未显示JComponent，未调用paintComponent

Question

I have a class which extends JComponent called Cell that doesn't get displayed, and its paintComponent is never called. Here is a short checklist of things I have done in an attempt to get it working:

• Cell constructor has this.setPreferredSize , this.setMaximumSize and this.setMinimumSize
• The object (extension of JPanel ) that holds the cell calls this.add(cell) for each cell as well as this.validate()
• The JPanel's that hold the other JPanels all call .validate() .
• The primary JPanel is added to the contentPane of the JFrame and the JFrame .pack() is called.

GUIFrontend.java

import java.awt.*;
public class GUIFrontend extends JFrame{
    private static final long serialVersionUID = -7074700257172600349L;
    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                GUIFrontend g = new GUIFrontend();
            }
        });
    }
    public GUIFrontend(){
        this.setDefaultCloseOperation(EXIT_ON_CLOSE);
        JPanel primary = new JPanel();

        //Map
        JPanel mapContainer = new JPanel();
        mapContainer.setMinimumSize(new Dimension(772, 600));
        GTGrid gtg = new GTGrid();
        gtg.validate();
        mapContainer.add(gtg);
        mapContainer.validate();

        JPanel controls = new JPanel();
        controls.setBackground(Color.green);
        controls.setPreferredSize(new Dimension(160,600));

        primary.add(mapContainer);
        primary.add(controls);
        primary.validate();
        this.validate();
        this.getContentPane().add(primary);
        this.pack();
        this.validate();
        this.setVisible(true);
    }

}

GTGrid.java

public class GTGrid extends Grid {
    private static final long serialVersionUID = -2787182463097088611L;

    public GTGrid() {
        this.height =  150;
        this.width = 193;
        //Minimum of 4x4 per cell
        this.setMinimumSize(new Dimension(width*4,height*4));
        GridLayout gl;
        this.setLayout(gl = new GridLayout(width,height));
        gl.setVgap(0);
        gl.setHgap(0);
        this.vmax = 2;
            //this.add(new JButton("Hello, World!"));
        this.intersections = new ArrayList<IntersectionController>();
        this.cells = new Cell[height][width];
        for(int dt=0;dt<this.height;dt++){
            for(int dl=0;dl<this.width;dl++){
                this.cells[dt][dl] = new Cell(dt,dl,this);
                this.cells[dt][dl].setDirection((dt%2==0)?Direction.East:Direction.West);
                //Paint detail
                this.add(cells[dt][dl]);
                this.validate();
            }
        }
    }
}

Cell.java (this method is huge, so only selections have been added)

    public Cell(int dt, int dl, Grid parent){
        //some deleted code unrelated to JComponent

        //Methods for Paint
        this.setPreferredSize(new Dimension(4,4));
        this.setMaximumSize(new Dimension(10,10));
        this.setMinimumSize(new Dimension(4,4));
    }
public void paintComponent(Graphics g){
    super.paintComponent(g);
    System.out.println("I'm printing!");
    Color oldColor = g.getColor();

    //Draw black border around cell;
    g.setColor(Color.black);
    g.drawRect(0, 0, this.getWidth(), this.getHeight());

    //Draw yellow line indicating direction of cell
    this.drawYellowLine(g);

    //Draw car if the cell has one
    if (this.hasCar())
        this.drawCar(g);

    g.setColor(oldColor);
}

Answer 1

Is there a problem with adding too many elements to that layout manager? (there would be 150*193=28,950 components)

One layout manager has a hardcoded limit of 512 components. Don't remember which one it is.

When you use a GridLayout the components are all resized to fit the space avaialable to the entire grid. Maybe not enough spaces is available so the actual size of each cell becomes (0, 0), in which case the paint method will never be called on the component.

Answer 2

The problem got resolved by removing the getWidth() and getHeight() methods from my Grid class (Which GTGrid extends). These were old methods that I rarely used anymore, and apparently they kept the GTGrid from being drawn because they were initialized to zero. In addition, I had to remove getWidth() & getHeight() from Cell.java