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How do I avoid breaking the Liskov substitution principle with a class that implements multiple interfaces?

Given the following class:
class Example implements Interface1, Interface2 {
...
}
When I instantiate the class using Interface1:
Interface1 example = new Example();
...then I can call only the Interface1 methods, and not the Interface2 methods, unless I cast:
((Interface2) example).someInterface2Method();
Of course, to make this runtime safe, I should also wrap this with an instanceof check:
if (example instanceof Interface2) {
((Interface2) example).someInterface2Method();
}
I'm aware that I could have a wrapper interface that extends both interfaces, but then I could end up with multiple interfaces to cater for all the possible permutations of interfaces that can be implemented by the same class. The Interfaces in question do not naturally extend one another so inheritance also seems wrong.
Does the instanceof/cast approach break LSP as I am interrogating the runtime instance to determine its implementations?
Whichever implementation I use seems to have some side-effect either in bad design or usage.

I'm aware that I could have a wrapper interface that extends both
interfaces, but then I could end up with multiple interfaces to cater
for all the possible permutations of interfaces that can be
implemented by the same class
I suspect that if you're finding that lots of your classes implement different combinations of interfaces then either: your concrete classes are doing too much; or (less likely) your interfaces are too small and too specialised, to the point of being useless individually.
If you have good reason for some code to require something that is both a Interface1 and a Interface2 then absolutely go ahead and make a combined version that extends both. If you struggle to think of an appropriate name for this (no, not FooAndBar) then that's an indicator that your design is wrong.
Absolutely do not rely on casting anything. It should only be used as a last resort and usually only for very specific problems (e.g. serialization).
My favourite and most-used design pattern is the decorator pattern. As such most of my classes will only ever implement one interface (except for more generic interfaces such as Comparable). I would say that if your classes are frequently/always implementing more than one interface then that's a code smell.
If you're instantiating the object and using it within the same scope then you should just be writing
Example example = new Example();
Just so it's clear (I'm not sure if this is what you were suggesting), under no circumstances should you ever be writing anything like this:
Interface1 example = new Example();
if (example instanceof Interface2) {
((Interface2) example).someInterface2Method();
}

Your class can implement multiple interfaces fine, and it is not breaking any OOP principles. On the contrary, it is following the interface segregation principle.
It is confusing why would you have a situation where something of type Interface1 is expected to provide someInterface2Method(). That is where your design is wrong.
Think about it in a slightly different way: Imagine you have another method, void method1(Interface1 interface1). It can't expect interface1 to also be an instance of Interface2. If it was the case, the type of the argument should have been different. The example you have shown is precisely this, having a variable of type Interface1 but expecting it to also be of type Interface2.
If you want to be able to call both methods, you should have the type of your variable example set to Example. That way you avoid the instanceof and type casting altogether.
If your two interfaces Interface1 and Interface2 are not that loosely coupled, and you will often need to call methods from both, maybe separating the interfaces wasn't such a good idea, or maybe you want to have another interface which extends both.
In general (although not always), instanceof checks and type casts often indicate some OO design flaw. Sometimes the design would fit for the rest of the program, but you would have a small case where it is simpler to type cast rather than refactor everything. But if possible you should always strive to avoid it at first, as part of your design.

You have two different options (I bet there are a lot more).
The first is to create your own interface which extends the other two:
interface Interface3 extends Interface1, Interface2 {}
And then use that throughout your code:
public void doSomething(Interface3 interface3){
...
}
The other way (and in my opinion the better one) is to use generics per method:
public <T extends Interface1 & Interface2> void doSomething(T t){
...
}
The latter option is in fact less restricted than the former, because the generic type T gets dynamically inferred and thus leads to less coupling (a class doesn't have to implement a specific grouping interface, like the first example).

