You can have as many interface s as you want; each one becomes an independent type that you can upcast to. The following example shows a concrete class combined with several interface s to produce a new class: Feedback. When you combine a concrete class with interfaces this way, the concrete class must come first, then the interfaces.
The compiler gives an error otherwise. Note that the signature for fight is the same in the interface CanFight and the class ActionCharacter , and that fight is not provided with a definition in Hero. If you want to create an object of the new type, it must be a class with all definitions provided. In class Adventure , you can see that there are four methods that take as arguments the various interfaces and the concrete class.
When a Hero object is created, it can be passed to any of these methods, which means it is being upcast to each interface in turn. Because of the way interfaces are designed in Java, this works without any particular effort on the part of the programmer. Keep in mind that the core reason for interfaces is shown in the preceding example: to be able to upcast to more than one base type. However, a second reason for using interfaces is the same as using an abstract base class: to prevent the client programmer from making an object of this class and to establish that it is only an interface.
This brings up a question: Should you use an interface or an abstract class? You can encounter a small pitfall when implementing multiple interfaces. In the preceding example, both CanFight and ActionCharacter have an identical void fight method. This is not a problem, because the method is identical in both cases. The difficulty occurs because overriding, implementation, and overloading get unpleasantly mixed together, and overloaded methods cannot differ only by return type.
When the last two lines are uncommented, the error messages say it all:. Using the same method names in different interfaces that are intended to be combined generally causes confusion in the readability of the code, as well. Strive to avoid it. You can easily add new method declarations to an interface by using inheritance, and you can also combine several interface s into a new interface with inheritance. In both cases you get a new interface , as seen in this example:. DangerousMonster is a simple extension to Monster that produces a new interface.
This is implemented in DragonZilla. The syntax used in Vampire works only when inheriting interfaces. Normally, you can use extends with only a single class, but since an interface can be made from multiple other interfaces, extends can refer to multiple base interfaces when building a new interface. As you can see, the interface names are simply separated with commas.
For example:. Notice the Java style of using all uppercase letters with underscores to separate multiple words in a single identifier for static final s that have constant initializers.
You can use the constants from outside the package by importing c Months just as you would with any other package, and referencing the values with expressions like Months. If you do want extra type safety, you can build a class like this: [33]. Month is a final class with a private constructor, so no one can inherit from it or make any instances of it. These objects are also used in the array month , which lets you iterate through an array of Month2 objects.
The number method allows you to select a Month by giving its corresponding month number. In main you can see the type safety; m is a Month object so it can be assigned only to a Month. The previous example Months. This works because there can be only one instance of each value of Month. This project also has many other potentially useful libraries. Fields defined in interfaces are automatically static and final. Since the fields are static , they are initialized when the class is first loaded, which happens when any of the fields are accessed for the first time.
The fields, of course, are not part of the interface but instead are stored in the static storage area for that interface. Interfaces may be nested within classes and within other interfaces. The syntax for nesting an interface within a class is reasonably obvious, and just like non-nested interfaces, these can have public or package-access visibility. You can also see that both public and package-access nested interfaces can be implemented as public , package-access, and private nested classes.
As a new twist, interfaces can also be private , as seen in A. D the same qualification syntax is used for nested interfaces as for nested classes. What good is a private nested interface? You might guess that it can only be implemented as a private inner class as in DImp , but A. DImp2 shows that it can also be implemented as a public class.
However, A. DImp2 can only be used as itself. You are not allowed to mention the fact that it implements the private interface, so implementing a private interface is a way to force the definition of the methods in that interface without adding any type information that is, without allowing any upcasting.
What can you do with the return value of this method? In main , you can see several attempts to use the return value, all of which fail. Interface E shows that interfaces can be nested within each other. NestingInterfaces shows the various ways that nested interfaces can be implemented. In particular, notice that when you implement an interface, you are not required to implement any interfaces nested within.
Also, private interfaces cannot be implemented outside of their defining classes. Initially, these features may seem like they are added strictly for syntactic consistency, but I generally find that once you know about a feature, you often discover places where it is useful. This is called an inner class. The inner class is a valuable feature because it allows you to group classes that logically belong together and to control the visibility of one within the other.
The inner classes, when used inside ship , look just like the use of any other classes. Here, the only practical difference is that the names are nested within Parcel1. More typically, an outer class will have a method that returns a reference to an inner class, like this:.
If you want to make an object of the inner class anywhere except from within a non- static method of the outer class, you must specify the type of that object as OuterClassName. The Addressable interface describes an addressable entity as containing an address.
This address is described by the nested Address class. The getAddress method is implemented by whatever class implements Addressable. Listing 2 presents the source code to a Letter class that implements Addressable. Letter stores a single address field of type Addressable. This nested class is instantiated by Letter 's constructor. The implemented getAddress method returns this object.
Now consider what happens when we add Postcard and Parcel classes to the Addressables application. Listing 3 presents the source code to an Addressables application, which demonstrates the Postcard , Parcel , and Letter types. The main method first creates an array of Addressable objects. It then iterates over these objects, invoking getAddress on each object.
The returned Addressable. Address object's toString method is invoked by System. Compile Listings 2 and 3 along with Postcard. Many developers believe that nesting a class in an interface violates object-oriented programming and the purpose for an interface. However, it's good to know about this capability because you might encounter it when maintaining someone else's code.
Create a free Team What is Teams? Learn more. Ask Question. Asked 6 years, 2 months ago. Active 3 years, 8 months ago. Viewed 1k times. Improve this question. Add a comment. Active Oldest Votes. The excerpt which i like to point is interface is meant to declare just to provide interface not for implementation. Yes, we can have classes inside interfaces. Improve this answer. Levit Just wondering, how the implemented class will look like? Would love to see an implementation in action for the above usage.
Thank you. SyntaxT3rr0r SyntaxT3rr0r Can you add some examples of usage? I've tested something similar some time ago and haven't understood what can I gain from using this construction.
Roman: well I remember I've encountered this on some project relatively clean project would I add but they weren't mine but I don't know if it's really clean or not. I've added a tiny example that looks like what I've seen but once again: this wasn't my code and I'm not using that construct so I'm not the most qualified to come up with valid examples : IIRC the class inside was always named, for example StateChecker and calls would always look like: A.
Using your narrowed definition, interface s can not have inner classes. This answer is wrong. Interfaces can have static nested classes but not inner classes. PaulBoddington You're right. Even removing 'static', the B class is a static nested class and not an inner class; interfaces get special treatment.
I couldn't find mention of this online, except for in the spec itself: "A member class of an interface is implicitly static so is never considered to be an inner class. Show 1 more comment. Community Bot 1 1 1 silver badge. Nad Nik Nad Nik 4 4 silver badges 2 2 bronze badges. This is probably the most concise answer of all. This is the answer that I was looking for.. Bachi Bachi 6, 4 4 gold badges 31 31 silver badges 30 30 bronze badges. And this is exactly why private member classes would make sense.
Michael Anderson Michael Anderson
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