泛型的上限和下限的Demo

Main Class

package Comparator.Bean;

import java.math.BigDecimal;
import java.util.List;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Iterator;
/**
 * ？ extends E : 接收E或者是E的子类对象。上限
 * ？ super E ：接收E或者E的父类对象。下限
 * @author asus
 *
 */
public class ComparatorMain {
    public static void main(String[] args) {
        System.out.println("----------extends---------------");
        List<People> peoples = new ArrayList<People>();
        peoples.add(new People("Tom",22));
        peoples.add(new People("Kitty",18));
        peoples.add(new People("Marry",20));
        printCollectionExtends(peoples);
        List<Worker> workers = new ArrayList<Worker>();
        workers.add(new Worker("Programmer", new BigDecimal(20000)));
        workers.add(new Worker("teacher", new BigDecimal(6000)));
        printCollectionExtends(workers);
        List<String> strings = new ArrayList<String>();
        strings.add("abc");
        strings.add("efg");
        strings.add("hij");;
        //printCollection(strings); 
        System.out.println("----------super---------------");
        printCollectionSuper(peoples);
        List<Student> students = new ArrayList<Student>();
        students.add(new Student(87, 96));
        students.add(new Student(100, 100));
        //printCollectionSuper(students);
        List<Human> humans = new ArrayList<Human>();
        humans.add(new Human("China", "1991-6-4"));
        humans.add(new Human("US", "2000-6-4"));
        printCollectionSuper(humans);
        //printCollectionExtends(humans);
    }
    
    public static void printCollectionExtends(Collection<? extends People> collection){
        Iterator<?> iterator = collection.iterator();
        while (iterator.hasNext()) {
            System.out.println(iterator.next().toString());
        }
    }
    
    public static void printCollectionSuper(Collection<? super People> collection){
        Iterator<?> iterator = collection.iterator();
        while (iterator.hasNext()) {
            System.out.println(iterator.next().toString());
        }
    }
}

People class

package Comparator.Bean;

public class People extends Human{
    private String name;
    private int age;
    private Student student;
    private Worker worker;
    
    public People() {
        super();
    }
    public People(String name, int age) {
        this.name = name;
        this.age = age;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
    public Student getStudent() {
        return student;
    }
    public void setStudent(Student student) {
        this.student = student;
    }
    public Worker getWorker() {
        return worker;
    }
    public void setWorker(Worker worker) {
        this.worker = worker;
    }
    
    public String toString(){
        return "name:"+name+";age:"+age;
    }
    
    
}

Human class

package Comparator.Bean;

public class Human {
    private String country;
    private String birthday;
    
    public Human(){}
    
    public Human(String country, String birthday) {
        this.country = country;
        this.birthday = birthday;
    }
    public String getCountry() {
        return country;
    }
    public void setCountry(String country) {
        this.country = country;
    }
    public String getBirthday() {
        return birthday;
    }
    public void setBirthday(String birthday) {
        this.birthday = birthday;
    }
    
    public String toString(){
        return "country:"+country+"birthday:"+birthday;
    }
}

Worker class

package Comparator.Bean;

import java.math.BigDecimal;

public class Worker extends People{
    private String occupation;
    private BigDecimal salary;
    
    public Worker(String occupation, BigDecimal salary) {
        this.occupation = occupation;
        this.salary = salary;
    }
    public String getOccupation() {
        return occupation;
    }
    public void setOccupation(String occupation) {
        this.occupation = occupation;
    }
    public BigDecimal getSalary() {
        return salary;
    }
    public void setSalary(BigDecimal salary) {
        this.salary = salary;
    }
    public String toString(){
        return "Occupation:"+occupation+";salary:"+salary;
    }
}

Student class

package Comparator.Bean;

public class Student extends People{
    private int ChineseScores;
    private int MathScores;
    
    public Student(int chineseScores, int mathScores) {
        ChineseScores = chineseScores;
        MathScores = mathScores;
    }
    public int getChineseScores() {
        return ChineseScores;
    }
    public void setChineseScores(int chineseScores) {
        ChineseScores = chineseScores;
    }
    public int getMathScores() {
        return MathScores;
    }
    public void setMathScores(int mathScores) {
        MathScores = mathScores;
    }
    
}