The core issue
Slightly tweaking your example so I can address the core issue:
public void DoTheThing(Interface1 example)
{
if (example instanceof Interface2)
{
((Interface2) example).someInterface2Method();
}
}
So you defined the method DoTheThing(Interface1 example). This is basically saying "to do the thing, I need an Interface1 object".
But then, in your method body, it appears that you actually need an Interface2 object. Then why didn't you ask for one in your method parameters? Quite obviously, you should've been asking for an Interface2
What you're doing here is assuming that whatever Interface1 object you get will also be an Interface2 object. This is not something you can rely on. You might have some classes which implement both interfaces, but you might as well have some classes which only implement one and not the other.
There is no inherent requirement whereby Interface1 and Interface2 need to both be implemented on the same object. You can't know (nor rely on the assumption) that this is the case.
Unless you define the inherent requirement and apply it.
interface InterfaceBoth extends Interface1, Interface2 {}
public void DoTheThing(InterfaceBoth example)
{
example.someInterface2Method();
}
In this case, you've required InterfaceBoth object to both implement Interface1 and Interface2. So whenever you ask for an InterfaceBoth object, you can be sure to get an object which implements both Interface1 and Interface2, and thus you can use methods from either interface without even needing to cast or check the type.
You (and the compiler) know that this method will always be available, and there's no chance of this not working.
Note: You could've used Example instead of creating the InterfaceBoth interface, but then you would only be able to use objects of type Example and not any other class which would implement both interfaces. I assume you're interested in handling any class which implements both interfaces, not just Example.
Deconstructing the issue further.
Look at this code:
ICarrot myObject = new Superman();
If you assume this code compiles, what can you tell me about the Superman class? That it clearly implements the ICarrot interface. That is all you can tell me. You have no idea whether Superman implements the IShovel interface or not.
So if I try to do this:
myObject.SomeMethodThatIsFromSupermanButNotFromICarrot();
or this:
myObject.SomeMethodThatIsFromIShovelButNotFromICarrot();
Should you be surprised if I told you this code compiles? You should, because this code doesn't compile.
You may say "but I know that it's a Superman object which has this method!". But then you'd be forgetting that you only told the compiler it was an ICarrot variable, not a Superman variable.
You may say "but I know that it's a Superman object which implements the IShovel interface!". But then you'd be forgetting that you only told the compiler it was an ICarrot variable, not a Superman or IShovel variable.
Knowing this, let's look back at your code.
Interface1 example = new Example();
All you've said is that you have an Interface1 variable.
if (example instanceof Interface2) {
((Interface2) example).someInterface2Method();
}
It makes no sense for you to assume that this Interface1 object also happens to implement a second unrelated interface. Even if this code works on a technical level, it is a sign of bad design, the developer is expecting some inherent correlation between two interfaces without actually having created this correlation.
You may say "but I know I'm putting an Example object in, the compiler should know that too!" but you'd be missing the point that if this were a method parameter, you would have no way of knowing what the callers of your method are sending.
public void DoTheThing(Interface1 example)
{
if (example instanceof Interface2)
{
((Interface2) example).someInterface2Method();
}
}
When other callers call this method, the compiler is only going to stop them if the passed object does not implement Interface1. The compiler is not going to stop someone from passing an object of a class which implements Interface1 but does not implement Interface2.

Your example does not break LSP, but it seems to break SRP. If you encounter such case where you need to cast an object to its 2nd interface, the method that contains such code can be considered busy.
Implementing 2 (or more) interfaces in a class is fine. In deciding which interface to use as its data type depends entirely on the context of the code that will use it.
Casting is fine, especially when changing context.
class Payment implements Expirable, Limited {
/* ... */
}
class PaymentProcessor {
// Using payment here because i'm working with payments.
public void process(Payment payment) {
boolean expired = expirationChecker.check(payment);
boolean pastLimit = limitChecker.check(payment);
if (!expired && !pastLimit) {
acceptPayment(payment);
}
}
}
class ExpirationChecker {
// This the `Expirable` world, so i'm using Expirable here
public boolean check(Expirable expirable) {
// code
}
}
class LimitChecker {
// This class is about checking limits, thats why im using `Limited` here
public boolean check(Limited limited) {
// code
}
}

