Develop Reference

Does java support "Soft" interfaces?

Consider the following scenario:
Say that you created an interface Foo:
public interface Foo {
public void bar();
}
And say that there is an old class SomeOldClass in a certain library that you want to use. It already has the bar() method, but does not explicitly implement Foo.
You have written the following code for all classed that implement Foo:
public <T extends Foo> T callBarOnThird(List<T> fooList){
return fooList.get(2).bar();
}
And now you want it to also work for SomeOldClass. You dont have access to the source code of this class, so you can't modify it.
Is there a way to declare Foo or something similar as some sort of "soft" interface, (as in where any class that implements all the required methods would be accepted as an implicit implementation of the soft interface)? If not, how would you solve this with code that is as clean as possible?

No, it does not.
You have to provide an adapter instance (there are several methods and tools to help with that, but Java does not do it "implicitly").

Java is statically typed and dynamically bind.
Dynamically bind: This means that the linking between a method signature and its implementation happens at runtime. For example.
For example
public interface MyInterface {
void doStuff();
}
public class MyFirstImpl implements MyInterface {
#Override
public void doStuff() {
// do some stuff here
}
}
public class MySecondImpl implements MyInterface {
#Override
public void doStuff() {
// do some stuff here
}
}
So if you would have the next snippet
MyInterface test; // pointing to either MyFirstImpl or MySecondImpl
test.doStuff();
The JVM will determine at runtime weather to call the doStuff method from MyFirstImpl or MySecondImpl based on the runtime type of the object.
Statically typed: This means that the JVM will check at compile time weather a there is a method to call regardless of the implementation.
For example:
public interface MyInterface {
void doStuff();
}
public class MyFirstImpl implements MyInterface {
// no override here
public void doStuff() {
// do some stuff here
}
}
public class MySecondImpl implements MyInterface {
// no override here
public void doStuff() {
// do some stuff here
}
}
So if you would have the next snippet
MyInterface test; // pointing to either MyFirstImpl or MySecondImpl
test.doStuff();
The compiler will complain because it can't ensure at compile time that regardless of the implementation of MyInterface there is a doStuff method to call (although in this case, both implementations of MyInterface define a doStuff method).
This ensures that you won't get a NoSuchMethodException at runtime, if you would pass, for example, the next implementation.
public class MySecondImpl implements MyInterface {
// no override here
// no doStuff method
}
This adds some type safety to the language at the cost of some rigidity (since you are able to determine the issue earlier than at runtime and therefore you have a shorter feedback loop, at the cost of the scenario in which all the implementations actually expose the method not working out of the box).
How you should refactor your code:
Create a wrapper over the third party library and expose the interface from the wrapper.
public interface Foo {
void bar();
}
public class ThirdPartyFooWrapper implements Foo {
private SomeOldClass oldClass;
public ThordPartyFooWrapper (SomeOldClass oldClass){
this.oldClass = oldClass;
}
#Override
public void bar() {
this.oldClass.bar();
}
}
Then, in your code use ThirdPartyFooWrapper instead of SomeOldClass.
Hope this answers your question!

Extension to Thilos answer.
You can also use the decorator to handle this
public <T extends Foo> T callBarOnThird(List<T> fooList){
return new BarDecorator(fooList.get(2)).bar();
}
Inside the decorator, you can check if given Object is the instance of Foo or not then do operations accordingly.

How to use an interface as a parameter for another interface's function?

Suppose I have this two interfaces...
public interface ObjectInterface { /* empty interface */ }
public interface ObjectHandler
{
void handle(ObjectInterface object);
}
...and also a concrete class which implements the ObjectInterface:
public class ConcreteObject implements ObjectInterface { /* ... */ }
Why is this not allowed in Java?
public class ConcreteHandler implements ObjectHandler
{
#Override
public void handle(ConcreteObject object) // <-- ObjectInterface implementation
{
// ...
}
}
A ConcreteObject implements the ObjectInterface interface afterall.
However, is there a way to design my classes in Java keeping this pattern of abstraction?

