Java基础之泛型的上限和下限

逃离我推掉我的手 2022-12-10 04:52 180阅读 0赞

package cn.itcast.p5.generic.advance.demo;
    
    import java.util.ArrayList;
    import java.util.Collection;
    import java.util.Iterator;
    
    import cn.itcast.p2.bean.Person;
    import cn.itcast.p2.bean.Student;
    import cn.itcast.p2.bean.Worker;
    
    public class GenericAdvanceDemo2 {
        
    
    	/**
    	 * @param args
    	 */
    	public static void main(String[] args) {
        
    
    		ArrayList<Person> al = new ArrayList<Person>();
    		
    		al.add(new Person("abc",30));
    		al.add(new Person("abc4",34));
    		
    		ArrayList<Student> al2 = new ArrayList<Student>();
    		
    		al2.add(new Student("stu1",11));
    		al2.add(new Student("stu2",22));
    		ArrayList<String> al3 = new ArrayList<String>();
    		
    		al3.add("stu3331");
    		al3.add("stu33332");
    		
    		printCollection(al2);
    		printCollection(al);
    	}
    
    	/**
    	 * 迭代并打印集合中元素。
    	 * 
    	 * 可以对类型进行限定：
    	 * ? extends E:接收E类型或者E的子类型对象。上限！
    	 * 
    	 * ? super E ：接收E类型或者E的父类型。下限！
    	 * @param al
    	 */
    	/*public static void printCollection(Collection<? extends Person> al) {//Collection<Dog> al = new ArrayList<Dog>()
    		Iterator<? extends Person> it = al.iterator();
    		
    		while(it.hasNext()){
    //			T str = it.next();
    //			System.out.println(str);
    //			System.out.println(it.next().toString());
    			Person p = it.next();
    			
    			System.out.println(p.getName()+":"+p.getAge());
    		}
    		
    	}*/
    	
    	public static void printCollection(Collection<? super Student> al){
        
    		Iterator<? super Student> it = al.iterator();
    		
    		while(it.hasNext()){
        
    			
    			System.out.println(it.next());
    		}
    	}
    
    }

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