Why the ArrayList implements the List interface

Why the ArrayList implements the List interface even though it extends the AbstractList class. Is it only for clarity of interface implemented?

899440 wrote:
Why the ArrayList implements the List interface even though it extends the AbstractList class. Is it only for clarity of interface implemented?I dont think there's any specific reason for that. In my opinion its redundant. May be its added for clarity so that from the documentation its obivous that ArrayList implements List.
May be there's something which I am missing regarding this?

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Why the HashMap implements the Map interface twice

See the type hierarchy below:
HashMap<K,V>
┃
┣AbstractMap<K,V>
┃┃
┃┗Map<K,V>
┗Map<K,V>
The HashMap extends AbstractMap, which implements Map. So the HashMap has already implements Map, and has the Map type. But the HashMap's declaration still contains "implements Map".
So why the HashMap implements the Map interface twice?
Thanks in advance.

georgemc wrote:
Maybe the guy who wrote HashMap wanted to make it clear that HashMap implemented Map, rather than rely on you looking further up the hierarchy to find outYes, it does make the Javadoc clearer. If they hadn't done that then there would have been legions of newbies who couldn't tell that HashMap implemented Map and thousands of "I have a doubt, does HashMap implement Map" questions here.

Why does ArrayList implement List when extending AbstractList

Hi
I am unable to understand why would the ArrayList class implement the List interface, when it is already extending the AbstarctList class. If anyone can explain, it would be a great help.

Why does ArrayList implement List when extendingAbstractList
It is a question of style. ArrayList'simplementing
List is a crucial piece of information, whereasits
extending AbstractList is a kind of implementation
detail. If would not matter much to the user if it
did not extend AbstractList but implemented List
directly.So why don't those subclasses of ArrayList step up
and implement List again? I agree, extending
ArrayList is an implementation detail, and in fact
perhaps they should have made ArrayList a field
rather than a superclass...Because those subclasses job is not to be a List but to be an extension of ArrayList. In general if you wanted to be a List you would be extending AbstractList. You don't just implement List because you happen to have the right methods, you implement List because you feel being a List is part of that classes main behavior.
Its a suttle point but I feel its more than a matter of style. Its a matter of correctness.
Also, ArrayList could use composition by having an AbstractList member within itself. In this case it would implement List as well and achieve the same result. Letting users know it intends to fill the role of a List.

Why does ArrayList "implement" List, when it extends AbstractList?

I never noticed this before, and so i was wondering if there's a good explanation for it: If you look at how ArrayList is defined, you'll see this:
===
public class ArrayList
extends AbstractList
implements List, RandomAccess, Cloneable, Serializable
===
But AbstractList, which ArrayList extends, already implements List, like this:
===
public abstract class AbstractList
extends AbstractCollection
implements List
===
So, why does ArrayList list List in the list of implements? (that's kind of a confusing sentence, read it twice) By extending AbstractList, it's already clear that it implements List - now it's like it's implementing it twice (which doesn't hurt of course, but it doesn't help either.) This seems inconsistent with other parts of the API, where usually in cases like this (with an abstract class being extended) the interface isn't listed again in the final concrete class. For instance:
public class TitledBorder
extends AbstractBorder
and:
public abstract class AbstractBorder
extends Object
implements Border, Serializable
but:
TiledBorder doesn't mention Border in its (non-existent) list of implements.
Any ideas?
-- Niek

Surely enough the AbstractList
takes care of all the burden (like I wrote before),
but the ArrayList (re)implements the List interface
for
very sound reasons. If it didn't, the
o.getClass().getInterfaces() would've missed the List
interface,Since AbstractList implements List, ArrayList has an implicit "implements List." However, as you say, getInterfaces() doesn't produce that.
So you're saying that the VM has to do something different to find the method? I don't think so. The compiler may have to work harder, but in both cases, the VM does an invokeinterface. The javap output looks the same in a copule of test classes I created.
public interface I1 {
    public void i1Meth();
    public void i1Meth2();
public abstract class IA implements I1 {
    public void i1Meth() {}
public class IC1 extends IA implements I1 {
    public void i1Meth2() {}
    void dummy() {
        I1 i1 = new IC1();
        i1.i1Meth();
        i1.i1Meth2();
import java.util.*;
public class IC2 extends IA {
    public void i1Meth2() {}
    void dummy() {
        I1 i1= new IC2();
        i1.i1Meth();
        i1.i1Meth2();
    public static void main(String[] args) {
        System.out.println(Arrays.asList(IC2.class.getInterfaces()));
        System.out.println(Arrays.asList(IC1.class.getInterfaces()));
Compiled from "IC1.java"
public class IC1 extends IA implements I1{
void dummy();
Code:
   0:   new     #2; //class IC1
   3:   dup
   4:   invokespecial   #3; //Method "<init>":()V
   7:   astore_1
   8:   aload_1
   9:   invokeinterface #4, 1; //InterfaceMethod I1.i1Meth:()V
   14: aload_1
   15: invokeinterface #5, 1; //InterfaceMethod I1.i1Meth2:()V
   20: return
Compiled from "IC2.java"
public class IC2 extends IA{
void dummy();
Code:
   0:   new     #7; //class IC2
   3:   dup
   4:   invokespecial   #8; //Method "<init>":()V
   7:   astore_1
   8:   aload_1
   9:   invokeinterface #9, 1; //InterfaceMethod I1.i1Meth:()V
   14: aload_1
   15: invokeinterface #10, 1; //InterfaceMethod I1.i1Meth2:()V
   20: return
...

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hi,
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is this casting.. i saw sumthing similar to this on an article about casting
Customer c = (Customer)customers.get(d);
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Why to take refrence of ArrayList in it's interface List

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Suppose you only need a "thing" with the functionality of a List. You
couldn't care less then if I gave you an ArrayList, LinkedList or whatever
list; as long as it implements the List interface.
kind regards,
Jos

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>
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Why the ArrayList implements the List interface

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