Usually, many, client-specific interfaces are fine, and somewhat part of the Interface segregation principle (the "I" in SOLID). Some more specific points, on a technical level, have already been mentioned in other answers.
Particularly that you can go too far with this segregation, by having a class like
class Person implements FirstNameProvider, LastNameProvider, AgeProvider ... {
#Override String getFirstName() {...}
#Override String getLastName() {...}
#Override int getAge() {...}
...
}
Or, conversely, that you have an implementing class that is too powerful, as in
class Application implements DatabaseReader, DataProcessor, UserInteraction, Visualizer {
...
}
I think that the main point in the Interface Segregation Principle is that the interfaces should be client-specific. They should basically "summarize" the functions that are required by a certain client, for a certain task.
To put it that way: The issue is to strike the right balance between the extremes that I sketched above. When I'm trying to figure out interfaces and their relationships (mutually, and in terms of the classes that implement them), I always try to take a step back and ask myself, in an intentionally naïve way: Who is going to receive what, and what is he going to do with it?
Regarding your example: When all your clients always need the functionality of Interface1 and Interface2 at the same time, then you should consider either defining an
interface Combined extends Interface1, Interface2 { }
or not have different interfaces in the first place. On the other hand, when the functionalities are completely distinct and unrelated and never used together, then you should wonder why the single class is implementing them at the same time.
At this point, one could refer to another principle, namely Composition over inheritance. Although it is not classically related to implementing multiple interfaces, composition can also be favorable in this case. For example, you could change your class to not implement the interfaces directly, but only provide instances that implement them:
class Example {
Interface1 getInterface1() { ... }
Interface2 getInterface2() { ... }
}
It looks a bit odd in this Example (sic!), but depending on the complexity of the implementation of Interface1 and Interface2, it can really make sense to keep them separated.
Edited in response to the comment:
The intention here is not to pass the concrete class Example to methods that need both interfaces. A case where this could make sense is rather when a class combines the functionalities of both interfaces, but does not do so by directly implementing them at the same time. It's hard to make up an example that does not look too contrived, but something like this might bring the idea across:
interface DatabaseReader { String read(); }
interface DatabaseWriter { void write(String s); }
class Database {
DatabaseConnection connection = create();
DatabaseReader reader = createReader(connection);
DatabaseReader writer = createWriter(connection);
DatabaseReader getReader() { return reader; }
DatabaseReader getWriter() { return writer; }
}
The client will still rely on the interfaces. Methods like
void create(DatabaseWriter writer) { ... }
void read (DatabaseReader reader) { ... }
void update(DatabaseReader reader, DatabaseWriter writer) { ... }
could then be called with
create(database.getWriter());
read (database.getReader());
update(database.getReader(), database.getWriter());
respectively.

