Java19-5.2 泛型通配符及限定上下线

public class Generic_Test6 {
    public static void main(String[] args) {
        List<String> list=new ArrayList<>();
        list.add("abc1");
        list.add("abc2");
        list.add("abc3");
        list.add("abc4");
        printCollection(list);
        Set<String> set=new TreeSet<>();
        set.add("abc1");
        set.add("abc2");
        set.add("abc3");
        set.add("abc4");
        printCollection(set);
    }

    private static void printCollection(Collection<String> collection) {
        for (Iterator<String> iterator = collection.iterator(); iterator.hasNext(); ) {
            String s = iterator.next();
            System.out.println(s);
        }
    }
}

为了同时可以迭代两个集合 我们将迭代范围改到Collection
但是此时我们依然无法迭代类型为学生的集合

 List<Student> list1=new ArrayList<>();
        list1.add(new Student());
        list1.add(new Student());
        list1.add(new Student());
        list1.add(new Student());
        printCollection(list1);

所以我们引入通配符<?>

//通配符<?>不明确传递参数类型
    private static void printCollection(Collection<?> collection) {
        for (Iterator<?> iterator = collection.iterator(); iterator.hasNext(); ) {
            Object o = iterator.next();
            System.out.println(o);
        }
    }

上限应用

import java.util.ArrayList;
import java.util.Iterator;
import java.util.TreeSet;

public class Gerneric_Test7 {
    public static void main(String[] args) {
        ArrayList<MStudent> list=new ArrayList<>();
        list.add(new MStudent("as",20));
        list.add(new MStudent("a2",10));
        list.add(new MStudent("as",19));
        TreeSet<Student> set=new TreeSet<Student>(list);
        for (Iterator<Student> iterator = set.iterator(); iterator.hasNext(); ) {
            Student next = iterator.next();
            System.out.println(next);
        }

    }
}

下限应用

public class Generic_Test8 {
    public static void main(String[] args) {
        Comparator<Student> comparator=new Comparator<Student>() {
            @Override
            public int compare(Student o1, Student o2) {
                int temp=o1.getName().compareTo(o2.getName());
                return temp==0? o1.getAge()-o2.getAge():temp;
            }
        };
        TreeSet<MStudent> set1=new TreeSet<>(comparator);
        set1.add(new MStudent("asd1",12));
        set1.add(new MStudent("zxcw",15));
        set1.add(new MStudent("xczx",13));
        set1.add(new MStudent("awqs",18));
        TreeSet<HStudent> set2=new TreeSet<>(comparator);
        set2.add(new HStudent("zxcz",12));
        set2.add(new HStudent("qwes",14));
        set2.add(new HStudent("zxwq",15));
        set2.add(new HStudent("zxcw",23));
        TreeSet<Student> set3=new TreeSet<>(comparator);
        set3.addAll(set1);
        set3.addAll(set2);
        for (Iterator<Student> iterator = set3.iterator(); iterator.hasNext(); ) {
            Student next = iterator.next();
            System.out.println(next);
        }

        }
    }