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Override an interface?

I am creating a mod for the game Minecraft, which has an interface to implement in-game commands. I need the mod to implement that interface, but override one of its methods with a non-compatible method (different return type). But I need to prevent a situation where other classes that implement that interface will not work or not be recognized by the game.
I think this would require overriding the interface with a new interface that is the same as the original, but with an overloaded version of that method to support the mod's needs. Is this possible (or is there another way I can accomplish this?)

One way to think about interfaces is as a contract.
Implementing classes must strictly adhere to this contract.
This means that the method signatures (including return values and parameter) must match exactly.
The entire point of interfaces is to define the interaction without strictly knowing the implementation.
If the interaction you are looking to implement is different then it's possible that you are trying to use something in a way it wasn't intended for.
Even if it is possible to subclass an interface, it's going to quickly get messy.
It's probably in you best interest to create a new interface (with all the other methods the same).
Since it's not going to be comparable with classes that use interface A, you are saving yourself trouble by separating it entirely.

The interfaces supplied by Mojang/forge team are intended for use within the mojang/forge code. They expect the result types to be returned that the interfaces return. If they don't get that as defined by the contract/interface the code will crash/not compile.
It seems as if you are trying to use an interface for a particular purpose for an own project/api.
Consider writing a new interface specifically for the purpose you intend to use it for. Don't modify core interfaces.
A class can inherit multiple interfaces, so that's not an issue. You can implement AND the forge/mojang interface AND your own.

You can implement the mod class. Then override the method. Also write another method which will overload the method in your implemented class. That way you won't have to change the interface.

Does really interfaces can be use to achieve multiple inheritance

I have this confusion for long time. Many people says we can achieve multiple inheritance by interfaces in languages like C# or Java which does not support it like C++ does. But my understanding of inheritance and interface says no. Because interfaces are contracts to validate an implementation which has nothing to do with behavior. Interface defines what something can do (not what something is). But inheritance is inheriting behavior and/or property from parents (like a child is getting some genetic behavior from his parent - which is inheritance). Now the child is learning a skills say painting and cooking and the interface (a certificate or contract) acts as a validation that the child is having such skills (that is what the child can do other than what he got from his parents - and that's not inheritance)
So am I understanding it wrong? And if not then why it is saying that we can achieve multiple inheritance using interfaces?

Interfaces give you multiple inheritance of a type, but not behaviour. A class implementing List and Map is a "ListMap", but the implementation has nothing (necessarily) to do with any existing List or Map implementation.
Of course using composition (which should be favored anyways), you could easily create a ListMap which delegates the calls accordingly to its list and map properties, while providing some presumably useful function that would combine their respective data.
With Java 8 interfaces are allowed default methods, so inheritance of behaviour is now also possible.

In Java you can create an interface for example Animal and an abstract class Bird.
then you can have a class MockingBird which extends the behavior of Bird and implements the actions of an Animal.
However, you can then address MockingBird as an Animal or as a Bird because it "inherits" from both.

No, interfaces cannot be used to achieve multiple inheritance
Not at all in Java, in C#, we can get closer.
I studied this matter when I wanted to implement an observer, and ended up in Robert Martin's blog: http://blog.cleancoder.com/uncle-bob/2015/01/08/InterfaceConsideredHarmful.html
After reading this post I realized he's talking about Java, but C# supports extension methods which allow you to attach behaviour on interfaces so I tried to make my implementation on some IObservable interface but obviously failed, even if I can attach behaviour in such interfaces extension methods I'm still not allowed for attaching state on them, if some day microsoft decides to implement extension properties then this combination (Interface + Extension methods + Extension properties) could be sufficient to truly simulate some useful multiple inheritance.
For now, we are stuck with duplicating the code, or the delegation code in all our observers as stated in the blog.

Why interfaces must be declared in Java?

