United StatesChange Country, Oracle Worldwide Web Sites Communities I am a... I want to...
Bug ID: 4187872 Can't implement Map and List in the same class because of remove(Object) method
4187872 : Can't implement Map and List in the same class because of remove(Object) method

Details
Type:
Enhancement
Submit Date:
1998-11-06
Status:
Closed
Updated Date:
1998-11-06
Project Name:
JDK
Resolved Date:
1998-11-06
Component:
core-libs
OS:
generic
Sub-Component:
java.util
CPU:
generic
Priority:
P4
Resolution:
Not an Issue
Affected Versions:
1.2.0
Fixed Versions:

Related Reports

Sub Tasks

Description

Name: tb29552			Date: 11/06/98


I have a need for a collection that maintains an
ordered list (like ArrayList) but allows fast
access by a key (like HashMap).  My particular
use for this is withing a TableModel that
represents database rows that may be changed by
other objects.  I would like to implement both the
List and Map interfaces.  It appears that the one
and only barrier to this is that both interfaces
contain a remove(Object) method, but they return
different things.  List returns boolean, while Map
returns Object.

You can't create a set of methods that will
compile for the following class declaration:

import java.util.*;
public class HashList
    implements List, Map
{
}

I know it's late in the release cycle to do this,
but if it's not done now -- when?

I'd like List to return the object removed, with
null for a failure to find it.
(Review ID: 42265)
======================================================================

Comments
WORK AROUND


tim.bell@Eng 2000-06-29

The source code (LinkedHashMap.java) mentioned
in the Evaluation is available as part of the
J2SDK 1.3 beta SCSL source bundle, which can
be obtained from:
   http://www.sun.com/software/java2/download.html

After you agree to the SCSL license and download
the source bundle, look for:
ext/jpda/src/share/classes/com/sun/tools/jdi/LinkedHashMap.java
2004-06-11 
PUBLIC COMMENTS

.
2004-06-10 
EVALUATION

   This should probably be an FAQ.  It is intentional that a class cannot
implement List and Map simultaneously.  Even if it were possible from a method
signature point-of-view, it would be impossible semantically.  In particular,
the equals contracts implies that it's not possible for a Map to be equal
to a List, or vice versa.  Similarly, the hashCode contracts would mandate two
different HashCode computations.

    It is possible to get a very similar effect by having an object that
implements Map and provides a List-view operation ("asList()"), or vice-versa.
Also, it's possible to implement a Map that guarantees insertion-order
iteration but does not implement the List interface.  I've attached source
code for such a class to this bug report.  Customers can use this code freely
(as-is).

joshua.bloch@Eng 1998-11-06
1998-11-06


Hardware and Software, Engineered to Work Together