Question

当我尝试构建下面的程序时，我收到以下错误：

Exception in thread "main" java.lang.NullPointerException

    at java.awt.Container.addImpl(Unknown Source)
    at java.awt.Container.add(Unknown Source)
    at miniCADFrame$CanvasPanel.<init>(miniCADFrame.java:71)
    at miniCADFrame.<init>(miniCADFrame.java:17)
    at miniCAD.<init>(miniCAD.java:12)
    at miniCAD.main(miniCAD.java:20)

显然我有一些NULL指针问题，但我不确定它们在哪里！我认为代码的来源如下（如果我避免使用下面代码中的类，程序将运行没有问题）。我在下面的问题点添加了行号。

完成后，程序将允许用户按下按钮以绘制各种形状。

import java.awt.BorderLayout;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JButton;
import javax.swing.JLabel;
import javax.swing.JPanel;

public class miniCADFrame extends JPanel {

    private CanvasPanel canvas = new CanvasPanel(); //LINE 17
    private ButtonPanel buttons = new ButtonPanel();

    public miniCADFrame() {
         //Constructor, creates the mother panel
         this.setLayout(new BorderLayout());
         this.add (canvas,BorderLayout.CENTER);
    }

    private class ButtonListener implements ActionListener {

        @Override
        public void actionPerformed(ActionEvent event) {    //Paint the figure associated with the button click
                canvas.add(new FigurePanel(FigurePanel.OVAL), BorderLayout.CENTER);
                canvas.repaint();
            }
    }

    class ButtonPanel extends JPanel {

        private JButton[] Jbuttons = new JButton[11];
        //Constructor
        ButtonPanel() {

            setLayout(new GridLayout(4, 4)); 

            // Create buttons to attach to the buttons panel
            Jbuttons[0] = new JButton("Change Colour");
            Jbuttons[1] = new JButton("Up");
            Jbuttons[2] = new JButton("Text");
            Jbuttons[3] = new JButton("Left");
            Jbuttons[4] = new JButton("Enlarge");
            Jbuttons[5] = new JButton("Right");
            Jbuttons[6] = new JButton("Rectangle");
            Jbuttons[7] = new JButton("Down");
            Jbuttons[8] = new JButton("Circle");
            Jbuttons[9] = new JButton("Save");
            Jbuttons[10] = new JButton("Load");

            //Add the buttons to the buttons panel
            for (int i=0; i<11; i++) {
                Jbuttons[i].addActionListener(new ButtonListener());
                buttons.add(Jbuttons[i]);
            }
    }
    }

class CanvasPanel extends JPanel {

        //Constructor
        CanvasPanel() {
            // Create "canvas" to hold a label for the buttons panel along with the button panel itself
             this.setLayout(new BorderLayout());
             this.add(new JLabel("CONTROL PANEL"),BorderLayout.NORTH);
             this.add(buttons, BorderLayout.WEST); //LINE 71 add the button panel to the canvas panel

             //test
             this.add(new FigurePanel(FigurePanel.RECTANGLE), BorderLayout.CENTER);
        }
}
}

Answer 1

我认为这是因为你在按钮之前实例化了画布。 buttons变量用于CanvasPanel的构造函数

空指针和私有成员，带有swing的基本Java GUI

1 个答案: