学以致用——Java源码——使用Ellipse2D.Double类绘制圆（Circles Using Class Ellipse2D.Double）

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程序功能：

用户输入圆的半径（单位为像素数），点击“开始绘制”按钮，程序将画出相应大小的圆，并绘制其直径、周长及面积信息。

运行结果：

源码：

1. 实体类

//Creating JFrame to display DrawPanel.
import java.awt.Graphics2D;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.geom.Ellipse2D;
import javax.swing.JButton;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTextField;
import java.awt.BasicStroke;
import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.FlowLayout;
import java.awt.Graphics;

/**
 * 13.17 (Circles Using Class Ellipse2D.Double) Write an application that asks
 * the user to input the radius of a circle as a floating-point number and draws
 * the circle, as well as the values of the circle’s diameter, circumference and
 * area. Use the value 3.14159 for π. [Note: You may also use the predefined
 * constant Math.PI for the value of π. This constant is more precise than the
 * value 3.14159. Class Math is declared in the java.lang package, so you need
 * not import it.] Use the following formulas (r is the radius): 
 * diameter = 2r
 * circumference = 2πr 
 * area = πr2 
 * The user should also be prompted for a set of
 * coordinates in addition to the radius. Then draw the circle and display its
 * diameter, circumference and area, using an Ellipse2D.Double object to
 * represent the circle and method draw of class Graphics2D to display it.
 * 
 * @author [email protected]
 * @Date Jan 23, 2019, 5:07:39 PM
 *
 */
public class CirclesInfoJPanel extends JPanel
{
	private JLabel radiusJLabel;
	private JTextField radiusJTextField;
	private JButton drawJButton;
	private JPanel topJPanel;
	private float radius;
	private float diameter;
	private float perimeter;
	private float area;
	private boolean initFlag;

	CirclesInfoJPanel(){
		initFlag = true;	//初始化状态
		radiusJLabel = new JLabel("圆的半径：");
		radiusJTextField = new JTextField("0.0",10);
		drawJButton = new JButton("开始绘制");
		topJPanel = new JPanel();
		
		drawJButton.addActionListener(new ActionListener() {
			
			public void actionPerformed(ActionEvent event) {
				initFlag = false; //用户触发了动作，结束初始化状态
				repaint();	//绘图
			}
		}
			
	);
		
		topJPanel.setLayout(new FlowLayout(FlowLayout.CENTER));
		topJPanel.add(radiusJLabel);
		topJPanel.add(radiusJTextField);
		topJPanel.add(drawJButton);
		this.add(topJPanel,BorderLayout.NORTH);

	}
 
	public void paint(Graphics g)
	{

		
	       Graphics2D g2d = (Graphics2D) g;	
		   super.paint(g);
		   double width = getWidth(); // total width   
		   double height = getHeight(); // total height
		   int rRed;
		   int rGreen;
		   int rBlue;
		   radius =Float.parseFloat(radiusJTextField.getText());
		   diameter = 2 * radius;
		   perimeter = (float) (2 * Math.PI * radius);
		   area = (float) (Math.PI * Math.pow(radius,2));

		   //Color of arc, below color is called "Taibao Lan"
		   rRed = 21;
		   rGreen = 101;
		   rBlue = 192;
		   Color color=new Color(rRed, rGreen, rBlue);
		   
		   //画1个圆，并输出其直径、周长、面积
		   if (! initFlag){	//初始界面中不显示以下内容
			   g2d.setColor(color);
			   g2d.setStroke(new BasicStroke(5));	//设置画笔的粗细
			   g2d.draw(new Ellipse2D.Double(width/2-radius, height/2-radius, 
					   2*radius, 2*radius));
			   g2d.drawString("直径："+String.format("%.2f", diameter),(int)(width/2+radius+20),
					   (int)(height/2-15));	//输出直径大小
			   g2d.drawString("周长："+String.format("%.2f", perimeter),(int)(width/2+radius+20),
					   (int)(height/2));	//输出周长大小
			   g2d.drawString("面积："+String.format("%.2f", area),(int)(width/2+radius+20),
					   (int)(height/2+15));	//输出面积大小

			   }	
		   
	}
		
} 

2. 测试类 

import java.awt.BorderLayout;
import javax.swing.JFrame;

public class DrawCircleInfo {
	
	public static void main(String[] args)
	{
	 // create a panel that contains our drawing
		CirclesInfoJPanel panel = new CirclesInfoJPanel();
		
	 // create a new frame to hold the panel
	 JFrame application = new JFrame();
	 application.setTitle("绘制圆");
	 
	 // set the frame to exit when it is closed
	 application.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
	 
	 application.add(panel,BorderLayout.CENTER); // add the panel to the frame
	 application.setSize(586, 586); // set the size of the frame
	 application.setVisible(true); // make the frame visible    
	} 
	

}