Java-对象数组

对象数组实际上是引用变量构成的数组。
所以调用circleArray[1].getArea()就包含两个层次的引用，如下图所示。

circleArray引用整个数组。circleArray[1]引用一个Circle对象。

例题：汇总圆的面积
编写程序，将一个Circle数组的面积相加。
程序创建由5个Circle对象组成的数组
然后以随机数初始化圆的半径
显示数组中所有圆的总面积

public class CircleWithPrivateDataFields {
     
  private double radius = 1;
  private static int numberOfObjects = 0;
  public CircleWithPrivateDataFields() {
     
    numberOfObjects++;
  }
  public CircleWithPrivateDataFields(double newRadius) {
     
    radius = newRadius;
    numberOfObjects++;
  }
  public double getRadius() {
     
    return radius;
  }
  public void setRadius(double newRadius) {
     
    radius = (newRadius >= 0) ? newRadius : 0;
  }
  public static int getNumberOfObjects() {
     
    return numberOfObjects;
  }  
  public double getArea() {
     
    return radius * radius * Math.PI;
  }
}

public class TotalArea {
     

  public static void main(String[] args) {
     
    CircleWithPrivateDataFields[ ] circleArray;
    circleArray = createCircleArray();
    printCircleArray(circleArray);
  }
//createCircleArray()方法功能：创建实体，调用完方法这个实体就得存在，
//返回对象的引用，此对象是对象数组实体，返回值类型是CircleWithPrivateDataFields类的数组引用
  public static CircleWithPrivateDataFields[ ] createCircleArray() {
     
    CircleWithPrivateDataFields[ ] circleArray = 
			new CircleWithPrivateDataFields[5]; //5个引用变量
    for (int i = 0; i < circleArray.length; i++) {
     
      circleArray[i] = 
	new CircleWithPrivateDataFields(Math.random() * 100);
    }
    return circleArray;
  }  
  
  public static void printCircleArray(CircleWithPrivateDataFields[] circleArray) {
     
    System.out.printf("%-30s%-15s\n", "Radius", "Area");
    for (int i = 0; i < circleArray.length; i++) {
     
      System.out.printf("%-30f%-15f\n", circleArray[i].getRadius(),
        circleArray[i].getArea());
    }
    System.out.println("-----------------------------------------");
    System.out.printf("%-30s%-15f\n", "The total areas of circles is",
      	sum(circleArray) );
  }

  public static double sum(CircleWithPrivateDataFields[] circleArray) {
     
    double sum = 0;
    for (int i = 0; i < circleArray.length; i++)
      sum += circleArray[i].getArea();
    return sum;
  }
}

实验结果：