Unchecked or unsafe operation was used

Source code

import java.util.*;
public class Example02
{
    
    
	public static void main(String[] args){
    
    
		LinkedList link=new LinkedList();
		link.add("stu1");
		link.add("stu2");
		link.add("stu3");
		link.add("stu4");
		System.out.println(link.toString());
		link.add("Stu5");
		link.addFirst("First");
		System.out.println(link);
		System.out.println(link.getFirst());
		link.remove(3);
		link.removeFirst();
		System.out.println(link);
	}
}

Compile time

After correcting the source code

import java.util.*;
public class Example02
{
    
    
	public static void main(String[] args){
    
    
		LinkedList<String>link=new LinkedList<String>();
		link.add("stu1");
		link.add("stu2");
		link.add("stu3");
		link.add("stu4");
		System.out.println(link.toString());
		link.add("Stu5");
		link.addFirst("First");
		System.out.println(link);
		System.out.println(link.getFirst());
		link.remove(3);
		link.removeFirst();
		System.out.println(link);
	}
}

Reasons after compilation

对于集合类，需要指定其类型。