Trouble Drawing Line on JFrame

Question

I'm trying to write a program that displays a bunch of lines on a screen, the coordinates of which will be determined from another program.

In doing so, I'm trying to modify jasssuncao's code from here, so that I don't have to click on any buttons in order to get lines: How to draw lines in Java

Here is what I have now:

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.LinkedList;

import javax.swing.JButton;
import javax.swing.JComponent;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class LineDrawing extends JComponent{

  private static class Line{
      final int x1; 
      final int y1;
      final int x2;
      final int y2;   
      final Color color;

      public Line(int x1, int y1, int x2, int y2, Color color) {
          this.x1 = x1;
          this.y1 = y1;
          this.x2 = x2;
          this.y2 = y2;
          this.color = color;
      }               
  }

  private final LinkedList<Line> lines = new LinkedList<Line>();

  public void addLine(int x1, int x2, int x3, int x4) {
      addLine(x1, x2, x3, x4, Color.black);
  }

  public void addLine(int x1, int x2, int x3, int x4, Color color) {
      lines.add(new Line(x1,x2,x3,x4, color));        
      repaint();
  }

  public void clearLines() {
      lines.clear();
      repaint();
  }

  @Override
  protected void paintComponent(Graphics g) {
      super.paintComponent(g);
      for (Line line : lines) {
          g.setColor(line.color);
          g.drawLine(line.x1, line.y1, line.x2, line.y2);
      }
  }

  public static void main(String[] args) {
      JFrame testFrame = new JFrame();
      testFrame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
      final LineDrawing comp = new LineDrawing();
      comp.setPreferredSize(new Dimension(1000, 400));
      testFrame.getContentPane().add(comp, BorderLayout.CENTER);
      comp.addLine(100, 100, 100, 100, new Color(24, 24, 24));
    testFrame.pack();
    testFrame.setVisible(true);
  }
  }
}

However, doing so does not display any a line. Why isn't the code displaying anything? Thanks!

score 6 · Accepted Answer · edited May 23 '17 at 12:19

6

Kudos for a complete example, but a few things bear mentioning:

Your line needs a non-zero size; note the coordinates required by drawLine():
```
comp.addLine(10, 10, 100, 100, Color.blue);
```
Your JComponent may need to be opaque:
```
comp.setOpaque(true);
```
Construct and manipulate Swing GUI objects only on the event dispatch thread, for example.
Don't use setPreferredSize() when you really mean to override getPreferredSize().

edited May 23 '17 at 12:19

Community

1
1

answered Aug 31 '16 at 02:24

trashgod

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Thanks so much for the help. I read your link on the event dispatch thread, and if I'm not mistaken, I just need to schedule out the tasks, right? If so, why is my code not on the event dispatch thread? I'm having trouble understanding how creating something in the main differs from constructing it in the event dispatch thread. – Valerie Z Aug 31 '16 at 02:41
Glad to help; I've added a link to a `EventQueue.invokeLater()` example. – trashgod Aug 31 '16 at 16:22

Trouble Drawing Line on JFrame

1 Answers1