LinkedList Insert AT first/last/insertAfter, Java

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//'main' method must be in a class 'Rextester'.
//Compiler version 1.8.0_111

import java.util.*;
import java.lang.*;

class Rextester
{  
    Node head;
    class Node{
    
        int data;
        Node next;
        
        Node(int d){
         data=d;
         next=null;   
        }
    }
    
    void printlist(){
    
        Node n=head;
        while(n!=null){
        System.out.print(n.data +" ");
            n=n.next;
        }
    }
    
    void insertAtFirst(int new_data){
        Node new_node = new Node(new_data);
        new_node.next=head;
        head=new_node;
    }
    
    void insertAtEnd(int new_data){
    
        Node new_node = new Node(new_data);
        if(head==null){
        head=new Node(new_data);
            return;
        }
        new_node.next=null;
        Node last=head;
        while(last.next!=null){

last=last.next;
            
        }
        last.next=new_node;
        return;
        
    }
    
    void insertAfter(Node prev_node,int new_data){
        if(prev_node==null){
        
            System.out.println(" the given previous node can not be null");
            return;
            
        }
        Node new_node=new Node(new_data);
        new_node.next=prev_node.next;
        prev_node.next=new_node;
    }
    
    public static void main(String args[])
    {
        Rextester r=new Rextester();
        for(int i=1;i<=10;i++){
            r.insertAtFirst(i);
            r.insertAtEnd(i);
        }
        r.insertAfter(r.head.next.next.next.next,66);
        r.printlist();
    }
}

[ + ] Show input
Compilation time: 0.72 sec, absolute running time: 0.23 sec, cpu time: 0.24 sec, memory peak: 18 Mb, absolute service time: 0,97 sec