With the help of various posts and comments on this page, a solution has been produced, which I feel is correct for my scenario.
The following shows the iterative changes to the solution to meet SOLID principles.
Requirement
To produce the response for a web service, key + object pairs are added to a response object. There are lots of different key + object pairs that need to be added, each of which may have unique processing required to transform the data from the source to the format required in the response.
From this it is clear that whilst the different key / value pairs may have different processing requirements to transform the source data to the target response object, they all have a common goal of adding an object to the response object.
Therefore, the following interface was produced in solution iteration 1:
Solution Iteration 1
ResponseObjectProvider<T, S> {
void addObject(T targetObject, S sourceObject, String targetKey);
}
Any developer that needs to add an object to the response can now do so using an existing implementation that matches their requirement, or add a new implementation given a new scenario
This is great as we have a common interface which acts as a contract for this common practise of adding response objects
However, one scenario requires that the target object should be taken from the source object given a particular key, "identifier".
There are options here, the first is to add an implementation of the existing interface as follows:
public class GetIdentifierResponseObjectProvider<T extends Map, S extends Map> implements ResponseObjectProvider<T, S> {
public void addObject(final T targetObject, final S sourceObject, final String targetKey) {
targetObject.put(targetKey, sourceObject.get("identifier"));
}
}
This works, however this scenario could be required for other source object keys ("startDate", "endDate" etc...) so this implementation should be made more generic to allow for reuse in this scenario.
Additionally, other implementations may require more context information to perform the addObject operation... So a new generic type should be added to cater for this
Solution Iteration 2
ResponseObjectProvider<T, S, U> {
void addObject(T targetObject, S sourceObject, String targetKey);
void setParams(U params);
U getParams();
}
This interface caters for both usage scenarios; the implementations that require additional params to perform the addObject operation and the implementations that do not
However, considering the latter of the usage scenarios, the implementations that do not require additional parameters will break the SOLID Interface Segregation Principle as these implementations will override getParams and setParams methods but not implement them. e.g:
public class GetObjectBySourceKeyResponseObjectProvider<T extends Map, S extends Map, U extends String> implements ResponseObjectProvider<T, S, U> {
public void addObject(final T targetObject, final S sourceObject, final String targetKey) {
targetObject.put(targetKey, sourceObject.get(U));
}
public void setParams(U params) {
//unimplemented method
}
U getParams() {
//unimplemented method
}
}
Solution Iteration 3
To fix the Interface Segregation issue, the getParams and setParams interface methods were moved into a new Interface:
public interface ParametersProvider<T> {
void setParams(T params);
T getParams();
}
The implementations that require parameters can now implement the ParametersProvider interface:
public class GetObjectBySourceKeyResponseObjectProvider<T extends Map, S extends Map, U extends String> implements ResponseObjectProvider<T, S>, ParametersProvider<U>
private String params;
public void setParams(U params) {
this.params = params;
}
public U getParams() {
return this.params;
}
public void addObject(final T targetObject, final S sourceObject, final String targetKey) {
targetObject.put(targetKey, sourceObject.get(params));
}
}
This solves the Interface Segregation issue but causes two more issues... If the calling client wants to program to an interface, i.e:
ResponseObjectProvider responseObjectProvider = new GetObjectBySourceKeyResponseObjectProvider<>();
Then the addObject method will be available to the instance, but NOT the getParams and setParams methods of the ParametersProvider interface... To call these a cast is required, and to be safe an instanceof check should also be performed:
if(responseObjectProvider instanceof ParametersProvider) {
((ParametersProvider)responseObjectProvider).setParams("identifier");
}
Not only is this undesirable it also breaks the Liskov Substitution Principle - "if S is a subtype of T, then objects of type T in a program may be replaced with objects of type S without altering any of the desirable properties of that program"
i.e. if we replaced an implementation of ResponseObjectProvider that also implements ParametersProvider, with an implementation that does not implement ParametersProvider then this could alter the some of the desirable properties of the program... Additionally, the client needs to be aware of which implementation is in use to call the correct methods
An additional problem is the usage for calling clients. If the calling client wanted to use an instance that implements both interfaces to perform addObject multiple times, the setParams method would need to be called before addObject... This could cause avoidable bugs if care is not taken when calling.
Solution Iteration 4 - Final Solution
The interfaces produced from Solution Iteration 3 solve all of the currently known usage requirements, with some flexibility provided by generics for implementation using different types. However, this solution breaks the Liskov Substitution Principle and has a non-obvious usage of setParams for the calling client
The solution is to have two separate interfaces, ParameterisedResponseObjectProvider and ResponseObjectProvider.
This allows the client to program to an interface, and would select the appropriate interface depending on whether the objects being added to the response require additional parameters or not
The new interface was first implemented as an extension of ResponseObjectProvider:
public interface ParameterisedResponseObjectProvider<T,S,U> extends ResponseObjectProvider<T, S> {
void setParams(U params);
U getParams();
}
However, this still had the usage issue, where the calling client would first need to call setParams before calling addObject and also make the code less readable.
So the final solution has two separate interfaces defined as follows:
public interface ResponseObjectProvider<T, S> {
void addObject(T targetObject, S sourceObject, String targetKey);
}
public interface ParameterisedResponseObjectProvider<T,S,U> {
void addObject(T targetObject, S sourceObject, String targetKey, U params);
}
This solution solves the breaches of Interface Segregation and Liskov Substitution principles and also improves the usage for calling clients and improves the readability of the code.
It does mean that the client needs to be aware of the different interfaces, but since the contracts are different this seems to be a justified decision especially when considering all the issues that the solution has avoided.

