double Yposition = 64;
 
         public Square(){
 
             Xposition = (int) (Math.random() * room_width);
             Yposition = (int) (Math.random() * room_height);
             }
 
         int XpositionINT = (int) Xposition;
         int YpositionINT = (int) Yposition;
 
         public int returnXposition(){
             return XpositionINT;
         }
 
         public int returnYposition(){
             return YpositionINT;
         }
 
 
}

The important part I did:

     Square squareRed = new Square();
     squareRed.Xposition(55);

but that does not work either

Could you explain a small point(I only find floating-point in google)

Sorry for the late post. I only meant that class names should start with a big (uppercase) letter: like MyPanel, Square. ("small" == a minor thing that is not the cause of your problem).

Originally Posted by Norm

That is the syntax for calling the Xposition() method and passing it the arg: 55
The class needs to define the method: Xposition(int x)

Normally methods that set the values of class variables have names that start with: set
So the method should be named: setXposition(int x)

So where should I initialize "int x" for it to function?

And how should the setXposition look like, I did this inside class Square, I want squareRed.setXposition(double x); to give the square its new x-position in a double format:

     public static void setXposition(double x){
 
         double Xposition = x;
 
             }

That's close. But you shouldn't declare Xposition again. It is already part of the Square class, so you just say:

public static void setXposition(double x){
    Xposition = x;
}

Then you can say:

Square squareRed = new Square();
squareRed.setXposition(64);

okey, I think I got it to work now

package objectclasstrain;
import javax.swing.SwingUtilities;
import javax.swing.JFrame;
import javax.swing.JPanel;
import java.awt.Dimension;
import java.awt.Color;
import java.awt.Graphics;
 
 
 
 
public class ObjectClassTrain {
 
 
 
 
 
 
    public static void main(String[] args) {
 
 
 
        SwingUtilities.invokeLater(new Runnable(){
            @Override
            public void run(){
                createAndShowGUI();
            }
        });
 
    }
 
    public static void createAndShowGUI(){
 
 
 
 
 
 
        JFrame f = new JFrame("JFrame test");
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.setResizable(false);
        f.add(new myPanel());
        f.pack();
        f.setVisible(true);
 
    }
 
}
 
class myPanel extends JPanel{
 
 
     int room_width = 640;
     int room_height = 480;
 
 
 
 
    public myPanel(){
 
 
 
    }
 
 
 
 
 
 
    @Override
    public Dimension getPreferredSize(){
 
        return new Dimension(room_width, room_height);
 
    }
 
     Square squareRed = new Square();
 
 
 
    @Override
    public void paintComponent(Graphics g){
        int x = 55;
        int y = 125;
        squareRed.setXposition(x); 
        squareRed.setYposition(y); 
        super.paintComponent(g);        
        g.setColor(Color.BLACK);
        g.fillRect(squareRed.returnXposition(), squareRed.returnYposition(), 32, 32);
 
    }
 
 
 
 
}
 
 
class Square{
    private static int Xposition;
    private static int Yposition;
 
     public static void setXposition(int x){
 
          Xposition = x;
             System.out.println(Xposition);
             }
 
 
     public static void setYposition(int y){
 
          Yposition = y;
             System.out.println(Xposition);
             }
 
 
    int room_width = 640;
     int room_height = 480;
 
 
        /* public Square(){
 
             Xposition = (int) (Math.random() * room_width);
             Yposition = (int) (Math.random() * room_height);
             }*/
 
         //int XpositionINT = (int) Xposition;
 
         //int YpositionINT = (int) Yposition;
 
         public int returnXposition(){
             return Xposition;
         }
 
         public int returnYposition(){
             return Yposition;
         }
 
 
 
 
 
}

Thanks for the help everyone! I really appreciate it