Gui 图形未出现在面板中

Gui graphic does not appear in Panel

我正在尝试使用我设置为 NorthEast 的两个按钮进行 drawOval 移动,这样球将在 JButton 之间移动,位于中心.

为什么没有出现在面板上?

我也在考虑使用一个函数来实现 x=x+;和 y=y+1 当我向左或向右按​​时。

我不知道我能做什么。

所以这是我编写的代码:

public class Main extends JFrame implements ActionListener {

    JButton left;
    JButton right;
    JPanel p;

    Main(){ 
    JButton left = new JButton("left"); 
    left.addActionListener(this);
    left.addActionListener(new ActionListener() {
        @Override
        public void actionPerformed(ActionEvent ae) {

        //The first way I think is better to make it move.  
        }
    });
    JButton right = new JButton("right");   
    right.addActionListener(this);


    Panel p = new Panel();
    p.setLayout(new BorderLayout());

    p.add("West",left);// to the left
    p.add("East",right);//to the right 

    Container c = getContentPane();     
    c.add(p);

    }           
    public static void main(String[] args) {
        Main f=new Main();
        f.setTitle("Heracles");
        f.setSize(500, 500);
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.setVisible(true);     //this is the window
    }

    public void paintComponent (Graphics g) {
           super.paintComponents(g);
           Graphics2D g1=(Graphics2D) g;
            g.drawOval(3, 5, 45, 46); // The ball
            g.fillOval(20, 30, 40, 40);     

        }
    @Override
    public void actionPerformed(ActionEvent e) {
        // TODO Auto-generated method stub

    }   
}

Why does not appear at the panel?

要显示您创建的图形,请按照以下步骤使用,

删除 paintComponent 方法并将其替换为以下代码..

public JComponent createOvel() {
    return new JComponent() {
        @Override
        protected void paintComponent(Graphics g) {
            Graphics2D g1 = (Graphics2D) g;
            g.drawOval(3, 5, 45, 46); // The ball
            g.fillOval(20, 30, 40, 40);
        }
    };
}

然后在Main()构造函数中调用,

p.add("Center", createOvel());

这将显示您创建的图形。

要了解它为什么不起作用,您需要了解绘画系统的实际工作原理

只要看一下这个片段,就应该很明显出了点问题。

public class Main extends JFrame implements ActionListener {
    //...    
    public void paintComponent (Graphics g) {
        super.paintComponents(g);
        //...
    }
}

您已经声明了一个名为 paintComponent 的方法,但正在调用超级方法 paintComponents(注意末尾的 s)。

此外,当你 "think" 覆盖一个方法时,你应该使用 #Override 属性,这会在你做错事情时导致编译器错误

public class Main extends JFrame implements ActionListener {
    //... 
    @Overrride   
    public void paintComponent (Graphics g) {
        super.paintComponents(g);
        //...
    }
}

上面的代码现在将无法编译,因为 JFrame 没有声明 paintComponent 方法。

作为一般规则,您应该避免直接从 JFrame(或其他顶级容器)扩展,它们是复合组件并且具有复杂的层次结构和功能。

更好的起点可能是 JPanel

import java.awt.BorderLayout;
import java.awt.Dimension;
import java.awt.EventQueue;
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;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;

public class Main {

    public static void main(String[] args) {
        new Main();
    }

    public Main() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
                    ex.printStackTrace();
                }

                JFrame frame = new JFrame("Testing");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.add(new TestPane());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public class TestPane extends JPanel {

        JButton left;
        JButton right;
        JPanel paintPane;

        public TestPane() {
            JButton left = new JButton("left");
            left.addActionListener(new ActionListener() {
                @Override
                public void actionPerformed(ActionEvent ae) {
                }
            });
            JButton right = new JButton("right");
            right.addActionListener(new ActionListener() {
                @Override
                public void actionPerformed(ActionEvent e) {
                }
            });

            paintPane = new PaintPane();

            setLayout(new BorderLayout());
            add(left, BorderLayout.WEST);
            add(right, BorderLayout.EAST);
            add(paintPane);
        }