Sometimes we have several classes that have some methods with the same signature, but that don't correspond to a declared Java interface. For example, both JTextField and JButton (among several others in javax.swing.*) have a method
public void addActionListener(ActionListener l)
Now, suppose I wish to do something with objects that have that method; then, I'd like to have an interface (or perhaps to define it myself), e.g.
public interface CanAddActionListener {
public void addActionListener(ActionListener l);
}
so that I could write:
public void myMethod(CanAddActionListener aaa, ActionListener li) {
aaa.addActionListener(li);
....
But, sadly, I can't:
JButton button;
ActionListener li;
...
this.myMethod((CanAddActionListener)button,li);
This cast would be illegal. The compiler knows that JButton is not a CanAddActionListener, because the class has not declared to implement that interface ... however it "actually" implements it.
This is sometimes an inconvenience - and Java itself has modified several core classes to implement a new interface made of old methods (String implements CharSequence, for example).
My question is: why this is so? I understand the utility of declaring that a class implements an interface. But anyway, looking at my example, why can't the compiler deduce that the class JButton "satisfies" the interface declaration (looking inside it) and accept the cast? Is it an issue of compiler efficiency or there are more fundamental problems?
My summary of the answers: This is a case in which Java could have made allowance for some "structural typing" (sort of a duck typing - but checked at compile time). It didn't. Apart from some (unclear for me) performance and implementations difficulties, there is a much more fundamental concept here: In Java, the declaration of an interface (and in general, of everything) is not meant to be merely structural (to have methods with these signatures) but semantical: the methods are supposed to implement some specific behavior/intent. So, a class which structurally satisfies some interface (i.e., it has the methods with the required signatures) does not necessarily satisfies it semantically (an extreme example: recall the "marker interfaces", which do not even have methods!). Hence, Java can assert that a class implements an interface because (and only because) this has been explicitly declared. Other languages (Go, Scala) have other philosophies.

Java's design choice to make implementing classes expressly declare the interface they implement is just that -- a design choice. To be sure, the JVM has been optimized for this choice and implementing another choice (say, Scala's structural typing) may now come at additional cost unless and until some new JVM instructions are added.
So what exactly is the design choice about? It all comes down to the semantics of methods. Consider: are the following methods semantically the same?
draw(String graphicalShapeName)
draw(String handgunName)
draw(String playingCardName)
All three methods have the signature draw(String). A human might infer that they have different semantics from the parameter names, or by reading some documentation. Is there any way for the machine to tell that they are different?
Java's design choice is to demand that the developer of a class explicitly state that a method conforms to the semantics of a pre-defined interface:
interface GraphicalDisplay {
...
void draw(String graphicalShapeName);
...
}
class JavascriptCanvas implements GraphicalDisplay {
...
public void draw(String shape);
...
}
There is no doubt that the draw method in JavascriptCanvas is intended to match the draw method for a graphical display. If one attempted to pass an object that was going to pull out a handgun, the machine can detect the error.
Go's design choice is more liberal and allows interfaces to be defined after the fact. A concrete class need not declare what interfaces it implements. Rather, the designer of a new card game component may declare that an object that supplies playing cards must have a method that matches the signature draw(String). This has the advantage that any existing class with that method can be used without having to change its source code, but the disadvantage that the class might pull out a handgun instead of a playing card.
The design choice of duck-typing languages is to dispense with formal interfaces altogether and simply match on method signatures. Any concept of interface (or "protocol") is purely idiomatic, with no direct language support.
These are but three of many possible design choices. The three can be glibly summarized like this:
Java: the programmer must explicitly declare his intent, and the machine will check it. The assumption is that the programmer is likely to make a semantic mistake (graphics / handgun / card).
Go: the programmer must declare at least part of his intent, but the machine has less to go on when checking it. The assumption is that the programmer is likely to might make a clerical mistake (integer / string), but not likely to make a semantic mistake (graphics / handgun / card).
Duck-typing: the programmer needn't express any intent, and there is nothing for the machine to check. The assumption is that programmer is unlikely to make either a clerical or semantic mistake.
It is beyond the scope of this answer to address whether interfaces, and typing in general, are adequate to test for clerical and semantic mistakes. A full discussion would have to consider build-time compiler technology, automated testing methodology, run-time/hot-spot compilation and a host of other issues.
It is acknowledged that the draw(String) example are deliberately exaggerated to make a point. Real examples would involve richer types that would give more clues to disambiguate the methods.

