AP计算机教程9-8：继承和构造函数

如果在child class中不能直接操作继承而来的那些private field，又该如何初始化它们呢？在Java中可以使用关键字super调用parent class的构造函数。如果要这么做的话，super()必须是child class构造函数的第一条语句。

class Person
{
   private String name;

   public Person(String theName)
   {
      this.name = theName;
   }

   public String getName()
   {
      return name;
   }

   public boolean setName(String theNewName)
   {
      if (theNewName != null)
      {
         this.name = theNewName;
         return true;
      }
      return false;
   }
}

public class Employee extends Person
{

   private static int nextId = 1;
   private int id;

   public Employee(String theName)
   {
      super(theName);
      id = nextId;
      nextId++;
   }

   public int getId()
   {
      return id;
   }

   public static void main(String[] args)
   {
      Employee emp = new Employee("Mark");
      System.out.println(emp.getName());
      System.out.println(emp.getId());
   }
}

以上例子中，Employee构造函数中的super(theName)会调用class Person的构造函数，利用字符串参数来设置姓名。

如果一个Java class没有构造函数，编译器会自动加上一个无参数的默认构造函数，如public Person()。注意构造函数没有返回类型，且名字要与class名一致。如果parent class有多个构造函数可供选择，super()的参数决定了具体是哪个构造函数被调用。当child class没有在自身构造函数的第一句调用super而parent class里又只定义了带参数的构造函数时，编译器会因为无法判定parent class构造函数的适用性而报错。解决的办法是在parent class中定义无参默认构造函数，或者在child class里显式用super()调用带参数的构造函数。

Given the class definitions of Point2D and Point3D below, which of the constructors that follow (labeled I, II, and III) would be valid in class Point3D?

class Point2D {
   public int x;
   public int y;

   public Point2D() {}

   public Point2D(int x,int y) {
     this.x = x;
     this.y = y;
   }
   // other methods
}

public class Point3D extends Point2D
{
   public int z;

   // other code
}

// possible constructors for Point3D
I.  public Point3D() {}
II. public Point3D(int x, int y, int z)
    {
       super(x,y);
       this.z = z;
    }
III. public Point3D(int x, int y)
     {
        this.x = x;
        this.y = y;
        this.z = 0;
     }

Given the class definitions of Point and NamedPoint below, which of the constructors that follow (labeled I, II, and III) would be valid in class NamedPoint?

class MPoint
{
   private int myX; // coordinates
   private int myY;

   public MPoint( )
   {
      myX = 0;
      myY = 0;
   }

   public MPoint(int a, int b)
   {
      myX = a;
      myY = b;
   }

   // ... other methods not shown

}

public class NamedPoint extends MPoint
{
   private String myName;
   // constructors go here
   // ... other methods not shown
}

//  Proposed constructors for this class:
I.   public NamedPoint()
     {
        myName = "";
     }
II.  public NamedPoint(int d1, int d2, String name)
     {
        myX = d1;
        myY = d2;
        myName = name;
     }
III. public NamedPoint(int d1, int d2, String name)
     {
        super(d1, d2);
        myName = name;
     }