Java Multiple inheritance, get rid of duplicate code

Question

There are library classes B and C, both inherit from class A. I have 2 classes that extend B & C, namely MyB & MyC.

    A
   / \    
  B   C 
 /     \
MyB   MyC

MyB & MyC share lots of common code and they are only slightly different.

I want to get rid of duplicate code, how can I do this in java? In C++ it would be possible by creating a common base class and putting everything that is common in it as follows:

    A
   / \  
  B   C 
   \ /
  MyBase
   / \
 MyB MyC

Check this out: http://stackoverflow.com/questions/17226364/java-alternative-to-multiple-inheritance — jsedano, Jul 24 '13 at 16:10

score 25 · Accepted Answer · edited May 23 '17 at 12:16

25

You could use composition:

create a new class MyCommon with the common code
add an instance of MyCommon in MyB and MyC and delegate the work to MyCommon.

edited May 23 '17 at 12:16

Community

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answered Jul 24 '13 at 16:08

assylias

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gah beat me to it. :) – Vivin Paliath Jul 24 '13 at 16:09
4

Beat everybody... ;-) – Thrakbad Jul 24 '13 at 16:09
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+1, good stuff. this is a helpful article that might be good to include in the answer: http://www.javaworld.com/javaworld/jw-10-2005/jw-1024-multiple.html – Paul Sanwald Jul 24 '13 at 16:10
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wow, this is the first time, i have seen +10 reps in less than a minute from answering – PermGenError Jul 24 '13 at 16:10
[This article](http://berniesumption.com/software/inheritance-is-evil-and-must-be-destroyed/) is what turned on the light bulb for me... – damian Jul 24 '13 at 16:10

score 3 · Answer 2 · answered Jul 24 '13 at 16:09

3

Instead of having all your logic in these classes, have all common logic inside class D. Now make it so that MyC and MyB each have a member that is an instance of D. That's called composition.

answered Jul 24 '13 at 16:09

Geeky Guy

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score 1 · Answer 3 · answered Jul 24 '13 at 16:09

1

A class can only extend from one class. However, you can implement multiple interfaces.

answered Jul 24 '13 at 16:09

But I'm Not A Wrapper Class

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score 1 · Answer 4 · answered Jul 24 '13 at 16:11

In Java you'll use something along the lines of:

Composition (pattern) to encapsulate instances of B and C "in" MyBase.
Refactor B and C (if necessary) to expose a separate interface, say IB and IC
MyBase to implement multiple interfaces: IB and IC, by "doing the right thing" to map methods on interface to internal B and C instances.
MyB and MyC to implement appropriate interface, and map calls to MyBase.

Java Multiple inheritance, get rid of duplicate code

4 Answers4