Java does not allow private or protected methods, so how do we ensure implementors of a bidirectional interface call the necessary methods?
Let's say we have an IModelListener interface as follows:
public interface IModelListener
{
public void handleChannelUpdate(int channel);
}
Then we have a ViewControl client as follows:
public class ViewControl implements IModelListener
ViewControl objects are going to work as long as we remember to have ViewControl call this:
model.registerChannelListener(this);
If Java allowed protected or private methods in an Interface, we could simply modify IModelListener to:
public interface IModelListener
{
public void handleChannelUpdate(int channel);
private void registerChannelListener( );
}
How can this be achieved?
Are there annotations that would do this?
Java does not support multiple inheritance so if Clients/Implementors are already a derived class (typical), then using an abstract class is not an option.
Thanks for helping,
Jeff
You probably miss the concept of interfaces. It can not contain private or protected methods, because the role of an interface is to provide accessible set of methods. You probably might, on the other hand, take look at abstract classes.
What you need is probably this:
public abstract class AbstractViewControler implements IModelListener {
protected abstract void registerChannelListener();
protected AbstractViewControler() {
this.registerChannelListener();
}
}
and then:
public class MyViewControler extends AbstractViewControler {
protected void registerChanelListener() {
//- Do what you need here.
}
}
and after that just:
IModelListener listner = new MyViewControler();
An interface is a way of providing a public contract to users of the class implementing the interface. How the class is implemented doesn't matter, as long as they are adhering to the contract. Therefore, it doesn't make sense to have private methods in an interface.
What you want is to enforce a default behavior on your class - in Java, abstract classes are the place to formulate default behavior inherited by all extending classes (see the template method design pattern for an application of this). Interfaces only describe, how your objects externally behave and how they can be used by others.
Interface in java intended to provide the signature of interface functionality in mean of signature that you implement in your classs so it should not to be private.
your need: you can have abstract method with some default statements that you want.
where you can have all type of access specifire.
In Java, you can't do it using an interface only.
If you want to achieve some sort of "autobinding" (i.e. call model.registerChannelListener(this); automatically on all pertinent objects), then you should have those objects implement an empty interface, retrieve the list of all the instances of classes implementing it via introspection and iterate on them.
You could do this periodically or at some specific point in the program. You could also use an #interface annotation to add a little syntactical sugar.
You might also want to invert the flow and create objects using dependency injection and/or a factory, so that you have that method called "automagically" just after creation (like with #PostConstruct).
Interfaces are the wrong place to look.
The usual solution in Java for problems like this one is to have a method in either object which returns the other:
public class ViewControl {
public IModel getModel() {...}
}
The method can then make sure that the model and the view are correctly hooked up. Now I guess that you don't really want to couple the view and the model. The solution is then to define a new ViewModel type which just delegates to the real model (most IDEs allow to create delegate types with 3-5 mouse clicks):
public class ViewControl {
public ViewModel getViewModel( IModel model ) {...}
}
You should be able to move this code to a base (abstract) view class which all views inherit from.
Related
I'm working with a database and I create a public interface called Dao with extends from AutoCloseabe, so I have a class which implements this interface but I want to create some private methods there but they still need Autocloseable.
So my question is, I can't create private methods in the interface without defining them in the interface. Which happens if I create a private method in the class but not overriding from Dao? They won't have autocloseable, will they?. And if not which solution can I implement?
The motivation behind the introduction in Java 9 of private methods in interfaces is for the same reasons you would use private methods in any other class body. It allows you to break up the code into reusable, manageable methods which are not inherited:
default public boolean tryHeads() {
return flip();
}
default public boolean tryTails() {
return !flip();
}
private boolean flip() {
return ThreadLocalRandom.current().nextBoolean();
}
This is grossly oversimplified. But see a similar question from me for more insight.
Private methods don't make a lot of sense in an interface since the main idea of an interface is to provide an interface for objects to interact with eachother. Since private methods are not visible to other objects they can't use them to communicate and send messages.
If you would like to have private methods I would suggest you use an abstract class instead.
As a sum up:
Q: So my question is, I can't create private methods in the interface without defining them in the interface.
