Java 泛型的限定

? 通配符。也可以理解为占位符。

泛型的限定；

？ extends E: 可以接收E类型或者E的子类型。上限。
？ super E: 可以接收E类型或者E的父类型。下限

import java.util.*;
class  GenericDemo6
{
public static void main(String[] args)
{
/*
ArrayList<String> al = new ArrayList<String>

();

al.add("abc1");
al.add("abc2");
al.add("abc3");

ArrayList<Integer> al1 = new

ArrayList<Integer>();
al1.add(4);
al1.add(7);
al1.add(1);

printColl(al);
printColl(al1);
*/

ArrayList<Person> al = new ArrayList<Person>

();
al.add(new Person("abc1"));
al.add(new Person("abc2"));
al.add(new Person("abc3"));
//printColl(al);

ArrayList<Student> al1 = new

ArrayList<Student>();
al1.add(new Student("abc--1"));
al1.add(new Student("abc--2"));
al1.add(new Student("abc--3"));
printColl(al1);  //ArrayList<? extends

Person> al = new ArrayList<Student>();error

}
public static void printColl(Collection<? extends

Person> al)
{
Iterator<? extends Person> it = al.iterator

();

while(it.hasNext())
{
System.out.println(it.next().getName

());
}
}
/*
public static void printColl(ArrayList<?>

al)//ArrayList al = new ArrayList<Integer>();error
{
Iterator<?> it = al.iterator();

while(it.hasNext())
{
System.out.println(it.next

().toString());
}
}
*/
}

class Person
{
private String name;
Person(String name)
{
this.name = name;
}
public String getName()
{
return name;
}
}

class Student extends Person
{
Student(String name)
{
super(name);
}

}

/*

class Student implements Comparable<Person>//<? super E>
{
public int compareTo(Person s)
{
this.getName()
}
}
*/
class Comp implements Comparator<Person>
{
public int compare(Person s1,Person s2)
{

//Person s1 = new Student("abc1");
return s1.getName().compareTo(s2.getName());
}
}

TreeSet<Student> ts = new TreeSet<Student>(new Comp());
ts.add(new Student("abc1"));
ts.add(new Student("abc2"));
ts.add(new Student("abc3"));

import java.util.*;
class GenericDemo7
{
public static void main(String[] args)
{

TreeSet<Student> ts = new TreeSet<Student>(new Comp());

ts.add(new Student("abc03"));
ts.add(new Student("abc02"));
ts.add(new Student("abc06"));
ts.add(new Student("abc01"));

Iterator<Student> it = ts.iterator();

while(it.hasNext())
{
System.out.println(it.next().getName());
}
/**/

TreeSet<Worker> ts1 = new TreeSet<Worker>(new Comp());

ts1.add(new Worker("wabc--03"));
ts1.add(new Worker("wabc--02"));
ts1.add(new Worker("wabc--06"));
ts1.add(new Worker("wabc--01"));

Iterator<Worker> it1 = ts1.iterator();

while(it1.hasNext())
{
System.out.println(it1.next().getName());
}
}
}

/*
class StuComp implements Comparator<Student>
{
public int compare(Student s1,Student s2)
{
return s1.getName().compareTo(s2.getName());
}
}

class WorkerComp implements Comparator<Worker>
{
public int compare(Worker s1,Worker s2)
{
return s1.getName().compareTo(s2.getName());
}
}
*/

class Comp implements Comparator<Person>
{
public int compare(Person p1,Person p2)
{
return p2.getName().compareTo(p1.getName());
}
}

class Person
{
private String name;
Person(String name)
{
this.name = name;
}
public String getName()
{
return name;
}
public String toString()
{
return "person :"+name;
}
}

class Student extends Person
{
Student(String name)
{
super(name);
}

}

class Worker extends Person
{
Worker(String name)
{
super(name);
}
}
————摘自《毕向东25天》