Multiple Inheritance

Hello,
I have been programming Java for last year,
evolved in quite some skills with it, and
really think it is great...
However, I was shocked to find out that there
is no multiple inheritance feature.
I know it is rare, and my case proves it
(1 year now, I never needed it)
HOWEVER, when one needs multiple inheritance,
then they really do need it.
I have interfaces which I would like implemented
in their respective class (ie ISomething be
implemented in CSomething), then some classes
I need to implement many of those interfaces...
Now I am forced to have those classes extend
multiple interfaces, and duplicate the interface
implementation code inside each of them.
I dont mind a little bit of copy/paste, nor
do I care about the compiled classes being
slightly bigger, BUT the problem is that
when I need to change some behaviour in those
interfaces, in the near (or far) future, I will
have their implementation scattered in many
classes... This is dangerously error prone and
not proffesional at all... And I do not think
that including multiple inheritance in the language
could be more error prone than this...
I think the Java team does a 100% perfect brilliant
job, but at this specific point, they "over-tried"
to "protect" the programmers from themselves...
Well, thats all,
I think some next version of Java should support
multiple inheritance. And the Java "warning" could be :
"if you havent missed it till now, then you probably
do not need anyway, so do not bother using it just
because it exists"
Thanks for reading my thoughts,
Dimitris

Personally I never need multiple inheritance of code and I try to avoid inheritance of code whenever possible. A common mistake in OO is too use inheritance as a way of reusing code. Code reuse is much easier, cleaner and more powerful by using composition instead. Only use inheritance for polymorhism (to use multiple implementations for the same interface). An example:
interface A {
void ma();
void maa();
interface B {
void mb();
class C implements A, B {
private A a;
private A c;
private B b;
public void ma() {
    a.ma();
public void maa() {
    c.maa();
public void mb() {
    b.mb();
}This is much more powerful than code reuse through inheritance. In this example I use one method from 'a' and one method from 'c' when I implement interface A. I can change the value of 'a', 'b' and 'c' during runtime, and I dont have to reuse all the code in 'a' and 'b', I can select which code to reuse. This is the power of composition and interfaces. Note that I only access 'a', 'b' and 'c' through the interfaces A and B, never directly through their implementations.
I would recommend you to look at your design and start to think about interfaces and inheritance, not about code reuse though inheritance.

Similar Messages

More about multiple inheritance

OK, you can solve problems where multiple inheritance is needed by using interfaces. But im facing a problem where it cant help me. Im constructing a system where there are componentes that need to extend JTextField as well Observable. I dont have interfaces above it in the hierarchy to substitute multiple inheritance. What can I do?
When you have a scenario that you have to use two (or more) third party classes, and need to inherit from both, how do interfaces can help? If ate least I had multiple inheritance from classes...

<< Begin Rant >>
I have seen more inherited code that is terribly designed because multiple inheritence was available.
The example provided is a perfect example of this: At first blush, it seems easy to combine the UI and data components by combining Observable and JTextArea. If you were able to do this, the person inheriting your code in 3 years will curse your name.
Nothing pisses me off more (well, I'm sure there are other things, but...) than attempting to debug C++ source code and finding that function calls are being made to multiple super classes.
Here's a fun one: try adding an innocuous method getInfo() to a class you've inherited, only to find that someone uses getInfo() in one of the super-classes, and it has been declared as 'friend' because the design is piss poor and it was the only way they could make the function available. Now, I have to go on a goose chase searching for all the places in the entire type hierarchy that getInfo() is used and change the code to explicitly call the other base class.
It gets to the point where its easier to name it getInfo2() (like that's good design) and get on with things.
MI is evil, evil, evil in any environment where you are trying to have code re-use and multiple teams.
I find that most programmers who insist that multiple inheritence is a good thing just don't know how to use the Composite design pattern.
Sun's decision to not support MI in Java is a sound one: the result is code that can be easily read and understood.
<< End Rant >>
Whew... I feel much better having said that...
- K

Problems of no multiple inheritance.

I have created two classes RECTANGLE with attributes Length and Height and PLANERECTANGLE, with various attributes required to specify the rectangle's center, an attribute that can be checked to see if it is inside an instance of rectangele. However, i am finding this following requirement difficult to understand.
In Question 5, we specified PlaneRectangle as a subclass of Rectangle. Suppose that we wanted the following generic behaviour to be implemented in a number of different �kinds of� shapes: being able to move a shape, check if a point is inside a shape, and check if another shape lies completely inside a specified instance of some shape. Java will not let us do this using multiple inheritance. How else could we specify this? Rewrite the Java code to illustrate use of this different method.
Thanks - Mark Costello.

The answer would be an interface
public interface Shape
public void moveShape();
public boolean containsPoint(int x, int y);
public boolean containsShape(Shape s);
Every shape class would then implement this interface:
public class Circle implements Shape
... and would need to implement those methods that
were specified (but not implemented) in the interface.

