Java笔记-Graphics实现简易画图板

点击按钮选择图形和颜色，使用鼠标在界面上绘制绘制。

• 使用Jframe创建窗口，设置大小和标题。
• 添加JButton，分为图形选择按钮和颜色选择按钮。

• 新建一个DrawListener类实现ActionListener和MouseListener接口
• 重写ActionListener的actionPerformed方法，为按钮添加监听addActionListener
• 重写MouseListener的mousePressed和mouseReleased方法，为界面添加鼠标监听addMouseListener
• 事件对象封装了事件的信息，包括时间、事件源对象 、事件命令、id等

• Graphics用于处理屏幕上的图形渲染
• 除了可以绘制线段，矩形，圆等基本图形，还可以绘制更为复杂的图形和字符串，本篇仅绘制了线段、矩形和圆
• Jframe对象包含一个Graphics对象，在绘制时需要获取Jframe的Graphics对象g，通过g来绘制

import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;

public class DrawListener implements ActionListener, MouseListener {

    private int x1,x2,y1,y2;
    private String name;
    private Color color;
    public Graphics g;

    @Override
    public void actionPerformed(ActionEvent e) {
        String command = e.getActionCommand();

        if (command.equals("")){
            //点击颜色按钮
            JButton jButton = (JButton)e.getSource();
            color = jButton.getBackground();
            g.setColor(color);
        }else {
            //点击图形按钮
            name = command;
        }
    }

    @Override
    public void mouseClicked(MouseEvent e) {

    }

    @Override
    public void mousePressed(MouseEvent e) {
        x1 = e.getX();
        y1 = e.getY();
    }

    @Override
    public void mouseReleased(MouseEvent e) {
        x2 = e.getX();
        y2 = e.getY();

        int w = x2-x1;
        int h = y2-y1;

        //绘制图形
        if (name.equals("直线")){
            g.drawLine(x1,y1,x2,y2);
        }else if (name.equals("矩形")){
            g.drawRect(Math.min(x1,x2),Math.min(y1,y2),Math.abs(w),Math.abs(h));
        }else if (name.equals("圆")){
            g.drawOval(Math.min(x1,x2),Math.min(y1,y2),Math.abs(w),Math.abs(h));
        }else if (name.equals("实心圆")){
            g.fillOval(Math.min(x1,x2),Math.min(y1,y2),Math.abs(w),Math.abs(h));
        }
    }

    @Override
    public void mouseEntered(MouseEvent e) {

    }

    @Override
    public void mouseExited(MouseEvent e) {

    }
}

import javax.swing.*;
import java.awt.*;

public class DrawTest {

    //可绘制的图形
    private String[] strs = {"直线","矩形","圆","实心圆"};
    //可选颜色
    private Color[] colors = {
            Color.BLACK,
            Color.LIGHT_GRAY,
            Color.GRAY,
            Color.WHITE,
            Color.YELLOW,
            Color.RED,
            Color.GREEN,
            Color.BLUE
    };

    public void show(){
        Jframe jf = new Jframe("图形绘制");

        jf.setSize(1000,1000);

        jf.setDefaultCloseOperation(Jframe.EXIT_ON_CLOSE);

        FlowLayout fl = new FlowLayout();
        jf.setLayout(fl);
        //设置监听
        DrawListener drawListener = new DrawListener();

        //设置大小
        Dimension dm = new Dimension(75,50);

        for (int i = 0; i < strs.length; i++) {
            JButton jButton = new JButton(strs[i]);
            jButton.setPreferredSize(dm);
            jButton.addActionListener(drawListener);
            jf.add(jButton);
        }

        for (int i = 0; i < colors.length; i++) {
            JButton jButton = new JButton();
            jButton.setBackground(colors[i]);
            jButton.setPreferredSize(dm);
            jButton.addActionListener(drawListener);
            jf.add(jButton);
        }

        jf.setVisible(true);

        //向drawListener中传入g必须在setVisible之后
        Graphics g = jf.getGraphics();
        drawListener.g = g;

        jf.addMouseListener(drawListener);
    }

    public static void main(String[] args) {
        new DrawTest().show();
    }
}