Exactly that's the main point for interface; they define a public API without caring about the internal implementation.
Q: What happens if I create a private method in the class but not overriding from Dao?
If you declare a method in an abstract class and you don't override it two things may happen:
The method was marked as abstract: you'll get a compile error saying that your implementation class doesn't satisfy the specification of the parent class.
1.1 If you declared a method in an interface and you don't implement it you will get the same error stating that your implementation class doesn't satisfy the interface it's trying to implement.
The method was public and not marked as final in the abstract class: You may or may not override it. If you don't override it, the parent class implementation is going to be used; if you override it completely your child class code will be executed. If you call super.method() and then your implementation, both codes will be executed.
Interfaces are great from a flexibility standpoint. But in case, where an interface is used by a large number of clients. Adding new methods to the interface while keeping the old mehtods intact will break all clients' code as new methods won't be present in clients. As shown below:
public interface CustomInterface {
public void method1();
}
public class CustomImplementation implements CustomInterface {
#Override
public void method1() {
System.out.println("This is method1");
}
}
If at some point later in time, we add another method to this interface all clients' code will break.
public interface CustomInterface {
public void method1();
public void method2();
}
To avoid this we have to explicitly implement new methods in all clients' code.
So I think of interfaces and this scenario as following:
Interfaces once written are like carving in stone. They are rarely supposed, and expected to change. And if they do, they come with a huge cost(rewriting the whole code) which programmers should be ready for.
In continuation with the point above, Is it possible to write interfaces that can stand the test of time?
How such a scenario is handled in interfaces where you expect additional functionality in future? That is anticipating change in the contract by which all clients are binded.
EDIT: Default method is indeed a nice addition to Java Interfaces which a lot of people have mentioned in their answers. But my question was more in the context of code design. And how forcing method implementation on the client is an intrinsic character of an interface. But this contract between an interface and a client seems fragile as functionality will eventually evolve.
One solution to this problem was introduced in Java 8 in the form of default methods in interfaces. It allowed to add new methods to existing Java SE interfaces without breaking existing code, since it supplied default implementation to all the new methods.
For example, the Iterable interface, which is widely used (it's a super interface of the Collection interface) was added two new default methods - default void forEach(Consumer<? super T> action) and default Spliterator<T> spliterator().
public interface CustomInterface {
public void method1();
}
public interface CustomInterface2 extends CustomInterface {
public void meathod2();
}
Other than default method you can use inheritance property as show above by which new interface will have all previous method along with new methods and use this interface in your required situation.
Java 8 has introduced default implementation for methods. These implementations reside in the interface. If a new method with a default implementation is created in an interface that is already implemented by many classes, there is no need to modify all the classes, but only the ones that we want to have a different implementation for the newly defined method than the default one.
Now, what about older Java versions? Here we can have another interface that extends the first one. After that, classes that we want to implement the newly-declared method will be changed to implement the new interface. As shown below.
public interface IFirst {
void method1();
}
public class ClassOne implements IFirst() {
public void method1();
}
public class ClassTwo implements IFirst() {
public void method1();
}
Now, we want method2() declared, but it should only be implemented by ClassOne.
public interface ISecond extends iFirst {
void method2();
}
public class ClassOne implements ISecond() {
public void method1();
public void method2();
}
public class ClassTwo implements IFirst() {
public void method1();
}
This approach will be ok in most cases, but it does have downsides as well. For example, we want method3() (and only that one) for ClassTwo. We will need a new interface IThird. If later we will want to add method4() that should be implemented by both ClassOne and ClassTwo, we will need to modify (but not ClassThree that also implemented IFirst) we will need to change both ISecond and IThird.
There rarely is a "magic bullet" when it comes to programming. In the case of interfaces, it is best if they don't change. This isn't always the case in real-life situations. That is why it is advised that interfaces offer just "the contract" (must-have functionality) and when possible use abstract classes.
A future interface change shouldn't break anything that has been working -- if it does, it's a different interface. (It may deprecate things, though, and a full cycle after deprecation it may be acceptable to say that throwing an Unimplemented exception is acceptable.)