Why can't the compiler deduce that the class JButton "satisfies" the interface declaration (looking inside it) and accept the cast? Is it an issue of compiler efficiency or there are more fundamental problems?
It is a more fundamental issue.
The point of an interface is to specify that there is a common API / set of behaviors that a number of classes support. So, when a class is declared as implements SomeInterface, any methods in the class whose signatures match method signatures in the interface are assumed to be methods that provide that behavior.
By contrast, if the language simply matched methods based on signatures ... irrespective of the interfaces ... then we'd be liable to get false matches, when two methods with the same signature actually mean / do something semantically unrelated.
(The name for the latter approach is "duck typing" ... and Java doesn't support it.)
The Wikipedia page on type systems says that duck typing is neither "nominative typing" or "structural typing". By contrast, Pierce doesn't even mention "duck typing", but he defines nominative (or "nominal" as he calls it) typing and structural typing as follows:
"Type systems like Java's, in which names [of types] are significant and subtyping is explicitly declared, are called nominal. Type systems like most of the ones in this book in which names are inessential and subtyping is defined directly on the structure of the types, are called structural."
So by Pierce's definition, duck typing is a form of structural typing, albeit one that is typically implemented using runtime checks. (Pierce's definitions are independent of compile-time versus runtime-checking.)
Reference:
"Types and Programming Languages" - Benjamin C Pierce, MIT Press, 2002, ISBN 0-26216209-1.

Likely it's a performance feature.
Since Java is statically typed, the compiler can assert the conformance of a class to an identified interface. Once validated, that assertion can be represented in the compiled class as simply a reference to the conforming interface definition.
Later, at runtime, when an object has its Class cast to the interface type, all the runtime needs to do is check the meta data of the class to see if the class that it is being cast too is compatible (via the interface or the inheritance hierarchy).
This is a reasonably cheap check to perform since the compiler has done most of the work.
Mind, it's not authoritative. A class can SAY that it conforms to an interface, but that doesn't mean that the actual method send about to be executed will actually work. The conforming class may well be out of date and the method may simply not exist.
But a key component to the performance of java is that while it still must actually do a form of dynamic method dispatch at runtime, there's a contract that the method isn't going to suddenly vanish behind the runtimes back. So once the method is located, its location can be cached in the future. In contrast to a dynamic language where methods may come and go, and they must continue to try and hunt the methods down each time one is invoked. Obviously, dynamic languages have mechanisms to make this perform well.
Now, if the runtime were to ascertain that an object complies with an interface by doing all of the work itself, you can see how much more expensive that can be, especially with a large interface. A JDBC ResultSet, for example, has over 140 methods and such in it.
Duck typing is effectively dynamic interface matching. Check what methods are called on an object, and map it at runtime.
All of that kind of information can be cached, and built at runtime, etc. All of this can (and is in other languages), but having much of this done at compile time is actually quite efficient both on the runtimes CPU and its memory. While we use Java with multi GB heaps for long running servers, it's actually pretty suitable for small deployments and lean runtimes. Even outside of J2ME. So, there is still motivation to try and keep the runtime footprint as lean as possible.

Duck typing can be dangerous for the reasons Stephen C discussed, but it is not necessarily the evil that breaks all static typing. A static and more safe version of duck typing lies at the heart of Go's type system, and a version is available in Scala where it is called "structural typing." These versions still perform a compile time check to make sure the object obeys the requirements, but have potential problems because they break the design paradigm that has implementing an interface always an intentional decision.
See http://markthomas.info/blog/?p=66 and http://programming-scala.labs.oreilly.com/ch12.html and http://beust.com/weblog/2008/02/11/structural-typing-vs-duck-typing/ for a discusion of the Scala feature.

