Java Applet设置颜色动作列表

Question

I have my 3 buttons, added them and have action listeners on each one. In the action performed section, they are suppose to change the g.setcolor to a certain color and repaint my oval. what am i doing wrong ?

import java.applet.*;
import java.awt.*;
import java.awt.event.*;

public class  zzz extends Applet implements ActionListener {

  Button a, b, c;

  public void init()
  {
  setLayout(new FlowLayout());

  a = new Button("Red");
  b = new Button("Blue");
  c = new Button("Green");
  add(a);
  add(b);
  add(c);
  a.addActionListener(this);
  b.addActionListener(this);
  c.addActionListener(this);

  }

  public void paint(Graphics g){

  g.drawOval(250,100,100,100);
  g.drawString("Circle",275,100);
  g.setColor(Color.white);
  g.fillOval(250,100,100,100);


  }

  public void actionPerformed (ActionEvent evt)
  {
  if (evt.getSource() == a){
  g.setColor(Color.red);
  repaint(); 
  }
  else if (evt.getSource() == b){
  g.setColor(Color.blue);
  repaint(); 
  }
  else if (evt.getSource() == c){
  g.setColor(Color.green);
  repaint(); 
  }
  }
} 

Answer 1

1. This is not how painting is done.
2. You should avoid painting directly to a top level container

Instead of trying to change g , which is undefined from the context of your actionPerformed method, you should set a variable to indicate the current color, something more like...

public void actionPerformed (ActionEvent evt)
{
    if (evt.getSource() == a){
        drawColor = Color.red;
    }
    else if (evt.getSource() == b){
        drawColor = Color.blue;
    }
    else if (evt.getSource() == c){
        drawColor = Color.green;
    }
    repaint();
}

Then you would need to update you paint method to look more like....

public void paint(Graphics g){
    super.paint(g);
    g.setColor(drawColor);
    g.drawOval(250,100,100,100);
    g.drawString("Circle",275,100);
    g.setColor(Color.white);
    g.fillOval(250,100,100,100);
}