Java泛型 继承中类型变量的关系

通过例子来看泛型类在继承中过程中类型变量应该具有怎样的关系。

首先给出几个辅助类：

package generic;
public class Animal {    }

package generic;

public class Person extends Animal {
	private String name;

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

	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}

	@Override
	public String toString() {
		return "Person [name=" + name + "]";
	}

	public Integer printAndReturn() {
		return 1;
	}
}

package generic;

public class Student extends Person {
	private String studentNumber;

	public Student(String name, String studentNumber) {
		super(name);
		this.studentNumber = studentNumber;
	}

	public String getStudentNumber() {
		return studentNumber;
	}

	public void setStudentNumber(String studentNumber) {
		this.studentNumber = studentNumber;
	}

	@Override
	public String toString() {
		return "Student [studentNumber=" + studentNumber + ", name="
				+ getName() + "]";
	}

}

package generic;

public class Building {
	private String name;

	public Building(String name) {
		super();
		this.name = name;
	}	

}

package generic;

/**
 * @version 1.00 2004-05-10
 * @author Cay Horstmann
 */
public class Pair<T extends Person>
{
   private T first;
   private T second;

   public Pair() { first = null; second = null; }
   public Pair(T first, T second) { this.first = first;  this.second = second; }

   public T getFirst() { return first; }
   public T getSecond() { return second; }

   public void setFirst(T newValue) { first = newValue; }
   public void setSecond(T newValue) { second = newValue; }

}

1.子类的类型变量表示的范围必须与父类一样或者是父类的子集

package generic;

public class PariSon<T extends Person> extends Pair<T> {

}

或

public class PariSon<T extends Student> extends Pair<T> {
}

2.当父类用具体类型代替类型变量时，具体变量必须是类型变量表示的范围内的一个元素

package generic;

public class PariSon extends Pair<Person> {
}

或

package generic;

public class PariSon extends Pair<Student> {
}