无法在JPanel中绘制Circle（扩展JComponent）

Question

I am trying to add a custom circle to a JPanel, see this:

    graphicPanel = new GraphicPanel();
    JTextArea text = new JTextArea("1233", 5, 10);
    graphicPanel.add(text);

    Circle circle = new Circle();
    circle.setX(30);
    circle.setY(30);
    circle.setDiameter(30);
    graphicPanel.add(circle);
    graphicPanel.repaint();
    graphicPanel.revalidate();

GraphicPanel is just a custom JPanel that doesn't do anything interesting yet (just holds a list that is not used yet)

GraphicPanel.java

public class GraphicPanel extends JPanel {

    private static final long serialVersionUID = -3813468764873993369L;
    private List<Node> nodes = new ArrayList<Node>();

    public GraphicPanel() {
    }

    public void addNode(Node node) {
        nodes.add(node);
    }

    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.setColor(Color.CYAN);
        g.fillOval((30 - 30 / 2), (30 - 30 / 2), 30, 30);
    }
}

Circle.java

public class Circle extends JComponent {
    private static final long serialVersionUID = 628299863960706428L;
    private int x;
    private int y;
    private int diameter;
    private Color color;

    public int getX() {
        return x;
    }

    public void setX(int x) {
        this.x = x;
    }

    public int getY() {
        return y;
    }

    public void setY(int y) {
        this.y = y;
    }

    public int getDiameter() {
        return diameter;
    }

    public void setDiameter(int diameter) {
        this.diameter = diameter;
    }

    public Color getColor() {
        return color;
    }

    public void setColor(Color color) {
        this.color = color;
    }

    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.setColor(color);
        g.fillOval((x - diameter / 2), (y - diameter / 2), diameter, diameter);
    }
}

The JTextArea appears, the circle does not. If I add the draw code for the circle directly to the paintComponent() of graphicPanel, then a circle appears:

    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.setColor(Color.CYAN);
        g.fillOval((30 - 30 / 2), (30 - 30 / 2), 30, 30);
}

So the draw code itself should be fine (I considered that maybe the circle is drawn somewhere where I cant see it but this is not the case). What do I have to change to make the circle appear? I want to draw it like this and not with g.fillOval() in paintComponent() of GraphicPanel. I am using Java 8

Answer 1

The first thing is that a JPanel has a FlowLayout as a default layout manager.

A FlowLayout honors the preferred size of a Component , but Circle doesn't have a specific one, so its size is (0,0).

You may want to override getPreferredSize to give it one, or use a layout manager that will still give a size to your Circle (eg a BorderLayout where you add your component to CENTER ) . For later , you may also want to override getMaximumSize and getMinimumSize .

The second thing is that getX and getY are existing methods from JComponent , that your code overrides (probably not on purpose). Those methods tell the position of this component within its container and would mess up the layout if you play with them (here your Circle is located at 30,30 inside GraphicPanel and gets hidden by the textarea).

getX()

the current x coordinate of the component's origin

In the following example, I changed the name and the accessor methods of x and y to avoid overriding getX and getY (there was actually no need to change the names of x and y variables, it is just to keep coherent with those accessor methods names).

A "preferred size" has also been set by adding an overriden getPreferredSize method, computing its optimal size.

import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;

import javax.swing.JComponent;

public class Circle extends JComponent {
    private static final long serialVersionUID = 628299863960706428L;
    private int xCoo;
    private int yCoo;
    private int diameter;
    private Color color;

    @Override
    public Dimension getPreferredSize() {

        return new Dimension(diameter + xCoo, diameter + yCoo);

    }

    /*  @Override
    public Dimension getMinimumSize() {

        return new Dimension(diameter + xCoo, diameter + yCoo);

    }

    @Override
    public Dimension getMaximumSize() {

        return new Dimension(diameter + xCoo, diameter + yCoo);

    }*/

    public int getXCoo() {
        return xCoo;
    }

    public void setXCoo(final int xCoo) {
        this.xCoo = xCoo;
    }

    public int getYCoo() {
        return yCoo;
    }

    public void setYCoo(final int yCoo) {

        this.yCoo = yCoo;
    }

    public int getDiameter() {
        return diameter;
    }

    public void setDiameter(final int diameter) {
        this.diameter = diameter;
    }

    public Color getColor() {
        return color;
    }

    public void setColor(final Color color) {
        this.color = color;
    }

    public void paintComponent(final Graphics g) {
        super.paintComponent(g);
        g.setColor(color);
        g.fillOval((xCoo - diameter / 2), (yCoo - diameter / 2), diameter, diameter);
    }
}

Also note that JComponent has set/getBackground and set/getForeground methods that you may find useful to set and get background and foreground Color .