The problem you describe often comes about through over-zealous application of the Interface Segregation Principle, encouraged by languages' inability to specify that members of one interface should, by default, be chained to static methods which could implement sensible behaviors.
Consider, for example, a basic sequence/enumeration interface and the following behaviors:
Produce an enumerator which can read out the objects if no other iterator has yet been created.
Produce an enumerator which can read out the objects even if another iterator has already been created and used.
Report how many items are in the sequence
Report the value of the Nth item in the sequence
Copy a range of items from the object into an array of that type.
Yield a reference to an immutable object that can accommodate the above operations efficiently with contents that are guaranteed never to change.
I would suggest that such abilities should be part of the basic sequence/enumeration interface, along with a method/property to indicate which of the above operations are meaningfully supported. Some kinds of single-shot on-demand enumerators (e.g. an infinite truly-random sequence generator) might not be able to support any of those functions, but segregating such functions into separate interfaces will make it much harder to produce efficient wrappers for many kinds of operations.
One could produce a wrapper class that would accommodate all of the above operations, though not necessarily efficiently, on any finite sequence which supports the first ability. If, however, the class is being used to wrap an object that already supports some of those abilities (e.g. access the Nth item), having the wrapper use the underlying behaviors could be much more efficient than having it do everything via the second function above (e.g. creating a new enumerator, and using that to iteratively read and ignore items from the sequence until the desired one is reached).
Having all objects that produce any kind of sequence support an interface that includes all of the above, along with an indication of what abilities are supported, would be cleaner than trying to have different interfaces for different subsets of abilities, and requiring that wrapper classes make explicit provision for any combinations they want to expose to their clients.

Interface method referencing a concrete class as parameter causes coupling?

I was thinking about programming to interfaces and not to concrete classes, but I had a doubt: should any interface method be able to hold references to concrete classes?
Suppose the following scenarios:
1)
public interface AbsType1 {
public boolean method1(int a); // it's ok, only primitive types here
}
2)
public interface AbsType2 {
public boolean method2(MyClass a); // I think I have some coupling here
}
Should I choose a different design here in order to avoid the latter? e.g.
public interface MyInterface {} // yes, this is empty
public classe MyClass implements MyInterface {
// basically identical to the previous "MyClass"
}
public interface AbsType2 {
public boolean method2(MyInterface a); // this is better (as long as the
// interface is really stable)
}
But there's still something that doesn't convince me... I feel uncomfortable with declaring an empty interface, though I saw someone else doing so.
Maybe and Abstract Class would work better here?
I am a little bit confused.
EDIT:
Ok, I'll try to be more specific by making an example. Let's say I'm desining a ShopCart and I want of course to add items to the cart:
public interface ShopCart {
public void addArticle(Article a);
}
Now, if Article were a concrete class, what if its implementation changes over time? This is why I could think of making it an Interface, but then again, it's probably not suitable at least at a semantic level because interfaces should specify behaviours and an Article has none (or almost none... I guess it's a sort of entity class).
So, probably I'm ending up right now to the conclusion that making Article an abstract class in this case would be the best thing... what do you think about it?

I would use interfaces because composition is much better than inheritance. "Should any interface method be able to hold references to concrete classes ?", why it shouldn't? Some classes within package are coupled, it's a fact and common use technique. When you marked this relation in interface then you see on which classes is dependent your implementation. Dependency or composition relations are not inheritance so a i would avoid abstract class.

In my opinion Interfaces are fine for all types where the implementation may vary. But if you define a module which introduces a new type, that isn't intended to have alternative implementations then there is no need to define it as an Interface in the first place. Often this would be over-design in my opinion. It depends on the problem domain and often on the way how support testing or AOP-weaving.
For example consider a 2D problem domain where you need to model a Location as a type. If it is clear that a Location is always represented by a x and y coordinate, you may provide it as a Class. But if you do not know which properties a Location could have (GPS data, x, y, z coordinates, etc.) but you rely on some behavior like distance(), you should model it as an Interface instead.