I can't say I know why certain design decisions were made by the Java development team. I also caveat my answer with the fact that those individuals are far smarter than I'll ever be with regards to software development and (particularly) language design. But, here's a crack at trying to answer your question.
In order to understand why they may not have chosen to use an interface like "CanAddActionListener" you have to look at the advantages of NOT using an interface and, instead, preferring abstract (and, ultimately, concrete) classes.
As you may know, when declaring abstract functionality, you can provide default functionality to subclasses. Okay...so what? Big deal, right? Well, particularly in the case of designing a language, it is a big deal. When designing a language, you will need to maintain those base classes over the life of the language (and you can be sure that there will be changes as your language evolves). If you had chosen to use interfaces, instead of providing base functionality in an abstract class, any class that implements the interface will break. This is particularly important after publication - once customers (developers in this case) start using your libraries, you can't change up the interfaces on a whim or you are going to have a lot of pissed of developers!
So, my guess is that the Java development team fully realized that many of their AbstractJ* classes shared the same method names, it would not be advantageous in having them share a common interface as it would make their API rigid and inflexible.
To sum it up (thank you to this site here):
Abstract classes can easily be extended by adding new (non-abstract) methods.
An interface cannot be modified without breaking its contract with the classes that implement it. Once an interface has been shipped, its member set is permanently fixed. An API based on interfaces can only be extended by adding new interfaces.
Of course, this is not to say that you could do something like this in your own code, (extend AbstractJButton and implement CanAddActionListener interface) but be aware of the pitfalls in doing so.

Interfaces represent a form of substitution class. A reference of type which implements or inherits from a particular interface may be passed to a method which expects that interface type. An interface will generally not only specify that all implementing classes must have methods with certain names and signatures, but it will generally also have an associated contract which specifies that all legitimate implementing classes must have methods with certain names and signatures, which behave in certain designated ways. It is entirely possible that even if two interfaces contain members with the same names and signatures, an implementation may satisfy the contract of one but not the other.
As a simple example, if one were designing a framework from scratch, one might start out with an Enumerable<T> interface (which can be used as often as desired to create an enumerator which will output a sequence of T's, but different requests may yield different sequences), but then derive from it an interface ImmutableEnumerable<T> which would behave as above but guarantee that every request would return the same sequence. A mutable collection type would support all of the members required for ImmutableEnumerable<T>, but since requests for enumeration received after a mutation would report a different sequence from requests made before, it would not abide by the ImmutableEnumerable contract.
The ability of an interface to be regarded as encapsulating a contract beyond the signatures of its members is one of the things that makes interface-based programming more semantically powerful than simple duck-typing.

Coding to interfaces? [duplicate]

This question already has answers here:
What does it mean to "program to an interface"?
(33 answers)
Closed 9 years ago.
I want to solidify my understanding of the "coding to interface" concept. As I understand it, one creates interfaces to delineate expected functionality, and then implements these "contracts" in concrete classes. To use the interface one can simply call the methods on an instance of the concrete class.
The obvious benefit is knowing of the functionality provided by the concrete class, irrespective of its specific implementation.
Based on the above:
Are there any fallacies in my understanding of "coding to interfaces"?
Are there any benefits of coding to interfaces that I missed?
Thanks.

Just one possible correction:
To use the interface one can simply call the methods on an instance of the concrete class.
One would call the methods on a reference of the type interface, which happens to use the concrete class as implementation:
List<String> l = new ArrayList<String>();
l.add("foo");
l.add("bar");
If you decided to switch to another List implementation, the client code works without change:
List<String> l = new LinkedList<String>();
This is especially useful for hiding implementation details, auto generating proxies, etc.
You'll find that frameworks like spring and guice encourage programming to an interface. It's the basis for ideas like aspect-oriented programming, auto generated proxies for transaction management, etc.

Your understanding seems to be right on. Your co-worker just swung by your desk and has all the latest pics of the christmas party starring your drunk boss loaded onto his thumbdrive. Your co-worker and you do not think twice about how this thumbdrive works, to you its a black box but you know it works because of the USB interface.
It doesn't matter whether it's a SanDisk or a Titanium (not even sure that is a brand), size / color don't matter either. In fact, the only thing that matters is that it is not broken (readable) and that it plugs into USB.
Your USB thumbdrive abides by a contract, it is essentially an interface. One can assume it fulfills some very basic duties:
Plugs into USB
Abides by the contract method CopyDataTo:
public Interface IUSB {
void CopyDataTo(string somePath); //used to copy data from the thumbnail drive to...
}
Abides by the contract method CopyDataFrom:
public Interface IUSB {
void CopyDataFrom(); //used to copy data from your PC to the thumbnail drive
}
Ok maybe not those methods but the IUSB interface is just a contract that the thumbnail drive vendors have to abide by to ensure functionality across various platforms / vendors. So SanDisk makes their thumbdrive by the interface:
public class SanDiskUSB : IUSB
{
//todo: define methods of the interface here
}
Ari, I think you already have a solid understanding (from what it sounds like) about how interfaces work.