You can parameterize ObjectHandler with the class of accepted ObjectInterfaces:
public interface ObjectHandler<T extends ObjectInterface>
{
void handle(T object);
}
and use it as follows:
public class ConcreteHandler implements ObjectHandler<ConcreteObject>
{
#Override
public void handle(ConcreteObject object) // <-- now this works
{
// ...
}
}

Interface method is the contract,
void handle(ObjectInterface object);
Therefore the implementation of the interface should adhere to the contract. So it won't let you limit the object type it accepts.
handle(ConcreteObject object){...}
Above limits the input to the method, as its just a one implementation of ObjectInterface. Think what happens if there's another implementation called, ConcreteObject2 which implements the ObjectInterface. According to your code handle method won't accepts the ConcreteObject2 type objects.
If you want to keep the pattern you can use java generics

Because you're breaking the contract of the interface, and thus breaking the Liskov substitutioon principle.
The interface says: handle() can take any instance of ObjectInterface as argument. That's the contract: any implementation of handle() must accept any instance of ObjectInterface.
But the class says: handle() only accepts instances of ConcreteHandler.

Java cast interface to class

I have a question about interface and class implementing interface.
This is my code:
interface iMyInterface {
public iMethod1();
}
public class cMyClass implements iMyInterface {
public iMethod1() {
// some code
}
protected iMethod2() {
// some code
}
}
I would like to create an instance of iMyInterface as this :
iMyInterface i = new cMyClass();
i.iMethod1();
It's ok, but how can I call iMethod2() from my interface instance? Is this working and safe:
((cMyClass)i).iMethod2();
Thanks for help.

Yes, that will work (if you change the declaration of cMyClass to implement iMyInterface) and it's safe so long as the reference really does refer to an instance of cMyClass.
However, it's a generally bad idea. The whole point of using an interface is to be able to work with any implementation - it's to separate the abstraction from the implementation. If you're then going to require a specific implementation, you might as well make the type of i just cMyClass to start with.
So suppose instead of calling the cMyClass constructor yourself, you receive a method parameter of type iMyInterface - it's a bad idea to cast to cMyClass at that point, as it could be a different implementation of the interface.
(On a separate note, it's a good idea to start following Java naming conventions, which state that classes and interfaces should be Pascal-cased - so ditch the c and i prefixes.)

It will work (provided that cMyClass implements iMyInterface and you are in scope of protected modifier) but that is not the correct OO approch.
If you want to use iMethod2 consider:
adding it to the interface
create another interface containing that method
Use cMyClass myClass = new cMyClass();

There is some another alternative to cast Interface to a class. Here is example how.
interface iMyInterface {
void iMethod1();
}
public class cMyClass implements iMyInterface {
private iMyInterface myInterface;
public cMyClass() {
myInterface = this;
}
public void iMethod1(){
System.out.println("Print from cMyClass iMethod1()");
}
protected void iMethod2() {
System.out.println("Print from cMyClass iMethod2()");
}
/**
* Getter so we can access to the interface methods.
* #return
*/
public iMyInterface getMyInterface() {
return myInterface;
}
}
And to get values from your interface, here is the code example.
public class Main {
public static void main(String[] args) {
cMyClass myClass = new cMyClass();
myClass.getMyInterface().iMethod1();
myClass.iMethod2();
}
}
Output:
Print from cMyClass iMethod1()
Print from cMyClass iMethod2()
I think this is a good example how you can separate interface code from the class code. Just create instance of the interface and every method use through getter with that interface.

Is there a pattern for this? Common base class with special actions for certain child classes

I have code that when given a thing it needs to sort out what specific kind of thing it is and then take special actions based on that. The possible classes it could be are all desc
public void doSomething(BaseThing genericThing)
{
if (genericThing instanceof SpecificThing)
{
SpecificThingProcessor stp = new SpecificThingProcessor((SpecificThing) genericThing);
}
else if (genericThing instanceof DifferentThing)
{
DifferentThingProcessor dtp = new DifferentThingProcessor((DifferentThing) genericThing);
}
else if (genericThing instanceof AnotherThing){
AnotherThingProcessor atp = new AnotherThingProcessor((AnotherThing) genericThing);
}
else
{
throw new IllegalArgumentException("Can't handle thing!");
}
}
Is there a pattern or better way of handling this? Unfortunately the operations being performed do not lend themselves to generalization around the BaseThing, they have to be done for each specific class of thing.