If there are no public methods which AbsType would access in MyClass then the empty interface is probably not a good way to go.
There is no interface declaration (contract) for static methods, which otherwise might make sense here.
So, if AbsType is not going to use any methods from MyClass/MyInterface, then I assume it's basically only storing the class object for some other purpose. In this case, consider using generics to make clear how you want AbsType to be used without coupling closely to the client's code, like
public class AbsType3<C extends Class<?>> {
public boolean method3(T classType) {...}
}
Then you can restrict the types of classes to allow if needed by exchanging the <C extends Class<?>> type parameter for something else which may also be an interface, like
<C extends Class<Collection<?>>>.
Empty interfaces are somewhat like boolean flags for classes: Either a class implements the interface (true) or it doesn't (false). If at all, these marker interfaces should be used to convey an significant statement about how a class is meant to be (or not to be) used, see Serializable for example.

When and where to use abstract classes and when and where to use interfaces in Java?

Abstract classes and interfaces play very a important role in Java and they have their own importance in certain situations. They possess certain special characteristics. There are some observable differences between them. Let me describe some a few of them.
One of the major differences between an interface and an abstract class is that an abstract class can never be instantiated, an interface however can.
Both of them can never be declared as final obviously, because they are to be inherited by some other non-abstract class(es).
Both of them can never have static methods. Neither concrete nor abstract (abstract static methods indeed and in fact, don't exist at all).
An interface can never never have concrete methods (a method with it's actual implementation), an abstract class however can have concrete methods too.
An interface can not have constructors, an abstract class can however can have.
Two obvious questions are likely to arise here.
An abstract class can never be instantiated because it is by nature, not a fully implemented class and it's full implementation requires it to be inherited by some other non-abstract class(es). If it is so then, an abstract class should not have a constructor of it's own because a constructor implicitly returns an object of it's own class and an abstract class by itself can not be instantiated hence, it should not be able to have a constructor of it's own.
An interface somewhat looks better and more appropriate to use than an abstract class, since it imposes less restrictions than what those are imposed by an abstract class. In which very specific situations, an interface is useful and in which very specific situations, an abstract class is appropriate? Hope! the boldface letters would be taken into much consideration.

First off, you're factually wrong in a few places:
One of the major differences between an interface and an abstract class is that an abstract class can never be instantiated, an interface however can.
Wrong. An abstract and interface can both be instantiated anonymously.
Both of them can never be declared as final obviously, because they are to be inherited by some other non-abstract class(es).
True, although I personally see no reason why interfaces couldn't have been able to be final so they couldn't be extended, but that's just me. I see why they made the decision they did.
Both of them can never have static methods. Neither concrete nor abstract (abstract static methods indeed and in fact, don't exist at all).
Abstract classes can have static methods; sorry!
An interface can never never have concrete methods (a method with it's actual implementation), an abstract class however can have concrete methods too.
Yes, that's one of the the primary differences between them.
An interface can not have constructors, an abstract class can however can have.
Yes, that's true.
Now, let's move on to your questions:
Your first paragraph doesn't have a question in it. What was the question there? If it was "Why allow abstract classes to have constructors if you can't instantiate them?" the answer is so child classes can use it. Here's an example
abstract class Parent {
String name;
int id;
public Parent(String n, int i) { name = n; id = i; }
}
class Child extends Parent {
float foo;
public Child(String n, int i, float f) {
super(n,i);
foo = f;
}
}
// later
Parent p = new Parent("bob",12); // error
Child c = new Child("bob",12); // fine!
Your second paragraph has a question but is malformed. I think you're simply missing an 'is' in there... :) The answer to it is as follows:
You use an interface when you want to define a contract. Here's a very specific example:
public interface Set<E> {
int size(); // determine size of the set
boolean isEmpty(); // determine if the set is empty or not
void add(E data); // add to the set
boolean remove(E data); // remove from the set
boolean contains(E data); // determine if set holds something
}
Four common methods to all sets.
You use an abstract class when you want to define SOME of the behavior, but still have the contract
public abstract class AbstractSet<E> implements Set<E> {
// we define the implementation for isEmpty by saying it means
// size is 0
public boolean isEmpty() { return this.size() == 0; }
// let all the other methods be determined by the implementer
}

