CODEWITHSUNDEEP
javalistarraylist
user9986762
user9986762

Reputation: 35

How do you implement a list vs. an arraylist

I'm a java beginner and I know that you should use list over arraylist but i'm not exactly sure how to change from arraylist to list. Before I had 

ArrayList<Homework3> hw = new ArrayList<Homework3>();

Which worked and then I tried:

List<Homework3> hw = new ArrayList<Homework3>();

Then I tried to implement the List interface with this:

public interface List<Homework3> // inheritance not shown 
{ 
 boolean add( Homework3 x ); 
 void add( int index, Homework3 x ); 
 Homework3 get( int index ); 
 Homework3 remove( int index ); 
 Homework3 set( int index, Homework3 x ); 
 int size(); 
}

But now it's saying incompatible types. I looked at other questions and discussions and they had the code just like this:

List<Object> list = new ArrayList<Object>();

And i'm following the same basic principle, can someone help explain why it isn't working and how I can fix this?

Upvotes: 1

Views: 169

Answers (2)

Makoto
Makoto

Reputation: 106389

You don't need to do any of this. Java takes care of this type resolution for you through generics.

Because List<E> is defined in a generic way, so too must its implementors be, and thus ArrayList<E> uses the same generic type declared by the interface.

To be explicit:
When you declare List<Homework3> hw = new ArrayList<Homework3>();, everywhere that E is used in the Javadoc is replaced by Homework3. You don't have to implement any of this because the language already has for you.

Upvotes: 1

Naman
Naman

Reputation: 31858

The class java.util.ArrayList implements java.util.List interface. Hence your code after defining your own List<Homework3> interface would have incompatible types.

    List<Object> list     =    new ArrayList<Object>();
//  ^^                               ^^
// your.package.List            java.util.ArrayList

Similar code in your question

    List<Homework3> hw  =   new ArrayList<Homework3>();
//  ^^                               ^^
// java.util.List            java.util.ArrayList

would have worked earlier since then you didn't introduce the List interface as mentioned in the question and you were referring to java.util.List itself.

Upvotes: 0

Related Questions