Java---泛型---高级应用_综合

泛型的通配符：?

当操作的不同容器中的类型都不确定的时候，而且使用的都是元素从Object类中继承的方法，这时泛型就用通配符?来表示即可。（助理解的比方: 泛型中的多态应用)

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;import org.junit.Test;public class GenericAdv1 {@Testpublic void demo1(){List<String> list1 = new ArrayList<String>();list1.add("aaaa");list1.add("bbbb");list1.add("cccc");Iterator<String> it = list1.iterator();while(it.hasNext()){String str = it.next();System.out.println(str);}List<Integer> list2 = new ArrayList<Integer>();list2.add(100);list2.add(200);list2.add(300);Iterator<Integer> it2 = list2.iterator();while(it2.hasNext()){Integer i = it2.next();System.out.println(i);}}@Testpublic void demo2(){List<String> list1 = new ArrayList<String>();list1.add("aaaa");list1.add("bbbb");list1.add("cccc");print1(list1);List<Integer> list2 = new ArrayList<Integer>();list2.add(100);list2.add(200);list2.add(300);print2(list2);}public void print1(List<String> list){Iterator<String> it = list.iterator();while(it.hasNext()){String str = it.next();System.out.println(str);}}public void print2(List<Integer> list){Iterator<Integer> it = list.iterator();while(it.hasNext()){Integer str = it.next();System.out.println(str);}}///高级泛型/@Testpublic void demo3(){List<String> list1 = new ArrayList<String>();list1.add("aaaa");list1.add("bbbb");list1.add("cccc");print(list1);List<Integer> list2 = new ArrayList<Integer>();list2.add(100);list2.add(200);list2.add(300);print(list2);}/*写泛型时，想用Object去通吃String、Integer等所有类型。* 想法很好，但行不通。因为这是多态思想，它是处于"编译+运行期"，* 而泛型只处于"编译期"。* 泛型中的该“多态”思想是用通配符“?”来实现的。以后记得：* 泛型中的通吃所有用"?",它类似多态中的"Object"*/public void print(List<?> list){Iterator<?> it = list.iterator();while(it.hasNext()){Object str = it.next();System.out.println(str);}}}

泛型的限定：

    对操作的类型限制在一个范围之内。比如：定义一个功能，只操作Person类型或者Person的子类型。这时可以用:
        ? extends E:接收E类型或者E的子类型。这就是上限。
        ? super E:   接收E类型或者E的父类型。这就是下限。

    一般情况下:
        只要是往容器中添加元素时，使用上限。 ? extends E
        只要是从容器中取出元素时，是用下限。 ? super E

演示过程采用了 7个类：

主类：

import java.util.HashSet;
import java.util.Set;
import java.util.TreeSet;import org.junit.Test;public class GenericAdv2 {@Testpublic void demo1(){Set<Student> students = new HashSet<Student>(); //new TreeSet<Student>();students.add( new Student("小王",22,"计算机"));students.add( new Student("张三",21,"软件工程"));students.add( new Student("李四",23,"外语"));students.add( new Student("老王",22,"电子商务"));print(students);Set<Worker> workers = new HashSet<Worker>(); //new TreeSet<Student>();workers.add( new Worker("Jack",22,"计算机"));workers.add( new Worker("Tom",21,"软件工程"));workers.add( new Worker("Mike",23,"外语"));workers.add( new Worker("Rose",22,"电子商务"));print(workers);//Set<Integer> set = new HashSet<Integer>();//print(set);//编译错误，因为print()方法参数用了“上限”}//本例参数中的泛型限制实参必须是Person的子类---高级泛型中的“上限”public void print(Set<? extends Person> set){for(Person p:set){System.out.println(p.getName()+","+p.getAge());}}@Test //使用比较器MyCompByName1 + MyCompByName2public void demo2(){Set<Student> students = new TreeSet<Student>( new MyCompByName1() );students.add( new Student("小王",22,"计算机"));students.add( new Student("张三",21,"软件工程"));students.add( new Student("李四",23,"外语"));students.add( new Student("老王",22,"电子商务"));print(students);Set<Worker> workers = new TreeSet<Worker>(  new MyCompByName2() );workers.add( new Worker("Jack",22,"计算机"));workers.add( new Worker("Tom",21,"软件工程"));workers.add( new Worker("Mike",23,"外语"));workers.add( new Worker("Rose",22,"电子商务"));print(workers);}@Test //使用比较器MyCompByNamepublic void demo3(){Set<Student> students = new TreeSet<Student>( new MyCompByName() );students.add( new Student("小王",22,"计算机"));students.add( new Student("张三",21,"软件工程"));students.add( new Student("李四",23,"外语"));students.add( new Student("老王",22,"电子商务"));print(students);Set<Worker> workers = new TreeSet<Worker>(  new MyCompByName() );workers.add( new Worker("Jack",22,"计算机"));workers.add( new Worker("Tom",21,"软件工程"));workers.add( new Worker("Mike",23,"外语"));workers.add( new Worker("Rose",22,"电子商务"));print(workers);}}

Person类：

public class Person {private String name;private int age;public Person(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;}@Overridepublic int hashCode() {final int prime = 31;int result = 1;result = prime * result + age;result = prime * result + ((name == null) ? 0 : name.hashCode());return result;}@Overridepublic boolean equals(Object obj) {if (this == obj)return true;if (obj == null)return false;if (getClass() != obj.getClass())return false;Person other = (Person) obj;if (age != other.age)return false;if (name == null) {if (other.name != null)return false;} else if (!name.equals(other.name))return false;return true;}@Overridepublic String toString() {return name + "," + age;}}

Student类：

public class Student extends Person {private String speciality;public Student(String name, int age, String speciality) {super(name, age);this.speciality = speciality;}@Overridepublic String toString() {return super.toString() +","+ speciality;}}

Worker类：

public class Worker extends Person {private String job;public Worker(String name, int age, String job) {super(name, age);this.job = job;}@Overridepublic String toString() {return super.toString() +","+ job;}}

MycompByName类：

import java.util.Comparator;public class MyCompByName implements Comparator<Person>{@Overridepublic int compare(Person o1, Person o2) {return o1.getName().compareTo(o2.getName());}}

MycompByName1类：

import java.util.Comparator;public class MyCompByName1 implements Comparator<Student>{@Overridepublic int compare(Student o1, Student o2) {return o1.getName().compareTo(o2.getName());}}

MycompByName2类：

import java.util.Comparator;public class MyCompByName2 implements Comparator<Worker>{@Overridepublic int compare(Worker o1, Worker o2) {return o1.getName().compareTo(o2.getName());}}