Most of the time, when the discussion comes up between deciding if you should use interfaces or abstract classes, it ends up with definitions of how to use them, but not always why and when? Also the obvious other concrete classes and utility classes that you may end up using as well aren't always brought up. Really, in my thinking the correct way to answer the question is to determine the context you are dealing with regarding the domain or entity objects, namely what is your use case?
From a very high level, Java consists of objects (entities or domain objects that can model objects in the real world) that communicate with each other using methods. In any event, you want to model behavior with interfaces and use abstract classes when you have inheritance.
In my personal experience, I do this using a top down and then bottom up approach. I start looking for inheritance by looking at the use case and seeing what classes I will need. Then I look to see if there is a superClassOrInterfaceType (since both classes and interfaces define types, I'm combining them into a one word for simplicity. Hopefully it doesn't make it more confusing) domain object that would encompass all the objects, as in a superClassOrInterfaceType of vehicle if I'm working on a use case dealing with subtypeClassOrInterfaceTypes like: cars, trucks, jeeps, and motorcycles for example. If there is a hierarchy relationship, then I define the superClassOrInterfaceType and subtypeClassOrInterfaceTypes.
As I said, what I generally do first is to look for a common domain superClassOrInterfaceType for the objects I'm dealing with. If so, I look for common method operations between the subtypeClassOrInterfaceTypes. If not, I look to see if there are common method implementations, because even though you may have a superClassOrInterfaceType and may have common methods, the implementations may not favor code reuse. At this point, if I have common methods, but no common implementations, I lean towards an interface. However, with this simplistic example, I should have some common methods with some common implementations between the vehicle subtypeClassOrInterfaceTypes that I can reuse code with.
On the other hand, if there is no inheritance structure, then I start from the bottom up to see if there are common methods. If there are no common methods and no common implementations, then I opt for a concrete class.
Generally, if there is inheritance with common methods and common implementations and a need for multiple subtype implementation methods in the same subtype, then I go with an abstract class, which is rare, but I do use it. If you just go with Abstract classes just because there is inheritance, you can run into problems if the code changes a lot. This is detailed very well in the example here: Interfaces vs Abstract Classes in Java, for the different types of domain objects of motors. One of them required a dual powered motor, that required multiple subtype implementation methods to be used in a single subtype class.
To sum it all up, as a rule you want to define behaviors (what the objects will do) with interfaces and not in Abstract classes. Abstract classes focus on an implementation hierarchy and code reuse.
Here are some links that go into greater details on this.
Thanks Type & Gentle Class
The Magic behind Subtype Polymorphism
Maximize Flexibility with Interfaces & Abstract Classes
Interfaces vs Abstract Classes in Java

Why is java.util.Observable not an abstract class?

I just noticed that java.util.Observable is a concrete class. Since the purpose of Observable is to be extended, this seems rather odd to me. Is there a reason why it was implemented this way?
I found this article which says that
The observable is a concrete class, so the class deriving from it must be determined upfront, as Java allows only single inheritance.
But that doesn't really explain it to me. In fact, if Observable were abstract, the user would be forced to determine the class deriving from it.

Quite simply it's a mistake that Observable is a class at all, abstract or otherwise.
Observable should have been an interface and the JDK should have provided a convenient implementation (much like List is an interface and ArrayList is an implementation)
There are quite a few "mistakes" in java, including:
java.util.Stack is a class, not an interface (like Observable, bad choice)
java.util.Properties extends java.util.Hashtable (rather than uses one)
The java.util.Date class is a bit of a mess, and is not immutable!
The java.util.Calendar class is a real mess
No unsigned 'byte' type (this is a real pain and the source of many low-level bugs)
java.sql.SQLException is a checked exception
Arrays don't use Arrays.toString(array) as their default toString() (how many SO questions has this caused?)
Cloneable shouldn't be a marker interface; it should have the clone() method and Object.clone() should not exist
While on the soapbox, in terms of the language itself, IMHO:
== should execute the .equals() method (this causes loads of headaches)
identity comparison == should either be === like javascript or a dedicated method like boolean isIdentical(Object o), because you hardly ever need it!
< should execute compareTo(Object o) < 0 for Comparable objects (and similarly for >, <=, >=)

