Why can LinkedList be assigned to a List variable?

Question

I am confused about the LinkedList. They do not really make sense to me. Could someone explain to me how it basically works. For example I saw this code (below) and it did not make sense to me. From what I'm guessing a string list (list1) is created and is put in a LinkedList. But what is diffrence between list and LinkedList ?? Thanks

List<String>list1 = new LinkedList<String>()

I suggest you begin by reading a basic text on data structures, and a basic text on Java collections. — NPE, Dec 03 '12 at 21:08
Please take a look on this [response][1] [1]: http://stackoverflow.com/questions/322715/when-to-use-linkedlist-over-arraylist — Maxim Shoustin, Dec 03 '12 at 21:09
http://docs.oracle.com/javase/6/docs/api/java/util/List.html — Mike G, Dec 03 '12 at 21:09
`list1` is a *reference* to a List, not a List. `new` creates a `LinkedList`, initialises it and returns a *reference* to it. — Peter Lawrey, Dec 03 '12 at 21:11

JustDanyul · Accepted Answer · 2012-12-03T21:17:58.420

LinkedList is a class implementing the List interface. A interface is not a actual class, its more like a convention/blueprint of what methods that classes implementing them needs to expose.

For example, if you have interface called Shape

interface Shape() {
     public void draw(); 
}

notice that this contain a method body for draw, rather, its a convention saying all classes which implents this interface must have a method called draw. Now, lets say you have two classes which both implements the above interface

class Box implements Shape {
    public void draw() {
        //do actual drawing in this method body
    }
}

class Circle implements Shape {
    public void draw() {
        //do actual drawing in this method body
    }
}

You can then cast instances of either Circle or Box into a Shape variable, and safely call the draw method.

score 2 · Answer 2 · answered Dec 03 '12 at 21:13

This line assigns an empty LinkedList object for containing String to a List interface.
The idea of the List interface is, that you later could exchange the LinkedList by an ArrayList whitout the need to change the rest of your code

E.g:

List<String> list1 = new ArrayList<String>();

list.add("element1");

list.add() works for both, for ArrayList and for LinkedList, thats the purpose of the List Interface

PermGenError · Answer 3 · 2012-12-03T21:13:42.517

1

java.util.List is an interface and java.util.LinkedList is class which implements List interface.

List<String>list1 = new LinkedList<String>();
SuperClass(Class or interface) ref= new SubClass(Implementing class/concrete class)

The above is polymorphic way to create an LinkedList.

Below are some of the characterstics of List:

Accepts duplicates
Can access using indexes.
Maintains Insertion Order

edited Dec 03 '12 at 21:13

answered Dec 03 '12 at 21:08

PermGenError

45,977
8
87
106

score 1 · Answer 4 · edited Dec 03 '12 at 21:39

LinkiedList implements the List interface so LinkedList is a List.

This is the definition of the LinkedList Class:

public class LinkedList<E> extends AbstractSequentialList<E> implements List<E>, Deque<E>, Cloneable, Serializable

As you can see the LinkedList class implements the List interface.

Therefore, as LinkedList is a List (or a child of List bot not exactly), you can use polymorphism to instantiate a List object like you are saying. The List interface say the behavior of the class that implement this interface, in this case LinkedList. Remember that the Interfaces are like a contract, so A*LL THE METHODS THAT ARE DEFINIED IN* List interface MUST BE IMPLEMENTED in the LinkedList class. In that way List interface has the same methods as the LinkedList class.

score 0 · Answer 5 · answered Dec 03 '12 at 21:18

I think what is confusing you is the use of generics.

Here it is without generics:

List list1 = new LinkedList();

This is pretty simple Java code. We're creating a LinkedList object and assigning it to a variable of type List. LinkedList implements the List interface so this works.

Now let's add the generics back:

List<String>list1 = new LinkedList<String>();

All that is different is that you're specifying what the LinkedList is going to contain.

To put something in the list:

list1.add("Something in my linked list");

Why can LinkedList be assigned to a List variable?

5 Answers5