Java class with generic data types

Question

I am trying to write a simple linked list class that uses generic data type. I am new to Java so I can't figure out how to debug the error message in the main method that arises when I try to insert data into an instance of my class. The class code and the main method are as follows:

import java.io.*; 
  
// Java program to implement 
// a Singly Linked List 
public class MyLinkedList<T> {
    

     Node head; // head of the list
     class Node<T> {
    
     T data;
     Node next;
     // Constructor
         Node(T d)
         {
             data = d;
             next = null;
         }
         }
      
        // Method to insert a new node 
    MyLinkedList insert(MyLinkedList list, T data)
    { 
        // Create a new node with given data 
            Node new_node = new Node(data); 
            new_node.next = null; 
      
            // If the Linked List is empty, 
        // then make the new node as head 
        if (list.head == null) { 
            list.head = new_node; 
        } 
        else { 
            // Else traverse till the last node 
            // and insert the new_node there 
                Node last = list.head; 
                while (last.next != null) { 
                    last = last.next; 
                } 
      
                // Insert the new_node at last node 
                last.next = new_node; 
            } 
      
            // Return the list by head 
        return list; 
    }    
    // Driver code 
    public static void main(String[] args) 
    { 
        /* Start with the empty list. */
        MyLinkedList list = new MyLinkedList();
        
        // Insert the values 
        list = insert(list, 1);
    } 
}

I think there is no problem with your code, could you show us the error message? — Ismail
– Ismail, Commented Jul 30, 2020 at 8:53
This is possibly not the code you intended to provide, as there's no generic data type usage to be seen; just integers as data. — Juha Laiho
– Juha Laiho, Commented Jul 30, 2020 at 8:55
I've run your code and it didn't give me any errors so there's nothing wrong with the code itself. — Amongalen
– Amongalen, Commented Jul 30, 2020 at 8:56
Furthermore, you should strongly consider, implementing it in an object-oriented fashion. Using your static method would not be considered clean or elegant in java.. — ehhc
– ehhc, Commented Jul 30, 2020 at 9:00

garima garg · Accepted Answer · 2020-08-02 12:25:03Z

3

You need to change int to T in class declaration and method signature.

 class MyLinkedList<T> {
 MyLinkedList() {
 head=null;
 }
 Node head; // head of list
 class Node<T> {

 T data;
 Node next;

 // Constructor
 Node(T d) {
     data = d;
     next = null;
 }

}

// Method to insert a new node

public void insert(T data) {
 // Create a new node with given data
 Node new_node = new Node(data);
 new_node.next = null;

 // If the Linked List is empty,
 // then make the new node as head
 if (this.head == null) {
     this.head = new_node;
 } else {
     Node last = this.head;
     while (last.next != null) {
         last = last.next;
     }
     // Insert the new_node at last node
     last.next = new_node;
 }

}

 protected void display() {
 Node myNode=head;
 System.out.println();
 while (myNode != null) {
     System.out.println(myNode.data);
     myNode=myNode.next;
 }

}

Change the insert method signature to the below:

public static void main(String[] args) {
 /* Start with the empty list. */
 MyLinkedList<Integer> list = new MyLinkedList<Integer>();

 // Insert the values
 list.insert(1);
 list.insert(3);
 list.insert(12);
 list.insert(11);
 list.insert(21);
 list.insert(22);
 list.insert(45);
 list.display();
 }

For clear coding and understanding I have changed class name as MyLinkedList

edited Aug 2, 2020 at 12:25

answered Jul 30, 2020 at 9:38

garima garg

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5 Comments

Junsui Bantsu Over a year ago

I did as you suggested and I am still getting an error. Try running my code that's above.

garima garg Over a year ago

I would say, either copy-paste the error message here or screenshot.

Junsui Bantsu Over a year ago

I can't send an error message because the code won't compile so I can't run it. I am using Eclipse and it highlights the error, it's this line in the code that won't compile list = insert(list, 1);

garima garg Over a year ago

@JunsuiBantsu: plz check my answer. I have added the complete code. It's working for me and it should work for you as well. All the best. (y) Also, you can not call a non-static method as static i.e. instance method must be called using class object only.

aeberhart Over a year ago

I assume you're trying to call "insert" from the main method. You cannot do this, since "main" is a static (global) method and "insert" is a method of "MyLinkedList" which you call as follows: list.insert(...) as @garima-garg points out.

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