To add things to an interface, the cleanest answer is to derive a new interface from it. That will allow using objects implementing the new behaviors with code expecting the old ones, while letting the user declare appropriately and/or typecast to get access to the new features. It's a bit annoying since it may require instanceof tests, but it's the most robust approach, and it's the one you'll see in many industry standards.
Interfaces are contracts between the developer and clients, so you're right - they are carved in stone and should not be changed. Therefore, an interface should expose (= demand) only the basic functionality that's absolutely required from a class.
Take the List interface for example. There are many implementations of lists in Java, many of which evolve over time (better under-the-hood algorithms, improved memory storage), but the basic "concept" of a list - add an item, search for an item, remove an item - should not and will not ever change.
So, to your question: Instead of writing interfaces which classes implement, you can use abstract classes. Interfaces are basically purely-abstract classes, in the sense that they do not provide any built-in functionality. However, one can add new, non-abstract methods to an abstract class that clients will not be required to implement (override).
Take this abstract class (= interface) for example:
abstract class BaseQueue {
abstract public Object pop();
abstract public void push(Object o);
abstract public int length();
public void clearEven() {};
}
public class MyQueue extends BaseQueue {
#Override
public Object pop() { ... }
...
}
Just like in interfaces, every class that extends BaseQueue is contractually bound to implement the abstract methods. The clearEven() method, however, is not an abstract method (and already comes with an empty implementation), so the client is not forced to implement it, or even use it.
That means that you can leverage the power of abstract classes in Java in order to create non-contractually-binding methods. You can add other methods to the base class in the future as much as you like, provided that they are not abstract methods.
I think your question is more about design and techniques, so java8 answers are a bit misleading. This problem was known long before java8, so there are some other solutions for it.
First, there are no absolutely chargeless ways to solve a problem. The size of inconviniences that come from interface evolving depends on how the library is used and how deliberate your design is.
1) No techniques will help, if you designed an interface and forgot to include a mandatory method in it. Plan your design better and try to anticipate how clients will use your interfaces.
Example: Imagine Machine interface that has turnOn() method but misses turnOff() method. Introducing a new method with default empty implementation in java8 will prevent compilation errors but will not really help, because calling a method will have no effect. Providing working implementation is sometimes impossible because interface has no fields and state.
2) Different implementations usually have things in common. Don't be afraid to keep common logic in parent class. Inherit your library classes from this parent class. This will enforce library clients to inherit their own implementations from your parent class as well. Now you can make small changes to the interface without breaking everything.
Example: You decided to include isTurnedOn() method to your interface. With a basic class, you can write a default method implementation that would make sence. Classes that were not inherited from parent class still need to provide their own method implementations, but since method is not mandatory, it will be easy for them.
3) Upgrading the functionality is usually achieved by extending the interfaces. There's no reason to force library clients to implement a bunch of new methods because they may not need them.
Example: You decided to add stayIdle() method to your interface. It makes sence for classes in your library, but not for custom client classes. Since this functionality is new, it's better to create a new interface that will extend Machine and use it when it's needed.
It is said that Java has an alternative to achieve multiple inheritance by implementing number of interfaces. According to the Java documentation about inheritance found here:
A subclass inherits all the members (fields, methods, and nested classes) from its superclass . The idea of inheritance is simple but powerful: When you want to create a new class and there is already a class that includes some of the code that you want, you can derive your new class from the existing class. In doing this, you can reuse the fields and methods of the existing class without having to write (and debug!) them yourself.
and about interfaces here, it says that interfaces don't have methods implemented. They just contain method declarations. Also, as far as I know, interfaces don't contain nested classes either. One more point, not all the interfaces have constant fields, many a time the interfaces (which I use for my work) contain just method declarations.
In that case, what do we really inherit from them, if we have to define all the methods ourselves? I know the other uses of interfaces very well. But I am not getting what it has to do with inheritance.
In that case, what do we really inherit from them, if we have to define all the methods ourselves?
In classic Java (7 and earlier), implementing an interface gives you:
an "is-a" relationship with an abstraction,
a set of method signatures that have to be implemented (to satisfy the "is-a" relationship), and
an implied "contract" that says what the methods are supposed to do.