The main advantage is that the use of an interface loosely couples a class with it's dependencies. You can then change a class, or implement a new concrete interface implementation without ever having to change the classes which depend on it.

To use the interface one can simply call the methods on an instance of the concrete class.
Typically you would have a variable typed to the interface type, thus allowing only access to the methods defined in the interface.
The obvious benefit is knowing of the functionality provided by the concrete class, irrespective of its specific implementation.
Sort of. Most importantly, it allows you to write APIs that take parameters with interface types. Users of the API can then pass in their own classes (which implement those interfaces) and you code will work on those classes even though they didn't exist yet when it was written (such as java.util.Arrays.sort() being able to sort anything that implements Comparable or comes with a suitable Comparator).
From a design perspective, interfaces allow/enforce a clear separation between API contracts and implementation details.

The aim of coding against interfaces is to decouple your code from the concrete implementation in use. That is, your code will not make assumptions about the concrete type, only the interface. Consequently, the concrete implementation can be exchanged without needing to adjust your code.

You didn't list the part about how you get an implementation of the interface, which is important. If you explicitly instantiate the implementing class with a constructor then your code is tied to that implementation. You can use a factory to get an instance for you but then you're as tied to the factory as you were before to the implementing class. Your third alternative is to use dependency injection, which is having a factory plug the implementing object into the object that uses it, in which case you escape having the class that uses the object being tied to the implementing class or to a factory.

I think you may have hinted at this, but I believe one of the biggest benefits of coding to an interface is that you are breaking dependency on concrete implementation. You can achieve loose coupling and make it easier to switch out specific implementations without changing much code. If you are just learning, I would take a look at various design patterns and how they solve problems by coding to interfaces. Reading the book Head First: Design Patterns really helped things click for me.

As I understand it, one creates interfaces to delineate expected functionality, and then implements these "contracts" in concrete classes.
The only sort of mutation i see in your thinking is this - You're going to call out expected contracts, not expected functionality. The functionality is implemented in the concrete classes. The interface only states that you will be able to call something that implements the interface with the expected method signatures. Functionality is hidden from the calling object.
This will allow you to stretch your thinking into polymorphism as follows.
SoundMaker sm = new Duck();<br/>
SoundMaker sm1 = new ThunderousCloud();
sm.makeSound(); // quack, calls all sorts of stuff like larynx, etc.<br/>
sm1.makeSound(); // BOOM!, completely different operations here...

Is OOP & completely avoiding implementation inheritance possible?