Alternative for multiple inheritance (AbstractQueue and AbstractList)

Hello all,
What is the best alternative for multiple inheritance? I want to make a list, which supports queue operations. Both AbstractQueue en AbstractList are useful, so I would like to use them both, instead of implementing the methods myself or copying the code.
thanks

Do you mean you want a class just like LinkedList?
Why don't you look at the code for LinkedList. Perhaps you could even use it.
Most Queue methods have trivial implmentations using a List.
From the source for LinkedList
public class LinkedList<E>
extends AbstractSequentialList<E>
implements List<E>, Queue<E>, Cloneable, java.io.Serializable

How java support multiple inheritance by the use of interface.

As per my understanding, Interface is just having the signatures of the methods not the implementation.
So How java support multiple inheritance by the use of interface?
Answer 1: we can institate interface reference by its implemented
class.
          �� interface inf...
          �� class aa implements inf..
          �� class bb implements inf....
           Now, inf i = new aa();
           inf i = new bb();
Answer 2: We can extends as many interface as we want in the
single
           interface.
           i.e. interface infFirst....
           interface infSecond....
           interface infThird....
           Now ,
           interface ingMulti extends infFrist, infThird...
By above two answers its not prity clear as per the multiple inheritance in C or C++.
           i.e.
           class first{
           method abc();....}
           class second{
           method bbc()......}
           class multi::first::second{
           we can call to abc();.....as well as bbc();
-Please give your important suggstion on the same.(Hope I explain it well.)
-Jeff

The keyword implement is used only for interfaces not
for abstract class. If i am wrong correct me.I believe your right, but I will double check.
As for the multiple inheritence think about the following code:
class Animal {
    // Animal generic stuff in this class
interface Eat {
    // Generic stuff that models eating behavior
interface Runs {
    // generic methods that model running behavior
public class Horse extends Animal implements Eat, Runs {
    // Stuff specific to a horse
}The Animal class is generic but has stuff in it common to all animals.
The Eat interface models behavior that is generic to eating, all living things have to eat something to survive. Herbavore are different from carnivores.
The Runs interface models generic behavior to running, such as speed. A cheeta definately runs faster than a human.
This brings us to the Horse class. It extends the Animal class because it "is-a" animal, and it implements the eat and runs interface because they are behaviors a horse has.
I hope that helps.
Extending an abstract class is the same as extending a regular class with the exception you MUST override all abstract methods in the abstract class. Thats not too difficult but I believe when designing classes, designing an abstract can be more diffecult than modeling the base class, and generic behaviors in interfaces. JMO.
JJ

Multiple inheritance in tagging interface? Is it possible?

I saw a code somewhere that goes like this:
public interface Node extends Serializable, Clonable
...Is it possible? I know that Java doesn't allow multiple inheritance and that Serializable and Clonable are tagging interfaces where no method must be implemented by the programmer.

KamenRiderZX wrote:
I know that Java doesn't allow multiple inheritanceMore exactly: Java doesn't allow multiple inheritance of implementations. Inheriting multiple interfaces ("implements" for classes, "extends" for interfaces) is fine 'though.

Question about multiple inheritance

Why does java not support multiple inheritance, but also give you the ability to use interfaces?
I've done a quick search on here which turned up the same thing as the books on java I've read - they tell me that java doesn't support multiple inheritance, and that it supports interfaces, but not why.
And from what I can see, the between multiple inheritance and single inheritance + interfaces make them seem almost equivalent, especially when you consider abstract classes. So why did the java designers make this decision?
Edit: Just to say I've never programmed in an OO language that supports multiple inheritance, so I've never had to deal with it. Also, single inheritance has never crippled any of my designs (not that there have been that many), I'm not whingeing, just asking.
Message was edited by:
Dross

Why does java not support multiple
inheritance, but also give you the ability to use
interfaces?It does support MI, just not MI of Implementation.
why.
class Beasty { }
class Horse extend Beasty {
public void gallop() { System.out.println( "horse" ); }
class Donkey extend Beasty {
public void gallop() { System.out.println( "donkey" ); }
class Mule extend House, Donkey {
Mule mule = new Mule();
mule.gallop();what would this print out.
MI of implementation makes life harder, but adds very little to the party. So why add it?

INF Looking for means to dialog C#-Dev team about multiple inheritance.

Please help.
I really need multiple inheritance with C#. Is there any forum/means to have a dialog with the dev's for C# about this? The amount of extra work and maintenance costs of not having multiple inheritance has been a big problem, but lately, it has really become
a burden for not having. The maintainability and reuse-ability of code is drastically reduced without it.

