java - 为什么我的 JComponent Paint(Graphics g) 不工作/不被调用？

Question

我一直在尝试编写这个用点制作多边形的应用程序。当我第一次尝试运行它时，我认为我的点有问题。但是当我开始四处寻找问题时，我意识到 JComponent 中的绘制方法甚至没有被调用。我应该怎么做才能解决这个问题？

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.MouseInfo;
import java.awt.Point;
import java.awt.RenderingHints;
import java.awt.Shape;
import java.awt.event.ItemEvent;
import java.awt.event.ItemListener;
import java.awt.event.MouseEvent;
import java.awt.event.MouseMotionListener;
import java.awt.geom.Line2D;
import java.util.concurrent.ScheduledThreadPoolExecutor;
import java.util.concurrent.TimeUnit;
import javax.swing.JComponent;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JToggleButton;

public class PolygonBuilder extends JFrame{

    final static int WIDTH = 1280;
    final static int HEIGHT = 640;
    public static int xPointPos, yPointPos;
    public static void main(String[] args) {
        new PolygonBuilder();
    }
    public PolygonBuilder() {
        setTitle("Build a Polygon");
        this.setSize(WIDTH,HEIGHT);
        setResizable(false);
        JLabel placeholder = new JLabel();
        BuilderDrawingPanel BuilderPanel = new BuilderDrawingPanel();
        this.add(BuilderPanel, BorderLayout.CENTER);

        JToggleButton DotOnOff = new JToggleButton("Dot");
        DotOnOff.addItemListener(new ItemListener() {
            @Override
            public void itemStateChanged(ItemEvent arg0) {
                // TODO Auto-generated method stub
                if(DotOnOff.isSelected()) {
                    System.out.println("ON");
                } else {
                    System.out.println("OFF");
                }
            }
        });

        DotOnOff.setBounds(100, 100, 90, 35);

        ScheduledThreadPoolExecutor executor = new ScheduledThreadPoolExecutor(5);
        executor.scheduleAtFixedRate(new RepaintTheBoard(this), 0L, 20L, TimeUnit.MILLISECONDS);

        addMouseMotionListener(new MouseMotionListener() {
            @Override
            public void mouseDragged(MouseEvent arg0) {}

            @Override
            public void mouseMoved(MouseEvent arg0) {
                Point p = MouseInfo.getPointerInfo().getLocation();
                xPointPos = (int) p.getX();
                yPointPos = (int) p.getY();
                System.out.println(xPointPos+","+yPointPos);
            }
        });

        this.add(DotOnOff);
        this.add(placeholder);
        setVisible(true);
    }
}
class RepaintTheBoard implements Runnable {
    PolygonBuilder theBuilder;

    public RepaintTheBoard(PolygonBuilder theBuilder) {
        this.theBuilder = theBuilder;
    }

    @Override
    public void run() {
        theBuilder.repaint();
    }
}

@SuppressWarnings("serial")
class BuilderDrawingPanel extends JComponent{
    static Graphics2D graphicSettings;
    static Shape drawLine;
    public BuilderDrawingPanel() {
        //stuff to draw at start
        System.out.println("Drawing..");
        drawLine =  new Line2D.Float(480,480,0,0);
    }
    @Override
    public void paint(Graphics g) {

        graphicSettings = (Graphics2D)g;
        System.out.println("Drawing..");
        graphicSettings.setColor(Color.RED);
        graphicSettings.fillRect(0, 0, getWidth(), getHeight());
        graphicSettings.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
        graphicSettings.setPaint(Color.BLUE);
        graphicSettings.draw(drawLine);
    }
}

Answer 1

虽然 c0der 提供了一些有值(value)的信息和链接，但我认为应该在答案中解决其中的某些方面。

• JFrame(的内容 Pane )的默认布局是 BorderLayout。
• 添加到没有约束的边框布局中的任何组件都默认位于CENTER。
• 边框布局的每个区域(布局约束)只能显示一个组件。

那么我们就来看看这部分代码:

this.add(DotOnOff);
this.add(placeholder);

JVM 将尝试将它们添加到已添加构建器绘图面板的内容 Pane 的中心。这就是为什么它甚至不会出现(或被绘制)。

其他提示:

