プログラムを起動して 1 つの色でペイントすることはできますが、ボタンの 1 つをクリックしても色が変わりません。ヌル ポインター例外が発生し続けます。グラフィック、画像、またはその両方を初期化していないことが原因である可能性があります。それがその問題である場合、それを修正する方法がよくわかりません。私はプログラミングにかなり慣れていないので、どんな助けでも大歓迎です。
コード：
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class Paint extends JFrame{
//Class Variables
private JFrame frame;
PaintPanel paint_panel;
private JPanel btn_panel;
private JButton red_btn, green_btn, blue_btn, clear_btn, erase_btn ;
//class to paint the panel in the frame
public class PaintPanel extends JComponent{
//Class Variables
private Graphics2D g;
private int x1, y1, x2, y2;
private Cursor paint_cursor, select_cursor;
private Image image;
//Constructor with Mouse listeners in it
public PaintPanel(){
addMouseListener(new MouseAdapter(){
public void mousePressed(MouseEvent e){
x1 = e.getX();
y1 = e.getY();
}
});
addMouseMotionListener(new MouseMotionAdapter(){
public void mouseDragged(MouseEvent e){
x2 = e.getX();
y2 = e.getY();
if(g != null)
g.drawLine(x1, y1, x2, y2);
repaint();
x1 = x2;
y1 = y2;
}
});
}
//PaintPanel Method that sets the cursor and does the drawing
public void paintComponent(Graphics gr){
super.paintComponent(gr);
gr.setColor(Color.black);
gr.fillRect(0, 0, this.getWidth(), this.getHeight());
paint_cursor=new Cursor(Cursor.CROSSHAIR_CURSOR);
select_cursor=new Cursor(Cursor.HAND_CURSOR);
paint_panel.setCursor(paint_cursor);
btn_panel.setCursor(select_cursor);
if(image == null){
image = createImage(this.getWidth(), this.getHeight());
g = (Graphics2D)image.getGraphics();
clear1();
}
gr.drawImage(image, 0, 0, null);
}
public void clear1(){
g.setPaint(Color.black);
g.fillRect(0, 0, this.getWidth(), this.getHeight());
g.setPaint(Color.blue);
repaint();
}
public void red(){
g.drawLine(x1, y1, x2, y2);
g.setPaint(Color.red);
x1 = x2;
y1 = y2;
repaint();
}
public void blue(){
g.drawLine(x1, y1, x2, y2);
g.setPaint(Color.blue);
x1 = x2;
y1 = y2;
repaint();
}
public void green(){
g.drawLine(x1, y1, x2, y2);
g.setPaint(Color.green);
x1 = x2;
y1 = y2;
repaint();
}
}
//Constructor
public Paint(){
frame = new JFrame("Paint Program");
frame.setSize(500,500);
frame.setLocation(500,100);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
red_btn = new JButton("RED");
green_btn = new JButton("GREEN");
blue_btn = new JButton("BLUE");
clear_btn = new JButton("CLEAR");
erase_btn = new JButton("ERASE");
btn_panel = new JPanel();
paint_panel = new PaintPanel();
btn_panel.setLayout(new GridLayout(5,1));
btn_panel.add(red_btn);
btn_panel.add(green_btn);
btn_panel.add(blue_btn);
btn_panel.add(clear_btn);
btn_panel.add(erase_btn);
frame.add(BorderLayout.CENTER, paint_panel);
frame.add(BorderLayout.EAST, btn_panel);
final PaintPanel pp1 = new PaintPanel();
red_btn.addActionListener(new ActionListener(){
public void actionPerformed(ActionEvent e) {
pp1.red();
}
});
blue_btn.addActionListener(new ActionListener(){
public void actionPerformed(ActionEvent e) {
pp1.blue();
}
});
green_btn.addActionListener(new ActionListener(){
public void actionPerformed(ActionEvent e) {
pp1.green();
}
});
clear_btn.addActionListener(new ActionListener(){
public void actionPerformed(ActionEvent e) {
pp1.clear1();
}
});
erase_btn.addActionListener(new ActionListener(){
public void actionPerformed(ActionEvent e) {
pp1.green();
}
});
}
//Makes the frame visible and working
public void launch(){
frame.setVisible(true);
}
//Main Method
public static void main(String[] args){
Paint p1 = new Paint();
p1.launch();
}
}