Btw, I think that this has been discussed
many times before. Almost once or twice each year since .NET is released (that's 12 years by now).
I don't think you can make them allow this feature as tons of example trying to convince them this is needed has been proved not necessary to use multiple inheritance by them.
Many of the discussions ends with something like "Java also does not support multiple inheritance but there isn't seems to be a problem for them". Maybe you can get more luck to convince Oracle to include MI in Java first.
Btw, I found it hard to believe you need Multiple Implementation Inheritance to... improve maintainability of code? WTF??? I think Multiple Implementation Inheritance has it own place in the hall of fame for the bugs it caused in languages that supports
it, even in C++.

Alternatives to multiple inheritance for my architecture (NPCs in a Realtime Strategy game)?

Coding isn't that hard actually. The hard part is to write code that makes sense, is readable and understandable. So I want to get a better developer and create some solid architecture.
So I want to do create an architecture for NPCs in a video-game. It is a Realtime
Strategy game like Starcraft, Age of Empires, Command & Conquers, etc etc.. So I'll have different kinds of NPCs. A NPC can have one to many abilities (methods) of these: Build(), Farm() and Attack().
Examples:
Worker can Build() and Farm()
Warrior can Attack()
Citizen can Build(), Farm() and Attack()
Fisherman can Farm() and Attack()
I hope everything is clear so far.
So now I do have my NPC Types and their abilities. But lets come to the technical / programmatical aspect.
What would be a good programmatic architecture for my different kinds of NPCs?
Okay I could have a base class. Actually I think this is a good way to stick with the DRY principle.
So I can have methods like WalkTo(x,y) in
my base class since every NPC will be able to move. But now lets come to the real problem. Where do I implement my abilities? (remember: Build(), Farm() and Attack())
Since the abilities will consists of the same logic it would be annoying / break DRY principle to implement them for each NPC (Worker,Warrior, ..).
Okay I could implement the abilities within the base class. This would require some kind of logic that verifies if a NPC can use ability X. IsBuilder, CanBuild,
.. I think it is clear what I want to express.
But I don't feel very well with this idea. This sounds like a bloated base class with too much functionality.
I do use C# as programming language. So multiple inheritance isn't an opinion here. Means: Having extra base classes like Fisherman
: Farmer, Attacker won't work.

Hi
PandoraElite，
You can inherit from multiple interfaces (and use explicit interface implementation), but not from classes in C#. You can almost simulate it:
In C# we don't support multiple inheritance
http://blogs.msdn.com/b/csharpfaq/archive/2004/03/07/why-doesn-t-c-support-multiple-inheritance.aspx
What would be a good programmatic architecture for my different kinds of NPCs?
In your scenario, we can define some interface ,An interface contains only the signatures of methods, properties, events or indexers. A class or struct that implements the interface must implement the members of the interface that are specified
in the interface definition.
How to use? Please refer to the following article.
http://www.codeproject.com/Articles/18743/Interfaces-in-C-For-Beginners
Best of luck!
Kristin
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Multiple inheritance in Java

Why it is sometimes said that interfaces provide a form of multiple inheritance?
Do you agree that interfaces can provide multiple inheritance? Explain.
Some people say that Java does not support multiple inheritance, and others: a class can implement more than 1 interface. Isn't that multiple inheritance?
Thanks

>
Some people say that Java does not support multiple
inheritance, and others: a class can implement more
than 1 interface. Isn't that multiple inheritance?Sort of, but you don't inherit any implementation from an interface.

No multiple inheritance in Java. Interfaces used.

Hi,
In java a class can extend only one class while the interface can extend any number of interfaces.
Class extending only one class avoids multiple inheritance.
Can you explain me the reason of avoiding this in classes and allowing interfaces to extend any number of interfaces ?

Hi,
In java a class can extend only one class while the
interface can extend any number of interfaces.
Class extending only one class avoids multiple
inheritance.
Can you explain me the reason of avoiding this in
classes and allowing interfaces to extend any number
of interfaces ?The real question is: do you have a need for multiple inheritance?
If so, I would be glad to hear about this concrete problem.

Multiple Inheritance problem persists in Interfaces

Hi,
I tentatively made a program and found that multiple inheritance problem of C++ persists even with interfaces. Although this is definetely a special case but I want to know what is this problem known as( i know that this is perhaps known as diamond problem in C++). And is there a way out of this thing.
interface one
     int i=10;
interface two
     int i=20;
interface z extends one,two
public class xyz implements z
     public static void main(String [] a)
     System.out.println(i);
}O/P
D:\Education\Java\JavaStudyRoom\Applets>javac xyz.java
xyz.java:16: reference to i is ambiguous, both variable i in one and variable i
in two match
System.out.println(i);
*^*
*1 error*
Thanks for replying

suvojit168 wrote:
I tentatively made a program and found that multiple inheritance problem of C++ persists even with interfaces. Although this is definetely a special case but I want to know what is this problem known as( i know that this is perhaps known as diamond problem in C++). And is there a way out of this thing. This is not the so called diamond inheritance problem. What you have here is an ordinary name clash. And as has been noted you can resolve it by qualifying which constant you're referring to, like
System.out.println(one.i);
For the diamond inheritance problem to apply both the one and the two interfaces would need to inherit a common ancestor (that's how the diamond is formed). Furthermore the common anscestor would need to carry implementation which would then be inherited two ways, once via one and once via two. This is the diamond inheritance problem Java is avoiding by allowing single inheritance of implementation only.
P.S. My previous post was posted my mistake.