As a first approach, one could think that this is done to allow the user to use composition instead of inheritance, which is very convenient if your class already inherits from another class, and you cannot inherit from Observable class also.
But if we look to the source code of Observable, we see that there is an internal flag
private boolean changed = false;
That is checked everytime the notifyObservers is invoked:
public void notifyObservers(Object arg) {
Object[] arrLocal;
synchronized (this) {
if (!changed) return;
arrLocal = obs.toArray();
clearChanged();
}
for (int i = arrLocal.length-1; i>=0; i--)
((Observer)arrLocal[i]).update(this, arg);
}
But from a class composed by this Observable, we cannot change this flag, since it is private, and the methods provided to change it are protected.
This means that the user is forced to subclass the Observable class, and I would say that the lack of the "abstract" keyword is just a "mistake".
I would say that this class is a complete screwup.

In java what does extending from a comparable mean

I see code like this
class A implements Comparable<A> {
}
What does this mean, what are the advantages and disadvantages of it?

It means that class is committed to respond to the methods defined by the "interface" Comparable.
The advantage you have with this ( and any other "implements" declaration ) it that you can "abstract" the type of the object and code to the interface instead.
Consider this
class A implements Comparable {
....
}
class B implements Comparable {
....
}
class C implements Comparable {
....
}
You then may code something that can use Comparable instead of a specific type:
public void doSomethingWith( Comparable c ) {
c.compareTo( other ); // something else...
}
And invoke it like:
doSomethingWith( new A() );
doSomethingWith( new B() );
doSomethingWith( new C() );
Because you don't really care what the type of the class is, you just care it does implement the interface.
This ( program to the interface rather to the implementation ) is one of the most powerful techniques in OO programming world, because it promotes low-coupling.

Implementing a comparable interface means that A can be compared with other instances of A.
Many operations in java that involve sorting use the methods defined in the Comparable interface to determine if instances of A are greater then less or equal to other instances.
By implementing these methods you are able to use a lot of handy features such as java sort, use instances of A as keys for binary trees, and more.

It means that class A can be sorted using the Comparable compareTo method:
A a1 = new A(1);
A a2 = new A(3);
// -1, 0, or 1 depending on whether a2 is less than, equal to, or greater than a1
int order = a1.compareTo(a2);
Comparable uses the natural ordering for your class.
Another way to go since Java 5 is Comparator. You can pass this object around and have more than one way to compare and sort the target class. For example, sometimes you might want to sort a Name class by first name, other times by last name. Comparable only gives you one way to do it, but you can have several Comparator instances.

It means that the class is one which can be operated on by functions which expect their arguments to be objects that can be compared with other objects of the same type (such as the pre-defined sorting functionality for lists).
Implementing the Comparable interface means that the class supports certain functions which the interface requires (specifically, the compareTo() method), that the sorting (or other) operations being performed on the class will utilize to do their work without having to care about the rest of the class.
For more details:
http://java.sun.com/j2se/1.4.2/docs/api/java/lang/Comparable.html

In addition to what everyone else said, by implementing an interface (or extending a class), you get compiler enforcement of the contract of the supertype. In the case of 'Comparable', that means that you get a compiler error if you fail to implement the 'int compareTo(A anA)' method in the implementing class. Adding the annotation '#Override' to the implementing method provides even more compile-time safety; if you fail to implement the method with the right signature the compiler will tell you. Compile-time errors are much, much easier and cheaper to fix than run-time errors. Furthermore, implementing an interface allows any instance of an implementing class to be treated as the interface type for methods (and constructors) that take the interface type as an argument or generic parameter. For example, the 'java.util.Collections.max(Collection coll)' method takes a collection whose base type must extend 'Comparable'.
http://download.oracle.com/javase/7/docs/api/java/util/Collections.html#max(java.util.Collection)

It means that objects of this class can be easily sorted in collections because they can be compared to each other. The other option is to implement a Comparator which is a class responsible for sorting other classes. The Comparable puts the sorting logic directly in the class to be sorted; the Comparator puts the sorting logic in a different class.