testinstanceandclassvariable.java

此源码为机械工业出版社出版的《Java语言程序设计》第三版所配套的书中所有源代码。

/*
// TestInstanceAndClassVariable.java: Demonstrate using instance and
// class variables
public class TestInstanceAndClassVariable
{
  // Main method
  public static void main(String[] args)
  {
    // Create circle1
    Circle circle1 = new Circle();

    // Display circle1 BEFORE circle2 is created
    System.out.println("Before creating circle2");
    System.out.print("circle1 is : ");
    printCircle(circle1);

    // Create circle2
    Circle circle2 = new Circle(5);

    // Change the radius in circle1
    circle1.setRadius(9);

    // Display circle1 and circle2 AFTER circle2 was created
    System.out.println("\nAfter creating circle2 and modifying " +
      "circle1's radius to 9");
    System.out.print("circle1 is : ");
    printCircle(circle1);
    System.out.print("circle2 is : ");
    printCircle(circle2);
  }

  // Print circle information
  public static void printCircle(Circle c)
  {
    System.out.println("radius (" + c.getRadius() +
      ") and number of Circle objects (" +
      c.getNumOfObjects() + ")");
  }
}

// Circle.java: Circle class with instance and class variables
class Circle
{
  private double radius;
  private static int numOfObjects = 0;  // Class variable

  // Default constructor
  public Circle()
  {
    radius = 1.0;
    numOfObjects++;
  }

  // Construct a circle with a specified radius
  public Circle(double r)
  {
    radius = r;
    numOfObjects++;
  }

  // Getter method for radius
  public double getRadius()
  {
    return radius;
  }

  // Setter method for radius
  public void setRadius(double newRadius)
  {
    radius = newRadius;
  }

  // Getter method for numOfObjects
  public static int getNumOfObjects()
  {
    return numOfObjects;
  }

  // Find circle area
  public double findArea()
  {
    return radius*radius*Math.PI;
  }
} */