Multiple inheritance and super

How to I get to a super function when using multiple inheritance? For example, the code below doesn't compile if Human extends Job, but works fine if Human only extends Animal.
package test;
import java.lang.System;
public class Job {
public class Animal {
public attribute isStanding:Boolean = false;
public function printStatus():Void {
System.out.println( if (isStanding) "Standing up" else "Laying down");
public class Human extends Animal, Job {
public attribute isAtWork:Boolean = false;
public function printStatus():Void {
System.out.println( if (isAtWork) "At the office" else "At home");
super.printStatus();
var person:Human = Human{};
person.printStatus();

Thanks! Works great. I never even tried that since my brain is wired to only use that syntax for calling a static method/function/field.
Note: If you change the class definition in the original sample from
public class Human extends Animal, Job {topublic class Human extends Animal {The original compiles and runs as you'd expect. So the 'super' keyword exists in some form and has the expected result for singular inheritance when used to call parent methods. I think you are thinking of using it to call constructors. In that usage I think you are right that it doesn't work at all in JavaFX.

Enumerate multiple inheritance scenarios and their java equivalents?

hi,
ppl have often said things like
"java doesn't support multiple inheritance but this is ok, since there are other mechanisms for doing the same thing (by which we nearly always mean interfaces)"
how solid a statement is this? are there any formal methods available eg smt like "over all possible inheritance trees over all possible classes, only a handful of cases are distinct when viewed from the converting-to-single-inheritance scheme"?
the two things mentioned as harder to workaround are mixins and the diamond problem - are there more?
also what other mechanism apart from interfaces (if any) are useful?
any help appreciated,
asjf

What I say is that it doesn't matter since there is
almost never any need for MI. Most of the time it is
used it is used because the developer/designer did not
understand what they were doing and it should not have
been used in the first place.
That leaves one with very few cases where it should
ever be used. And that coupled with the fact that a
developer should never use it unless they are very
experienced (so that they actually know that it should
be used,) means that practicing programmers should
leave discussion of such usages to scholarly
journals.thanks :) I guess my problem is that often with computer stuff you don't have to rely on other peoples experience about things - you can go and test it yourself
I've done very little C++ development, and so have never come across real-world multiple inheritance. I bumped into the first situation with some java code where it might've been a neat solution recently but this could easily fit into the "designer did not understand what they were doing" category from above..
will have a casual look around the scholarly journals if I can find any that look promising :)
asjf

Replacement for multiple inheritance in MovieClip class

Hello
I need your opinion about a problem.
Commonly, when you need to replace multiple inheritance, you do it by making an aggregation with one of the class involved.
But in case of MovieClip attached class, there is one more class involved in the process : the MovieClip class, and this class can't be the one aggregated because every class attached to a MovieClip need to inherits from it.
The problem is if the other class can't be also aggregated because it has some abstract class behaviour, for instance, call of a virtual function.
A solution could be making the abstract class inherit from the MovieClip class, but what if you want to reuse its behaviour in a class which have nothing to do with MovieClip ?

This is Not Supported in WebLogic that the Remote Interface extends other Interfaces. Because Annotation Processor just looks up inside the implemented interface methods. The actual interface which is Implemented by the Bean Class. So the Methods declared inside the Interface B and Interface C will be ignored and will not be available as part of the generated Stubs. Thats why u are getting NoSuchMethodError.
You can even contact Oracle Support on this...there are 3-4 Cases on it. And the Solution is Work As Designed.
Workaround is : edit your interface A as following
Declare all the Business Methods only in the Remote Interface and not inside it's Super Interfaces.
Example:
@Stateless(name="A")
@Remote({A.class})
@TransactionAttribute(TransactionAttributeType.REQUIRED)
public class AImpl implements A {
@Override
public void writeA() {
System.out.println("A");
@Override
public void writeB() {
System.out.println("B");
@Override
public void writeC() {
System.out.println("C");
@Remote
@JNDIName(A.JNDI_NAME)
public interface A extends B, C {
public static String JNDI_NAME = "A_JNDI_NAME";
void writeA();
void writeB();
void writeC();
Thanks
Jay SenSharma
http://jaysensharma.wordpress.com (WebLogic Wonders Are Here)

Multiple Inheritance

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