理解我的代码有什么问题的问题

Question

我一直在尝试修复这个错误，但我似乎无法弄清楚到底出了什么问题。我想编写一个程序，提示用户输入 x- 和中心的 y 位置和半径。当用户点击“绘制” 按钮，在组件中以该圆心和半径绘制一个圆。

import java.awt.Dimension;
import java.awt.FlowLayout;
import java.awt.Graphics;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTextArea;
import javax.swing.JTextField;

public class CircleMakerVeiwer {

public static void main(String[] args) {
    // stuff that displays that will help display the circle
    JPanel panel = new JPanel();
    JPanel pane2 = new JPanel();
    JFrame frame = new JFrame();
    JButton button = new JButton("Draw Circle");

    // the obj that will be drawn
    CircleMaker circle = new CircleMaker();

    // panel is set to a flowLayout
    panel.setLayout(new FlowLayout());
    // panel's size is selected
    panel.setPreferredSize(new Dimension(800, 800));

    // now here's where the fun begins

    // labels
    JLabel x_cor = new JLabel("X Cordinate");
    JLabel y_cor = new JLabel("Y Cordinate");
    JLabel rad = new JLabel("Radius");

    // JTextFeilds
    JTextField x_input = new JTextField(3);
    JTextField y_input = new JTextField(3);
    // [in surfer dude voice] It's so RADDDD bROOOo
    JTextField rad_input = new JTextField(3);

    // all de panel adding
    panel.add(x_cor);
    panel.add(x_input);
    panel.add(y_cor);
    panel.add(y_input);
    panel.add(rad);
    panel.add(rad_input);


    // time for the action listener
    class DrawCircleListener implements ActionListener {

        public void actionPerformed(ActionEvent arg0) {
            int x, y, r;
            System.out.println("action preformed (sort of)");
            x = Integer.parseInt(x_input.getText());
            y = Integer.parseInt(y_input.getText());
            r = Integer.parseInt(rad_input.getText());

            circle.setCircle(r, x, y);

        }// end of actionPerformed

    }// end of DrawCircleListener
    DrawCircleListener listener = new DrawCircleListener();



    button.addActionListener(listener);
    panel.add(button);

    panel.add(circle);

    frame.setSize(800, 800);
    frame.setTitle("Circle Button");
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.add(panel);

    frame.setVisible(true);

   }// end of main

}// end of CircleMakerVeiwer.java

我也将提供 CircleMaker 对象以防万一。

    import java.awt.Graphics;
    import java.awt.Graphics2D;
    import java.awt.geom.Ellipse2D;

    import javax.swing.JComponent;

    /*
     * This program will construct a circle it will also 
     * paint and set the parameters of the circle outside the constructor 
     */

public class CircleMaker extends JComponent {

private Ellipse2D.Double c;
    private int radius, x_cor, y_cor;

public CircleMaker() {
    c= new Ellipse2D.Double();
}

public void setCircle(int r, int x_cordinate, int y_cordinate) {
    //Graphics g;
    c = new Ellipse2D.Double(x_cordinate - r, y_cordinate - r, r * 2, r * 2);
    System.out.println("set circle was called");
    //repaint();
}

public void paintComponent(Graphics g) {
    Graphics2D g2 = (Graphics2D) g;
    g2.draw(c);

}


}

Answer 1

您正在将 CircleMaker 添加到使用 FlowLayout 的 JPanel，它尊重组件的 preferredSize，但 CircleMaker 的默认 preferredSize 为 0x0，因此该组件实际上是不可见的
调用 setCircle 时您没有调用 repaint，因此组件不知道应该更新它
您在进行自定义绘画之前未调用 super.paintComponent，这违反了绘画方法链契约

首先覆盖 CircleMaker 的 getPreferredSize 方法...

public static class CircleMaker extends JComponent {
    //...
    @Override
    public Dimension getPreferredSize() {
        return new Dimension(400, 400);
    }

这将使布局管理器了解您希望组件通常有多大

更新 setCircle 方法以调用 repaint 并将 super.paintComponent 添加到您的 paintComponent 方法

    public void setCircle(int r, int x_cordinate, int y_cordinate) {
        //Graphics g;
        c = new Ellipse2D.Double(x_cordinate - r, y_cordinate - r, r * 2, r * 2);
        System.out.println("set circle was called");
        repaint();
    }

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        Graphics2D g2 = (Graphics2D) g;
        g2.draw(c);

    }

最后，将CircleMaker的实例添加到框架的CENTER位置，默认使用BorderLayout...

    //panel.add(circle);

    frame.setTitle("Circle Button");
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.add(panel, BorderLayout.NORTH);
    frame.add(circle);

    frame.pack();
    frame.setVisible(true);

哦，去掉panel.setPreferredSize(new Dimension(800, 800));

理解我的代码有什么问题的问题

Issue in understanding whats wrong with my code

java

layout

swing

paintcomponent