AP计算机教程9-7：访问继承的private field

继承意味着child class自动包含有parent class中定义的field和method。但如果继承的field是private的，child class则不能直接访问它们，需要通过parent class中所提供的一些public method（术语叫做getter或accessor）来读取它们，以及另一些public method（术语叫做setter或mutator）来操作它们。

例如，如果parent中有一个名为name的private field，则一般也会提供如下所示的getName和setName这两个public method。在setName中，先检查传入的字符串参数是否为null，然后再进行更新操作并返回是否成功。class Employee继承了name，但是需要通过getName和setName来操作它。

class Person
{
   private String name;

   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()
   {
      id = nextId;
      nextId++;
   }

   public int getId()
   {
      return id;
   }

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

Given the following class definitions which of the following would not compile if it was used in place of the missing code in the main method?

class Item
{
   private int x;

   public void setX(int theX)
   {
      x = theX;
   }
   // ... other methods not shown
}

public class EnhancedItem extends Item
{
   private int y;

   public void setY(int theY)
   {
      y = theY;
   }

   // ... other methods not shown

   public static void main(String[] args)
   {
      EnhancedItem currItem = new EnhancedItem();
      // missing code
   }
}