I will choose Java as an example, most people know it, though every other OO language was working as well.
Java, like many other languages, has interface inheritance and implementation inheritance. E.g. a Java class can inherit from another one and every method that has an implementation there (assuming the parent is not abstract) is inherited, too. That means the interface is inherited and the implementation for this method as well. I can overwrite it, but I don't have to. If I don't overwrite it, I have inherited the implementation.
However, my class can also "inherit" (not in Java terms) just an interface, without implementation. Actually interfaces are really named that way in Java, they provide interface inheritance, but without inheriting any implementation, since all methods of an interface have no implementation.
Now there was this article, saying it's better to inherit interfaces than implementations, you may like to read it (at least the first half of the first page), it's pretty interesting. It avoids issues like the fragile base class problem. So far this makes all a lot of sense and many other things said in the article make a lot of sense to me.
What bugs me about this, is that implementation inheritance means code reuse, one of the most important properties of OO languages. Now if Java had no classes (like James Gosling, the godfather of Java has wished according to this article), it solves all problems of implementation inheritance, but how would you make code reuse possible then?
E.g. if I have a class Car and Car has a method move(), which makes the Car move. Now I can sub-class Car for different type of cars, that are all cars, but are all specialized versions of Car. Some may move in a different way, these need to overwrite move() anyway, but most would simply keep the inherited move, as they move alike just like the abstract parent Car. Now assume for a second that there are only interfaces in Java, only interfaces may inherit from each other, a class may implement interfaces, but all classes are always final, so no class can inherit from any other class.
How would you avoid that when you have an Interface Car and hundred Car classes, that you need to implement an identical move() method for each of them? What concepts for code reuse other than implementation inheritance exist in the the OO world?
Some languages have Mixins. Are Mixins the answer to my question? I read about them, but I cannot really imagine how Mixins would work in a Java world and if they can really solve the problem here.
Another idea was that there is a class that only implements the Car interface, let's call it AbstractCar, and implements the move() method. Now other cars implement the Car interface as well, internally they create an instance of AbstractCar and they implement their own move() method by calling move() on their internal abstract Car. But wouldn't this be wasting resources for nothing (a method calling just another method - okay, JIT could inline the code, but still) and using extra memory for keeping internal objects, you wouldn't even need with implementation inheritance? (after all every object needs more memory than just the sum of the encapsulated data) Also isn't it awkward for a programmer to write dummy methods like
public void move() {
abstractCarObject.move();
}
?
Anyone can imagine a better idea how to avoid implementation inheritance and still be able to re-use code in an easy fashion?

Short answer: Yes it is possible. But you have to do it on purpose and no by chance ( using final, abstract and design with inheritance in mind, etc. )
Long answer:
Well, inheritance is not actually for "code re-use", it is for class "specialization", I think this is a misinterpretation.
For instance is it a very bad idea to create a Stack from a Vector, just because they are alike. Or properties from HashTable just because they store values. See [Effective].
The "code reuse" was more a "business view" of the OO characteristics, meaning that you objects were easily distributable among nodes; and were portable and didn't not have the problems of previous programming languages generation. This has been proved half rigth. We now have libraries that can be easily distributed; for instance in java the jar files can be used in any project saving thousands of hours of development. OO still has some problems with portability and things like that, that is the reason now WebServices are so popular ( as before it was CORBA ) but that's another thread.
This is one aspect of "code reuse". The other is effectively, the one that has to do with programming. But in this case is not just to "save" lines of code and creating fragile monsters, but designing with inheritance in mind. This is the item 17 in the book previously mentioned; Item 17: Design and document for inheritance or else prohibit it. See [Effective]
Of course you may have a Car class and tons of subclasses. And yes, the approach you mention about Car interface, AbstractCar and CarImplementation is a correct way to go.
You define the "contract" the Car should adhere and say these are the methods I would expect to have when talking about cars. The abstract car that has the base functionality that every car but leaving and documenting the methods the subclasses are responsible to handle. In java you do this by marking the method as abstract.
When you proceed this way, there is not a problem with the "fragile" class ( or at least the designer is conscious or the threat ) and the subclasses do complete only those parts the designer allow them.
Inheritance is more to "specialize" the classes, in the same fashion a Truck is an specialized version of Car, and MosterTruck an specialized version of Truck.
It does not make sanse to create a "ComputerMouse" subclase from a Car just because it has a Wheel ( scroll wheel ) like a car, it moves, and has a wheel below just to save lines of code. It belongs to a different domain, and it will be used for other purposes.
The way to prevent "implementation" inheritance is in the programming language since the beginning, you should use the final keyword on the class declaration and this way you are prohibiting subclasses.
Subclassing is not evil if it's done on purpose. If it's done uncarefully it may become a nightmare. I would say that you should start as private and "final" as possible and if needed make things more public and extend-able. This is also widely explained in the presentation"How to design good API's and why it matters" See [Good API]
Keep reading articles and with time and practice ( and a lot of patience ) this thing will come clearer. Although sometime you just need to do the work and copy/paste some code :P . This is ok, as long you try to do it well first.
Here are the references both from Joshua Bloch ( formerly working in Sun at the core of java now working for Google )
[Effective]
Effective Java. Definitely the best java book a non beginner should learn, understand and practice. A must have.
Effective Java
[Good API]Presentation that talks on API's design, reusability and related topics.
It is a little lengthy but it worth every minute.
How To Design A Good API and Why it Matters
Regards.
Update: Take a look at minute 42 of the video link I sent you. It talks about this topic:
"When you have two classes in a public API and you think to make one a subclass of another, like Foo is a subclass of Bar, ask your self , is Every Foo a Bar?... "
And in the minute previous it talks about "code reuse" while talking about TimeTask.

