单击按钮绘制文本将不起作用，除非调整框架大小

Question

如果在下面的代码中，如果我省略了布尔变量bDrawText，那么程序一启动，文本就会显示出来。因此，我将该变量设置为false，以便只在单击按钮时绘制文本。但它不起作用，只有调整了框架的大小，文本才会出现。我认为我对图形的使用是错误的。。。

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.EventQueue;
import java.awt.FlowLayout;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
public class Demo1 {
public static void main(String[] args) {
EventQueue.invokeLater(new Runnable() {
@Override
public void run() { 
justAnyFrame f = new justAnyFrame("Draw text",200,100,600,400);                         
f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
f.setVisible(true);       
}
});
}
}
////////////////////////////////////////////////////////////////
class justAnyFrame extends JFrame { 
private JButton bDrawText;
private boolean drawNow=false;
public justAnyFrame(String title,int left,int top,int width,int height) {   
setTitle(title);
setLocation(left,top);
setSize(width,height);
centralPanel cp=new centralPanel();
cp.setBackground(Color.LIGHT_GRAY);
add(cp,BorderLayout.CENTER);
lowerPanel lp=new lowerPanel();
add (lp, BorderLayout.SOUTH);       
}
//////////////////////////////////////////////////////////////////
class lowerPanel extends JPanel  implements ActionListener {
public lowerPanel() {
setLayout(new FlowLayout(FlowLayout.LEFT));
bDrawText=new JButton("Draw Text");
bDrawText.addActionListener(this);  
add(bDrawText);
}

public void actionPerformed(ActionEvent e) {
Object clicked=e.getSource();
if(clicked==bDrawText) {
drawNow=true;
repaint();
}
}
}
////////////////////////////////////////////////////////////////
class centralPanel extends JPanel {
public void paintComponent(Graphics g) {
super.paintComponent(g);
Graphics2D g2 = (Graphics2D) g; 
if(drawNow) {
g2.setColor(Color.BLUE);
Font myFont=new Font("Helvetica",Font.BOLD,40);
g2.setFont(myFont);
g2.drawString("QWERTY",50,100);
}
}           
}   
}

Answer 1

您为lowerPanel调用了重新绘制，但有必要重新绘制centralPanel。