In Java 8, you also inherit default methods declared in the interface.
It is true that a class the implements an interface doesn't inherent any state declarations, or methods that can access the state declarations, as it would if it extend a class. But the flip-side is that a class can implement multiple interfaces ... which you can't do with a extend.
In short, implements allows you to do things you can't do with extends and vice versa.
Interfaces offer a contract, meaning that when a class implements an interface, then, by definition it provides a set of methods. How these are implemented is left to the class implementing the interface.
Consider:
public interface IPrint
{
public void print(String str);
}
public interface IClean
{
public void clean();
}
public class CanonPrint implements IPrint, IClean //Prints to a physical canon printer
{
public void print(String str)
{
//Send data to canon printer
}
public void clean()
{
//Clean printer buffer, etc
}
}
public class Console implements IPrint, IClean //Prints to a console
{
public void print(String str)
{
//Send data to console
}
public void clean()
{
//Clean console buffer, etc
}
}
The above example, you know that the CanonPrint and ConsolePrint classes can print and clean after themselves, they have the logic to do it, the fact that they implement the IPrint and IClean interfaces guarantees it. Nothing is inherited, there is no functionality being extended, just a guaranteed a set of functions. The logic, etc, is delegated.
I already read the post of research effort required to post a SO question. I am ashamed again to post this question to a pile of million questions. But I still don't get the idea of interfaces in java. They have unimplemented methods and then defined for every class in which they are implemented. I searched about it. Interfaces were used to support multiple inheritance in java and also to avoid (Deadly) Diamond Death of inheritance. I also came across Composition vs Inheritance and that inheritance is not for code reuse and its for polymorphism. So when I have a common code as a class to extend it will not be supported due to multiple inheritance which gives the option to use Interfaces(Correct me if I am wrong). I also came across that its not possible in most cases to define a generic implementation. So what is the problem in having a common definition (not a perfect generic implementation) of the interface method and then Override it wherever necessary and why doesn't java support it. Eg. When I have 100 classes that implements an interface 70 of them have a common implementation while others have different implementation. Why do I have to define the common method in interface over 70 classes and why can't I define them in Interface and then override them in other 30 classes which saves me from using same code in 70 classes. Is my understanding of interfaces wrong?
First, an interface in Java (as of Java 7) has no code. It's a mere definition, a contract a class must fulfill.
So what is the problem in having a common definition (not a perfect
generic implementation) of the interface method and then Override it
wherever necessary and why doesn't java support it
Yes you can do that in Java, just not with interfaces only. Let's suppose I want from this Example interface to have a default implementation for method1 but leave method2 unimplemented:
interface Example {
public void method1();
public String method2(final int parameter);
}
abstract class AbstractExampleImpl implements Example {
#Override
public void method1() {
// Implement
}
}
Now classes that want to use this method1 default implementation can just extend AbstractExampleImpl. This is more flexible than implementing code in the interface because if you do so, then all classes are bound to that implementation which you might not want. This is the advantage of interfaces: being able to reference a certain behavior (contract) without having to know how the class actually implements this, for example:
List<String> aList = MyListFactory.getNewList();
MyListFactory.getNewList() can return any object implementing List, our code manipulating aList doesn't care at all because it's based on the interface.
What if the class that uses interface already is a Sub-class. Then we
can't use Abstract class as multiple inheritance is not supported
I guess you mean this situation:
class AnotherClass extends AnotherBaseClass
and you want to extend AbstractExampleImpl as well. Yes, in this case, it's not possible to make AnotherClass extend AbstractExampleImpl, but you can write a wrapped inner-class that does this, for example:
class AnotherClass extends AnotherBaseClass implements Example {
private class InnerExampleImpl extends AbstractExampleImpl {
// Here you have AbstractExampleImpl's implementation of method1
}
}
Then you can just internally make all Example methods being actually implemented by InnerExampleImpl by calling its methods.
Is it necessary to have the interface in AnotherClass?
I guess you mean AnotherClass implements Example. Well, this is what you wanted: have AnotherClass implement Example with some default implementation as well as extend another class, or I understood you wrong. Since you cannot extend more than one class, you have to implement the interface so you can do
final Example anotherClass = new AnotherClass();
Otherwise this will not be possible.