The best option I can think of is to abstract the functionality in to an Interface and have each type implement that Interface.
If you add a little more detail about what you're trying to do based on the types, I could make a better suggestion (possibly with some sample code).
EDIT
After the edit, there is definitely a clear way to do this. Each Processor will implement a specific Interface:
public interface IProcessor
{
void Process();
}
public class SpecificThingProcessor : IProcessor
{
public void Process() { /* Implementation */ }
}
public class DifferentThingProcessor : IProcessor
{
public void Process() { /* Implementation */ }
}
public class AnotherThingProcessor : IProcessor
{
public void Process() { /* Implementation */ }
}
Each BaseThing must implement a method to return the specific processor:
public abstract class BaseThing
{
public abstract IProcessor GetProcessor();
}
public class SpecificThing : BaseThing
{
public override IProcessor GetProcessor()
{
return new SpecificThingProcessor();
}
}
public class DifferentThing : BaseThing
{
public override IProcessor GetProcessor()
{
return new DifferentThingProcessor();
}
}
And then your method will simply be:
public void doSomething(BaseThing genericThing)
{
IProcessor processor = genericThing.GetProcessor();
processor.Process();
}

You should define a method in BaseThing to be overridden by the specific Things.
In other words you should be using a virtual function.
The operations being performed are not
being performed on the generic thing.
Depending on its specific type, a
"Producer" class needs to be
instantiated to deal with the correct
type of thing. It is not appropriate
to call the Producer from the
BaseThing subclasses
You can still do: thing.GetProcessor(), and have each thing return the specific processor its used for it. Processors would of course implement a common interface or base class.
For another alternative, this hits my java limit, but I'm sure you should be able to do something along these lines:
store a list/dictionary of type, processor constructor.
Get the type of genericThing instance you are receiving
search for the type in the list and call the corresponding constructor.

The visitor pattern is exactly what you're trying to achieve. However, a "good old-fashioned polymorphism" should do just fine for what you need. For example :
abstract class BaseThing {
abstract public void doSomething();
}
class ThingA extends BaseThing {
public void doSomething() {
System.out.println("ThingA...");
}
}
class ThingB extends BaseThing {
public void doSomething() {
System.out.println("ThingB...");
}
}
class ThingC extends BaseThing {
public void doSomething() {
throw new UnsupportedOperationException("Cannot call this on ThingC");
}
}
and then
class ThingHandler {
public void doSomething(BaseThing thing) {
try {
thing.doSomething();
} catch (UnsupportedOperationException e) {
throw new IllegalArgumentException("Can't handle thing!");
}
}
}
Thus
ThingHandler handler = new ThingHandler();
handler.doSomething(new ThingA()); // -> ThingA...
handler.doSomething(new ThingB()); // -> ThingB...
handler.doSomething(new ThingC()); // -> IllegalArgumentException: Can't handle thing!
You have mentioned "it needs to sort out what specific kind of thing it is", so all you need now is have your BaseThing have an abstract method that will return a Comparator and each ThingA, etc. will implement it and return the proper comparator for the ThingHandler class to sort. Each BaseThing implementation can perform the specific operations or return some kind of value that you'd need in ThingHandler (you could even pass the ThingHandler instance in the BaseThing.doSomething method...)
But if the Visitor pattern is really what you need, here is an example for your need :
interface IThing {
public void accept(ThingHandler handler);
}
interface IThingHandler {
public void visit(ThingA a);
public void visit(ThingB b);
//...
}
class ThingA implements IThing {
public void accept(IThingHandler h) {
h.visit(this);
}
public String getSomeValueA() {
return "Thing A";
}
}
class ThingB implements IThing {
public void accept(IThingHandler h) {
h.visit(this);
}
public String getSomeValueB() {
return "Thing B";
}
}
// ...
class ThingHandler implements IThingHandler {
public void visit(ThingA thing) {
// sort according to ThingA
System.out.println(thing.getSomeValueA() + " has visited");
doSomething(thing);
}
public void visit(ThingB thing) {
// sort according to ThingB
System.out.println(thing.getSomeValueB() + " has visited");
doSomething(thing);
}
private void doSomething(IThing thing) {
// do whatever needs to be done here
}
}
Then
IThingHandler handler = new ThingHandler();
new ThingA().accept(handler); // -> Thing A has visited
new ThingB().accept(handler); // -> Thing B has visited
//...
But since this means maintaining the IThingHandler interface every time a new IThing class is implemented, I prefer suggesting the first modified/simplified implementation of the pattern. However, feel free to adapt the pattern for your need and don't stop yourself because it doesn't exactly look like the described visitor pattern.
The two questions to ask are
"who is responsible to handle the operation?"
"who is responsible to hold the necessary data to perform the operation?"
I usually prefer keeping most of the concrete at the same place and generalize elsewhere; it helps maintaining (i.g. adding and removing features). Although the visitor pattern helps to centralize the operation in a same class...

