mjhjmtu

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:

1. Produce an enumerator which can read out the objects if no other iterator has yet been created.




2. Produce an enumerator which can read out the objects even if another iterator has already been created and used.




3. Report how many items are in the sequence




4. Report the value of the Nth item in the sequence




5. Copy a range of items from the object into an array of that type.




6. 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.