    }

    public class PaintPane extends JPanel {

        @Override
        public Dimension getPreferredSize() {
            return new Dimension(200, 200);
        }

        public void paintComponent(Graphics g) {
            super.paintComponents(g);
            Graphics2D g1 = (Graphics2D) g;
            g1.drawOval(3, 5, 45, 46); // The ball
            g1.fillOval(20, 30, 40, 40);

        }
    }

}

您应该花时间查看 Painting in Swing and Performing Custom Painting 了解更多详情。

您可能愿意花时间学习的其他一些概念:

Also I am thinking using a function that make this x=x+; and y=y+1 when I pressed left or right.

好的,这就是 MVC 的 "model" 部分发挥作用的地方。

所以让我们从定义我们希望模型支持的基本属性开始...

public interface ShapeModel {
    public Point getPoint();
    public void addChangeListener(ChangeListener listener);
    public void removeChangeListener(ChangeListener listener);
}

这里支持一个Point作为位置和一个ChangeListener作为观察者模式,它会通知感兴趣的人模型的状态发生了变化。

为什么要以 interface 开头?作为一般概念,您应该始终更喜欢为接口而不是实现编写代码。在这种情况下,尚未定义 interface 的一个方面是,Point 如何更新?对于大多数想要使用该模型的人来说,这没什么兴趣,他们只想知道它何时发生变化,模型的变化可以直接通过实现或 "mutable" interface 来表达从 this interface

延伸

接下来,我们定义一个默认实现...

public class DefaultShapeModel implements ShapeModel {

    private Point point = new Point(40, 40);

    private List<ChangeListener> listeners = new ArrayList<>(25);

    @Override
    public Point getPoint() {
        return point;
    }

    public void setPoint(Point point) {
        this.point = point;
        fireStateChanged();
    }

    protected void fireStateChanged() {
        ChangeEvent evt = new ChangeEvent(this);
        for (ChangeListener listener : listeners) {
            listener.stateChanged(evt);
        }
    }

    @Override
    public void addChangeListener(ChangeListener listener) {
        listeners.add(listener);
    }

    @Override
    public void removeChangeListener(ChangeListener listener) {
        listeners.remove(listener);
    }

}

这确实定义了绘画的更新方式。

最后,我们更新 TestPanePaintPane 以支持模型...

public class TestPane extends JPanel {

    JButton left;
    JButton right;
    JPanel paintPane;

    private DefaultShapeModel model;

    public TestPane() {
        model = new DefaultShapeModel();

        JButton left = new JButton("left");
        left.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent ae) {
                Point p = model.getPoint();
                p.x--;
                if (p.x > 0) {
                    p.x = 0;
                }
                model.setPoint(p);
            }
        });
        JButton right = new JButton("right");
        right.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                Point p = model.getPoint();
                p.x++;
                if (p.x + 40 > paintPane.getWidth()) {
                    p.x = paintPane.getWidth() - 40;
                }
                model.setPoint(p);
            }
        });

        paintPane = new PaintPane(model);

        setLayout(new BorderLayout());
        add(left, BorderLayout.WEST);
        add(right, BorderLayout.EAST);
        add(paintPane);
    }

}

public class PaintPane extends JPanel {

    private ShapeModel model;

    public PaintPane(ShapeModel model) {
        this.model = model;
        this.model.addChangeListener(new ChangeListener() {
            @Override
            public void stateChanged(ChangeEvent e) {
                repaint();
            }
        });
    }

    public ShapeModel getModel() {
        return model;
    }

    @Override
    public Dimension getPreferredSize() {
        return new Dimension(200, 200);
    }

    public void paintComponent(Graphics g) {
        super.paintComponents(g);
        Graphics2D g1 = (Graphics2D) g;
        Point p = getModel().getPoint();
        g1.fillOval(p.x, p.y, 40, 40);
        g1.setColor(Color.WHITE);
        g1.drawOval(p.x, p.y, 40, 40);

    }
}