This sounds like one of the basic ideas of object-oriented programming. You create a superclass that declares doSomething, and then you create subclasses each of which implements it differently. That is:
public class BaseThing
{
abstract public void doSomething();
}
public class SpecificThing extends BaseThing
{
public void doSomething()
{
System.out.println("I'm a SpecificThing!");
}
}
public class DifferentThing extends BaseThing
{
public void doSomething()
{
System.out.println("I'm a DifferentThing!");
}
}
public class AnotherThing extends BaseThing
{
public void doSomething()
{
System.out.println("I'm an AnotherThing!");
}
}
If you really need to pass the "thing" as a parameter for some reason, okay. Do the above, then write:
void doSomething(BaseThing genericThing)
{
genericThing.doSomething();
}
If some of your subclasses can't do the function and should give an error message instead, then just instead of making it abstrct in the supertype, make the supertype do the "invalid" processing, like:
public void BaseThing
{
public void doSomething()
throws IllegalArgumentException
{
throw new IllegalArgumentException("Can't handle this thing");
}
}

The question is almoust the text-book example of Strategy-pattern. You extract the specific behavoir into separate classes that al implement the same interface (with a method like doIt() of something). And then you give each specific class a reference to the "behavior"-object you want it to have.
Bonus:
1) You can change the behavior of an object at runtime by simply given it another "behavior"-object.
2) You don't have to override a method (danger with overriding methods could be class-booming).

This could be dealt with using plain old OO polymorphism before trying to force a pattern onto it.
You don't need to necessarily subclass the processors, you can overload the method declarations in a single Processor class keeping the method name the same but declaring the parameter for the specific type.
void foo(BaseTing ting) { System.out.println("Default " + ting.name); }
void foo(TingA ting) { System.out.println("AA " + ting.name); }
void foo(TingB ting) { System.out.println("BB " + ting.name); }
Java will resolve the method that most closely matches the parameter type, so if you have TingC that extends TingB, then foo(TingB) will be invoked until foo(TingC) is defined in the Processor class.
If you are going to add a lot more actions for each type of thing, i.e. baz(Ting), bar(Ting), bat(Ting) etc. then you may want to split you Processor classes by Ting subtype and use a factory method to create the specific processor a la Strategy pattern.
i.e. BaseProcessor, TingAProcessor, TingBProcessor.
The BaseProcessor would be a good candidate to house the factory method, and should provide default implementations for each of the methods, even if the default implementation is abstract or just throws an exception. The specialised Processors classes should extend from the BaseProcessor and inherit and override the default operations.

