inheritancetest.java

中国水利水电出版社java程序设计教程,附源码

// InheritanceTest.java
// 示范"is a" 关联
import java.text.DecimalFormat;
import javax.swing.JOptionPane;

public class InheritanceTest {
   public static void main( String args[] )
   {
      Point pointRef, p; 
      Circle circleRef, c;
      String output;

      p = new Point( 30, 50 );
      c = new Circle( 2.7, 120, 89 );

      output = "Point p: " + p.toString() +
               "\nCircle c: " + c.toString();
   
      // 使用"is a" 关联到有关的圆
      // with a Point reference
      pointRef = c;   // 圆赋值到点的定义

      output += "\n\nCircle c (via pointRef): " +
                pointRef.toString();

      // 使用继承 (实例一个父类定义到子类）
      // 子类赋值把一个点定义创建圆对象定义
      
      circleRef = (Circle) pointRef;

      output += "\n\nCircle c (via circleRef): " +
                circleRef.toString();

      DecimalFormat precision2 = new DecimalFormat( "0.00" );
      output += "\nArea of c (via circleRef): " +
                precision2.format( circleRef.area() );

      // 将圆定义为点对象后输出
      if ( p instanceof Circle ) {
         circleRef = (Circle) p; 
         output += "\n\ncast successful";
      }
      else
         output += "\n\np does not refer to a Circle";

      JOptionPane.showMessageDialog( null, output,
         "Demonstrating the \"is a\" relationship",
         JOptionPane.INFORMATION_MESSAGE );

      System.exit( 0 );
   }
}