The problem with most example against inheritance are examples where the person is using inheritance incorrectly, not a failure of inheritance to correctly abstract.
In the article you posted a link to, the author shows the "brokenness" of inheritance using Stack and ArrayList. The example is flawed because a Stack is not an ArrayList and therefore inheritance should not be used. The example is as flawed as String extending Character, or PointXY extending Number.
Before you extend class, you should always perform the "is_a" test. Since you can't say Every Stack is an ArrayList without being wrong in some way, then you should not inheirit.
The contract for Stack is different than the contract for ArrayList (or List) and stack should not be inheriting methods that is does not care about (like get(int i) and add()). In fact Stack should be an interface with methods such as:
interface Stack<T> {
public void push(T object);
public T pop();
public void clear();
public int size();
}
A class like ArrayListStack might implement the Stack interface, and in that case use composition (having an internal ArrayList) and not inheritance.
Inheritance is not bad, bad inheritance is bad.

You could also use composition and the strategy pattern.link text
public class Car
{
private ICar _car;
public void Move() {
_car.Move();
}
}
This is far more flexible than using inheritance based behaviour as it allows you to change at runtime, by substituting new Car types as required.

You can use composition. In your example, a Car object might contain another object called Drivetrain. The car's move() method could simply call the drive() method of it's drivetrain. The Drivetrain class could, in turn, contain objects like Engine, Transmission, Wheels, etc. If you structured your class hierarchy this way, you could easily create cars which move in different ways by composing them of different combinations of the simpler parts (i.e. reuse code).

To make mixins/composition easier, take a look at my Annotations and Annotation Processor:
http://code.google.com/p/javadude/wiki/Annotations
In particular, the mixins example:
http://code.google.com/p/javadude/wiki/AnnotationsMixinExample
Note that it doesn't currently work if the interfaces/types being delegated to have parameterized methods (or parameterized types on the methods). I'm working on that...

It's funny to answer my own question, but here's something I found that is pretty interesting: Sather.
It's a programming language with no implementation inheritance at all! It knows interfaces (called abstract classes with no implementation or encapsulated data), and interfaces can inherit of each other (actually they even support multiple inheritance!), but a class can only implement interfaces (abstract classes, as many as it likes), it can't inherit from another class. It can however "include" another class. This is rather a delegate concept. Included classes must be instantiated in the constructor of your class and are destroyed when your class is destroyed. Unless you overwrite the methods they have, your class inherits their interface as well, but not their code. Instead methods are created that just forward calls to your method to the equally named method of the included object. The difference between included objects and just encapsulated objects is that you don't have to create the delegation forwards yourself and they don't exist as independent objects that you can pass around, they are part of your object and live and die together with your object (or more technically spoken: The memory for your object and all included ones is created with a single alloc call, same memory block, you just need to init them in your constructor call, while when using real delegates, each of these objects causes an own alloc call, has an own memory block, and lives completely independently of your object).
The language is not so beautiful, but I love the idea behind it :-)

Inheritance is not necessary for an object oriented language.
Consider Javascript, which is even more object-oriented than Java, arguably. There are no classes, just objects. Code is reused by adding existing methods to an object. A Javascript object is essentially a map of names to functions (and data), where the initial contents of the map is established by a prototype, and new entries can be added to a given instance on the fly.

You should read Design Patterns. You will find that Interfaces are critical to many types of useful Design Patterns. For example abstracting different types of network protocols will have the same interface (to the software calling it) but little code reuse because of different behaviors of each type of protocol.
For some algorithms are eye opening in showing how to put together the myriad elements of a programming to do some useful task. Design Patterns do the same for objects.Shows you how to combine objects in a way to perform a useful task.
Design Patterns by the Gang of Four