You have few options:
* Abstract your functionality into an interface and let other classes implement that interface.
* You could use The Chain of responsibility pattern(consisting of a source of command objects and a series of processing objects).
* You could also use the Strategy design pattern( algorithms can be selected at runtime)

How to use java interfaces with multiple implementing classes

public interface Foo {
}
public class SpecificFoo implements Foo {
}
public interface SomeInterface {
void thisMethod(Foo someKindOfFoo);
}
public class SomeClass implements SomeInterface {
public void thisMethod(Foo someKindOfFoo) {
// calling code goes into this function
System.out.println("Dont go here please");
}
public void thisMethod(SpecificFoo specificFoo) {
// not into this function
System.out.println("Go here please");
}
}
public class SomeOlderClass {
public SomeOlderClass( SomeInterface inInterface ) {
SpecificFoo myFoo = new SpecificFoo();
inInterface.thisMethod(myFoo);
}
}
calling code:
SomeClass myClass = new SomeClass();
SomeOlderClass olderClass = new SomeOlderClass(myClass);
I have an interface (SomeInterface) that several classes call into (such as SomeOlderClass). I have a class that implements the interface, but I want to do type safe operations on the specific implementations that are passed into the generic interface.
As shown in the above code, I really want to able to make another method that matches the specific type passed in to the interface. This doesn't work. I assume it is because the calling code only knows about the interface, and not the implementation with the more specific methods (even though SpecificFoo implements Foo)
So how can I do this in the most elegant way? I can get the code working by adding an if statement in the class implementing the interface (SomeClass):
public void thisMethod(Foo someKindOfFoo) {
// calling code goes into this function
if ( someKindOfFoo.getClass().equals(SpecificFoo.class) )
thisMethod(SpecificFoo.class.cast(someKindOfFoo));
else
System.out.println("Dont go here please");
}
However, this is not elegant, as I have to add if statements everytime I add a new kind of Foo. And I might forget to do so.
The other option is to add SpecificFoo to the SomeInterface, and let the compiler sort out reminding me that I need implementations in SomeClass. The problem with this is that I end up adding quite a bit of boiler plate code. (If someone else implements the interface, they have to implement the new method, as well as any tests)
It seems that there should be another option I am missing, given that Foo and SpecificFoo are related. Ideas?
MORE INFO:
Well I actually worked for a while to try and simplify the question. As I add more details the complexity goes up by quite a bit. But whatever... I think I can explain it.
Basically, I am write a GWT web apps RPC servlet using the command pattern as explained by Ray Ryan in his talk
There are several implementations of it on google code, but many of them suffer this inherit problem. I thought it was a bug in the GWT-RPC code bugreport HOWEVER, as I was implementing further I noticed a similar problem happening purely on the client side, and while in hosted mode. (ie all java, no gwt javascript madness).
So I abstracted the basic ideas to a raw java command line case, and saw the same issue, as described above.
If you follow along with what Ray Ryan discusses, Foo is an Action, SpecificFoo is a specific action I want to call. SomeInterface is the client side RPC service and SomeClass is the server side RPC class. SomeOlderClass is a kind of rpc service that would know about cacheing and whatnot.
Obvious, right? Well as I said, I think all the GWT RPC nonsense just muddies up the waters on the base issue, which is why I tried to simplify it as best I could.

If you need to find out the actual type of an object at runtime, then the design is most probably wrong. That violates at least the Open Closed Principle and Dependency Inversion Principle.
(Because Java does not have multiple dispatch, the thisMethod(Foo)will be called instead of thisMethod(SpecificFoo). Double dispatch could be used to get around the language's limitations, but there might still be some design problem lurking there...)
Please give more information on what you are trying to accomplish. Right now the question does not provide enough information to come up with a right design.
A generic solution is that since the action depends on the runtime type of Foo, that method should be part of Foo so that its implementation can vary depending on Foo's type. So your example would be changed to something like below (possibly adding SomeInterface or other parameters to thisMethod()).
public interface Foo {
void thisMethod();
}
public class SpecificFoo implements Foo {
public void thisMethod() {
System.out.println("Go here please");
}
}