Also for every class that implements an interface do I have to design
an inner class?
No, it doesn't have to be an inner class, that was just an example. If you want multiple other classes have this default Example implementation, you can just write a separate class and wrap it inside all the classes you want.
class DefaultExampleImpl implements Example {
// Implements the methods
}
class YourClass extends YetAnotherClass implements Example {
private Example example = new DefaultClassImpl();
#Override
public void method1() {
this.example.method1();
}
#Override
public String method2(final int parameter) {
return this.example.method2(parameter);
}
}
You can create an abstract class to implement that interface, and make your those classes inherit that abstract class, that should be what you want.
A non abstract class that implements and interface needs to implement all the methods from the interface. A abstract class doesn't have to implement all the methods but cannot initiated. If you create abstract class in your example that implements all the interface methods except one. The classes that extend from these abstract class just have to implement the one not already implemented method.
The Java interfaces could have been called contracts instead to better convey their intent. The declarer promise to provide some functionality, and the using code is guaranteed that the object provides that functionality.
This is a powerful concept and is decoupled from how that functionality is provided where Java is a bit limited and you are not the first to notice that. I have personally found that it is hard to provide "perfect" implementations which just need a subclass or two to be usable in a given situation. Swing uses adapters to provide empty implementations which can then be overrides as needed and that may be the technique you are looking for.
The idea of the interface is to create a series of methods that are abstract enough to be used by different classes that implement them. The concept is based on the DRY principle (Don't repeat yourself) the interface allows you to have methods like run() that are abstract enough to be usuable for a game loop, a players ability to run,
You should understand the funda of interface first. Which is
It is use to provide tight coupling means tight encapsulation
It helps us to hide our code from the external environment i.e. from other class
Interface should have only definition and data which is constant
It provide facility to class open for extension. Hence it cannot be replace by the any other class in java otherwise that class will become close for extension. which means class will not be able to extend any other class.
I think you are struggling with the concept of Object Oriented Design more than anything. In your example above where you state you have 100 classes and 70 of them have the same method implementation (which I would be stunned by). So given an interface like this:
public interface Printable
{
void print();
}
and two classes that have the "same" implementation of print
public class First implements Printable
{
public void print()
{
System.out.println("Hi");
}
}
public class Second implements Printable
{
public void print()
{
System.out.println("Hi");
}
}
you would instead want to do this:
public abstract class DefaultPrinter implements Printable
{
public void print()
{
System.out.println("Hi");
}
}
now for First and Second
public class First extends DefaultPrinter
{
}
public class Second extends DefaultPrinter
{
}
Now both of these are still Printable . Now this is where it gets very important to understand how to properly design object hierarchies. If something IS NOT a DefaultPrinter YOU CANNOT AND SHOULD NOT make the new class extend DefaultPrinter
Let's say, for example, that I wanted to group some classes in a jar library that all fit the definition of my custom interface. Since I can't edit the classes inside the jar in order to implement my interface, is there any other way to use these classes as though they implemented my interface?
Example: I have uneditable classes A and B. I have a static method in a different class that accepts objects belonging to my interface I. How can I pass in A and B objects without making the method accept all Object objects. I want to give A and B new functionality that only they can have.
From your edit and based on my comment: create a class that will implement your interface I and serve as wrapper for classes A and B. This is Facade Design Pattern. Here's a pretty basic simple example in code:
public interface I {
void methodFromExternalAClass();
void methodFromExternalBClass();
}
public class MyClass implements I {
#Override
public void methodFromExternalAClass() {
new A().someMethod();
}
#Override
public void methodFromExternalBClass() {
new B().someMethod();
}
}
Use Adapter design pattern https://en.wikipedia.org/wiki/Adapter_pattern. Create a wrapper over a class from the library, make the wrapper implement your interface, make wrapper use methods of the class.
If what you want is to identify whether a given class is in a grouping of classes, similar to using a tagging interface, you could keep a list of Class objects and refer to that list to see if a particular class is in your grouping of classes. If that's not what you want than please clarify. I would comment for clarification, but I don't have the reputation.