Java--泛型（通配符）

通配符：表示当前类型可以是任何类型
它也会进行类型擦除的 擦除到Object

Object ArrayList
Number ArrayList
Integer ArrayList
库的开发 ：
写一个函数，打印容器内的元素 ArrayList:容器

class GenericList{
    //泛型写：
    public static<T> void printList1(ArrayList<T> list){
        for(T obj : list){
            System.out.print(obj+" ");
        }
        System.out.println();
    }

    public static<T> void printList2(ArrayList<Object> list){
        for(Object obj : list){
            System.out.print(obj+" ");
        }
        System.out.println();
    }

    public static<T> void printList(ArrayList<?> list){
        for(Object obj : list){
            System.out.print(obj+" ");
        }
        System.out.println();
    }
}

public class ArrayListDemo {

    public static void main(String[] args) {
        ArrayList<Integer> list = new ArrayList<Integer>(); 
        list.add(10);
        list.add(20);
        list.add(30);
        GenericList.printList(list);



        //ArrayList<Object> obj = new ArrayList<Integer>();
        //obj[0] = 10;           obj[1] = "梨";     
    }

}

---

通配符既有上界，又有下界
通配符的下界： ? super T
通配符的上界： ? extends T

class GenericAlg3{

    //泛型实现
    public static<T extends Comparable<T>> T findMaxPerson1(ArrayList<T> list){

        T maxPerson = list.get(0);
        for(int i = 1;i < list.size();i++){
            if(maxPerson.compareTo(list.get(i)) < 0){
                maxPerson = list.get(i);
            }
        }

        return maxPerson;
    }
    //下界     =====》   
    //? super T  找到是否有T类型的基类实现了Comparable接口
public static<T extends Comparable<? super T>> T findMaxPerson(ArrayList<T> list){

        T maxPerson = list.get(0);
        for(int i = 1;i < list.size();i++){
            if(maxPerson.compareTo(list.get(i)) < 0){
                maxPerson = list.get(i);
            }
        }

        return maxPerson;
    }

}


class Person implements Comparable<Person>{
    private String name;

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

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

    @Override
    public int compareTo(Person o) {
        // TODO Auto-generated method stub
        return name.compareTo(o.name);
    }

}

class Student extends Person{
    private int age;

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

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


}

public class TestDemo1 {


    public static void main3(String[] args) {
        ArrayList<? extends Person> list = new ArrayList<Student>();

        //上界用来读取的(查一下)

    }


    public static void main2(String[] args) {

        ArrayList<Student> list1 = new ArrayList<Student>();

        list1.add(new Student("wang",10));
        list1.add(new Student("gao",20));
        list1.add(new Student("a",30));

        System.out.println(GenericAlg3.findMaxPerson(list1));
        //汉字比较？？？？？？？？？？？？？？
    }

    public static void main(String[] args) {
        ArrayList<Person> list1 = new ArrayList<Person>();

        list1.add(new Person("wang"));
        list1.add(new Person("gao"));
        list1.add(new Person("abcd"));
        System.out.println(GenericAlg3.findMaxPerson(list1));

    }

}