Try using double dispatch: Add a method to the Foo interface that is called by SomeClass#thisMethod. Then place the code in the implementation of this method.
public interface Foo {
public void thatMethod(SomeClass a);
public void thatMethod(SomeOlderClass a);
}
public class SomeClass implements SomeInterface {
public void thisMethod(Foo someKindOfFoo) {
someKindOfFoo.thatMethod(this);
}
}

Sorry, I find the problem description far too abstract to be able to make a recommendation. You clearly have a design issue because you generally should not need to check the type of interface. I will give it a go though... First, I need to make your problem more concrete for my small brain to understand. Instead of Foos, how about Birds?
public interface Bird {
}
public class Ostrich implements Bird {
}
public interface BirdManager {
void fly(Bird bird);
}
public class AdvancedBirdManager implements BirdManager {
public void fly(Bird bird) {
System.out.println("I am in the air. Yay!");
}
public void fly(Ostrich ostrich) {
System.out.println("Sigh... I can't fly.");
}
}
public class ZooSimulation {
public ZooSimulation(BirdManager birdManager) {
Ostrich ostrich = new Ostrich();
birdManager.fly(ostrich);
}
}
public static void main(String[] args) {
AdvancedBirdManager advancedBirdManager = new AdvancedBirdManager();
ZooSimulation zooSimulation = new ZooSimulation(advancedBirdManager);
}
Here, the Ostrich will declare "I am in the air. Yay!" which is not what we want.
OK, so, ignoring the fact that I am failing basic OO here, the problem is that the BirdManager will look for the least-specific method that matches the type that is passed in. So no matter what kind of bird I give it, it will always match fly(Bird). We can put some if checks in there, but as you add more types of birds, your design will degrade further. Here's the tough part - I have no idea if this makes sense within the context of your problem, but consider this refactoring where I move the logic from the manager into bird:
public interface Bird {
void fly();
}
public class BasicBird implements Bird {
public void fly() {
System.out.println("I am in the air. Yay!");
}
}
public class Ostrich implements Bird {
public void fly() {
System.out.println("Sigh... I can't fly.");
}
}
public interface BirdManager {
void fly(Bird bird);
}
public class AdvancedBirdManager implements BirdManager {
public void fly(Bird bird) {
bird.fly();
}
}
public class ZooSimulation {
public ZooSimulation(BirdManager birdManager) {
Ostrich ostrich = new Ostrich();
birdManager.fly(ostrich);
}
}
public static void main(String[] args) {
AdvancedBirdManager advancedBirdManager = new AdvancedBirdManager();
ZooSimulation zooSimulation = new ZooSimulation(advancedBirdManager);
}
Our Ostrich now says the correct thing and the bird manager still treats it as just a bird. Again, bad OO (Ostriches should not have fly() methods) but it illustrates my thoughts.

As long as there are not too many implementations of Foo, I would declare an abstract method in SomeInterface for each subclass of Foo, and have an abstract class forward calls to a default method that is defined for the most general type:
public interface Foo {
}
public class SpecificFoo implements Foo {
}
public interface SomeInterface {
void thisMethod(Foo someKindOfFoo);
void thisMethod(SpecificFoo specificFoo);
void thisMethod(OtherSpecificFoo otherSpecificFoo);
}
public abstract class AbstractSomeInterface {
public void thisMethod(Foo wrongFoo) {
throw new IllegalArgumentException("Wrong kind of Foo!");
}
public void thisMethod(SpecificFoo specificFoo) {
this.thisMethod((Foo) specificFoo);
}
public void thisMethod(OtherSpecificFoo otherSpecificFoo) {
this.thisMethod((Foo) specificFoo);
}
}
public class SomeClass extends AbstractSomeInterface {
public void thisMethod(SpecificFoo specificFoo) {
// calling code goes into this function
System.out.println("Go here please");
}
}
public class SomeOlderClass {
public SomeOlderClass( SomeInterface inInterface ) {
SpecificFoo myFoo = new SpecificFoo();
inInterface.thisMethod(myFoo);
}
}