• 在任何 JComponent 中，重写自定义绘制的正确方法是 paintComponent(Graphics) 方法。
• 任何重写的绘制方法都应立即调用 super 方法，以确保绘制背景、边框和其他“自动”元素。
• Graphics 对象是 transient 的，因此不应存储为类的属性，更不应声明为static。
• 说到静态声明，它们(至少在GUI中)更多地是问题的根源而不是解决方案。不要将 GUI 属性声明为静态，除非您能解释这样做的原因。
• 请学习常见的 Java 命名法(命名约定 - 例如 EachWordUpperCaseClass、firstWordLowerCaseMethod()、firstWordLowerCaseAttribute，除非它是 UPPER_CASE_CONSTANT)并一致地使用它。

更新

这是问题中看到的来源，解决了最直接的问题。它并没有改变太多推荐的其他内容，也没有触及我在使用代码本身时看到的其他问题。查看评论以了解为使其发挥作用而进行的更改。

这就是它的样子，经过更改:

import java.awt.*;
import java.awt.event.*;
import java.awt.geom.Line2D;
import java.util.concurrent.*;
import javax.swing.*;

public class PolygonBuilder extends JFrame{

    final static int WIDTH = 640;
    final static int HEIGHT = 320;
    public static int xPointPos, yPointPos;
    public static void main(String[] args) {
        new PolygonBuilder();
    }
    public PolygonBuilder() {
        setTitle("Build a Polygon");
        // let's be nice to the user..
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        // this is almost certainly not the correct size, 
        // but in itself could deserve a question
        this.setSize(WIDTH,HEIGHT);
        setResizable(false);
        JLabel placeholder = new JLabel("Label!");
        BuilderDrawingPanel BuilderPanel = new BuilderDrawingPanel();
        this.add(BuilderPanel, BorderLayout.CENTER);

        JToggleButton DotOnOff = new JToggleButton("Dot");
        DotOnOff.addItemListener(new ItemListener() {
            @Override
            public void itemStateChanged(ItemEvent arg0) {
                if(DotOnOff.isSelected()) {
                    System.out.println("ON");
                } else {
                    System.out.println("OFF");
                }
            }
        });

        DotOnOff.setBounds(100, 100, 90, 35);

        ScheduledThreadPoolExecutor executor = new ScheduledThreadPoolExecutor(5);
        executor.scheduleAtFixedRate(new RepaintTheBoard(this), 0L, 20L, TimeUnit.MILLISECONDS);

        addMouseMotionListener(new MouseMotionListener() {
            @Override
            public void mouseDragged(MouseEvent arg0) {}

            @Override
            public void mouseMoved(MouseEvent arg0) {
                Point p = MouseInfo.getPointerInfo().getLocation();
                xPointPos = (int) p.getX();
                yPointPos = (int) p.getY();
                System.out.println(xPointPos+","+yPointPos);
            }
        });

        // create a new panel for these buttons..
        //this.add(DotOnOff);
        //this.add(placeholder);
        JPanel buttonPanel = new JPanel(); // default flow layout
        buttonPanel.add(DotOnOff);
        buttonPanel.add(placeholder);
        // now add that panel above the builder panel
        this.add(buttonPanel, BorderLayout.PAGE_START);
        setVisible(true);
    }
}
class RepaintTheBoard implements Runnable {
    PolygonBuilder theBuilder;

    public RepaintTheBoard(PolygonBuilder theBuilder) {
        this.theBuilder = theBuilder;
    }

    @Override
    public void run() {
        theBuilder.repaint();
    }
}

@SuppressWarnings("serial")
class BuilderDrawingPanel extends JComponent{
    static Graphics2D graphicSettings;
    static Shape drawLine;
    public BuilderDrawingPanel() {
        //stuff to draw at start
        System.out.println("Drawing..");
        drawLine =  new Line2D.Float(480,480,0,0);
    }
    @Override
    public void paint(Graphics g) {

        graphicSettings = (Graphics2D)g;
        System.out.println("Drawing..");
        graphicSettings.setColor(Color.RED);
        graphicSettings.fillRect(0, 0, getWidth(), getHeight());
        graphicSettings.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
        graphicSettings.setPaint(Color.BLUE);
        graphicSettings.draw(drawLine);
    }
}