布尔值不适用于图形 java

Question

Here is my code

package javaapplication7;

import java.awt.*;

import java.awt.event.ActionEvent;

import java.awt.event.ActionListener;

import javax.swing.*;

import java.awt.Graphics;

import java.awt.Graphics2D;``

import java.util.*;

import java.io.*;

public class JavaApplication7 extends JPanel implements ActionListener{

    JButton j = new JButton();

    boolean drawHello = true;

    boolean drawWorld = false;

   String hello = "Hello";

   String World = "World";


   public JavaApplication7(){

       this.setLayout(new BorderLayout());

        add(j,BorderLayout.NORTH);

    }

    public void actionPerformed(ActionEvent e) {

     boolean drawWorld = true;

      repaint();
}

    protected void paintComponent(Graphics g) {
    super.paintComponent(g);
    if (drawHello)
             g.drawString(hello, 50, 50);

    if (drawWorld)
             g.drawString(World, 70, 70);
}



    public static void main(String[] args) {
        JFrame f = new JFrame("Swing Paint Demo");
        JavaApplication7 j7 = new JavaApplication7();
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.setSize(250,250);
        f.setVisible(true);
        f.add(j7);
    }

}

Answer 1

I think you want

public void actionPerformed(ActionEvent e) {

  drawWorld = true;
  repaint();
}

If you re-declare it in your method, the global value will not be updated.

Answer 2

What I don't understand that you've defined a Boolean and then you create another one in actionPerformed() just remove the Boolean as follows:

public void actionPerformed(ActionEvent e) {

drawWorld = true;

  repaint();
}

Because you allready created it there :

public class JavaApplication7 extends JPanel implements ActionListener{

JButton j = new JButton();

boolean drawHello = true;

boolean drawWorld = false; //<------

Also add

j.addActionListener(this);

Like this :

public JavaApplication7(){

   this.setLayout(new BorderLayout());
   j.addActionListener(this);
   add(j,BorderLayout.NORTH);

}

Answer 3

you have two mistakes in your code.

The first one, you want to add the ActionListener to the JButton , otherwise you will never execute the actionPerformed method.(i simply guess you want the action to be performed on the button).

public JavaApplication7() {
    this.setLayout(new BorderLayout());

    add(j, BorderLayout.NORTH);
    j.addActionListener(this);
}

the second one is that you are using a local variable in the actionPerformed method, remove the type identifier there to assign the class variable. Otherwise you will only set the local variable drawWorld to true, which wont change the initial value of the class variable drawWorld . In this case the paintComponent wont notify the "change" you have done to the variable darwWorld because you never reassigned it as true .

public void actionPerformed(ActionEvent e) {
    drawWorld = true;
    repaint();
}