泛型固定上边界和泛型固定下边界

Dear 丶 2022-06-16 09:24 246阅读 0赞

##  泛型固定下边界    ? super E ##

类 TreeSet<E>构造方法
      public TreeSet(Collection<? extends E> c)
      public TreeSet(Comparator<? super E> comparator)
    类 TreeMap<K,V>构造方法
     public TreeMap(Comparator<? super K> comparator)
      public TreeMap(Map<? extends K,? extends V> m)

## 泛型固定上边界   ? extends E ##

接口 Collection<E>方法
     boolean addAll(Collection<? extends E> c)

import java.util.ArrayList;
    import java.util.Comparator;
    import java.util.TreeSet;
    import bean.Person;
    import bean.Student;
    public class k {
    
    	/**
    	 * person是student的子类
    	 */
    	public static void main(String[] args) {
    		
    		ArrayList<Student> list1 = new ArrayList<>();
    		list1.add(new Student("张三", 23));
    		list1.add(new Student("李四", 24));
    		
    		ArrayList<Person> list2 = new ArrayList<>();
    		list2.add(new Person("王五", 25));
    		list2.add(new Person("赵六", 26));
    		
    		list1.addAll(list2);//可以把子类对象添加到父类集合中区
    		System.out.println(list1);
    		
    		
    		TreeSet<Student> ts1 = new TreeSet<>(new CompareByAge());
    		ts1.add(new Student("张三", 33));
    		ts1.add(new Student("李四", 13));
    		ts1.add(new Student("王五", 23));
    		ts1.add(new Student("赵六", 43));
    		
    		TreeSet<Person> ts2 = new TreeSet<>(new CompareByAge());
    		ts2.add(new Person("张三", 33));
    		ts2.add(new Person("李四", 13));
    		ts2.add(new Person("王五", 23));
    		ts2.add(new Person("赵六", 43));
    		//比较器比较的是student对象，为什么student的子类person也可以比较
    		System.out.println(ts2);
    		
    		
    		
    	}
    
    }
    //比较器做成之后可以放入student及其子类
    class CompareByAge implements Comparator<Student> {
    
    	@Override
    	public int compare(Student s1, Student s2) {
    		int num = s1.getAge() - s2.getAge();
    		return num == 0 ? s1.getName().compareTo(s2.getName()) :  num;
    	}
    	
    }

student

package bean;
    
    public class Student implements Comparable<Student> {
    	
    	String name;
    	int age;
    	
    	public Student() {
    		super();
    		
    	}
    	public Student(String name, int age) {
    		super();
    		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;
    	}
    	@Override
    	public String toString() {
    		return "Student [name=" + name + ", age=" + age + "]";
    	}
    	
    	@Override
    	public int compareTo(Student obj) {
    		// TODO Auto-generated method stub
    		//return 0;
    		//根元素为13 ,传入的为23，23这个对象调用 compareTo方法 23-13》0，在13的右边
    		//return (this.age - obj.age);
    		int num = this.age - obj.age;//age是主要判据
    		//string里面重写了compareto方法，按字典顺序
    		return num == 0 ? this.name.compareTo(obj.name) : num;
    	}
    	/* (non-Javadoc)
    	 * @see java.lang.Object#hashCode()
    	 */
    	@Override
    	public int hashCode() {
    		final int prime = 31;
    		int result = 1;
    		result = prime * result + age;
    		result = prime * result + ((name == null) ? 0 : name.hashCode());
    		return result;
    	}
    	/* (non-Javadoc)
    	 * @see java.lang.Object#equals(java.lang.Object)
    	 */
    	@Override
    	public boolean equals(Object obj) {
    		if (this == obj)
    			return true;
    		if (obj == null)
    			return false;
    		if (getClass() != obj.getClass())
    			return false;
    		Student other = (Student) 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;
    	}
    		
    }

person

package bean;
    
    public class Person extends Student {
    	public Person() {
    		super();	
    	}
    	public Person(String name, int age) {
    		super(name ,age);
    	}
    }