如何使用重画来调用Java paintComponent

Question

In this video drawing() method is called in main class. When we remove drawing() in the main method it still draws the shape. How can we avoid this situation ?

shapes class:

import java.awt.*;
import javax.swing.*;
public class shapes{
public static void main(String[] args){
    JFrame frame = new JFrame("Test");
    frame.setVisible(true);
    frame.setSize(400,200);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    draw object = new draw();
    frame.add(object);

    object.drawing();
    }

}

Draw class:

import java.awt.*;
import javax.swing.*;

public class draw extends JPanel{
public void drawing(){
    repaint();
}
public void paintComponent(){
  super.paintComponent(g);
  g.setColor(Color.BLUE);
  g.fillRect(10,15,100,100);
  }
}

Answer 1

There are some minor issues with the code, but I assume that it's only a small snippet for demonstration purposes. For details, have a look at Performing Custom Painting .

Actually, this tutorial would also answer your question, but to summarize it:

The paintComponent method will be called automatically, "by the operating system", whenever the component has to be repainted. The call to repaint() only tells the operating system to call paintComponent again, as soon as possible. So you can call repaint() to make sure that something that you canged appears on the screen as soon as possible.

If you explicitly want to enable/disable certain painting operations, you can not influence this by preventing paintComponent from being called. It will be called anyhow. Instead, you'll introduce some flag or state indicating whether something should be painted or not.

In your example, this could roughly be done like this:

import java.awt.*;
import javax.swing.*;

public class Draw extends JPanel{
    private boolean paintRectangle = false;

    void setPaintRectangle(boolean p) {
        paintRectangle = p;
        repaint();
    }

    @Override
    public void paintComponent(){
        super.paintComponent(g);

        if (paintRectangle) {
            g.setColor(Color.BLUE);
            g.fillRect(10,15,100,100);
        }
    }
}

You can then call the setPaintRectangle method to indicate whether the rectangle should be painted or not.