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NOTE As of March , the Java SE Tutorials can be downloaded from the Sun Download Center. These historical download files will be available for some. Core Java Tutorial pdf, Core Java online Tutorial with reference manuals and examples. Core Java is a high-level programming language developed by Sun . This is a brief tutorial in Java for you who already know another .. The Java programming language was developed at Sun Microsystems and originally became.
You can have both abstract and concrete classes in your hierarchies. We can do that by defining the methods on Person to be abstract.
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Then our subclasses will have to implement those methods. We changed Person to make the methods abstract, forcing subclasses to implement them. We made Adult subclass Person, and implemented the methods. We made Baby subclass Person, and implemented the methods. When you declare a method to be abstract, you require subclasses to implement the method, or to be abstract themselves and pass along the implementation responsibility to their subclasses.
You can implement some methods on an abstract class, and force subclasses to implement others. If a subclass fails to implement an abstract method from a superclass, the compiler will complain. Now that both Adult and Baby subclass Person, we can refer to an instance of either class as being of type Person.
Refactoring to abstract behavior We now have Person, Adult, and Baby in our hierarchy. But notice that the behavior is the same again.
It makes sense to refactor the code to remove this code duplication. The most likely refactoring is to move move to Person. This is a simplistic example, so this back-and-forth might seem wasteful. But what we just experienced is a common occurrence when you write Java code.
You often see classes and methods change as your system grows, and sometimes you end up with code duplication that you can refactor to superclasses. You might even do that, decide it was a mistake, and put the behavior back down in the subclasses. You learn the right place for behavior as you go. When to abstract … and when not to Deciding when to abstract or create a hierarchy is a hotly debated topic in OO circles, especially among Java language programmers.
Certainly there are few right and wrong answers about how to structure class hierarchies. This is an area where conscientious and skilled practitioners can and often do disagree. That said, there are some good rules of thumb to follow with regard to hierarchies. Wait until the code tells you that you should. It is almost always better to refactor your way to an abstraction than to assume that you need it at the outset.
Many Java programmers overuse hierarchies.
Second, resist the use of abstract classes when you can. We used an abstract class to force our subclasses to implement certain behavior.
Quite possibly. You might have a method defined three or four classes up the chain. By the time you use it in a sub-sub-sub-subclass, you might have to hunt to discover what that method does. That can make debugging frustrating. There are many coding patterns that make use of the Java language abstract method and abstract class concepts, such as the Gang of Four Template Method pattern see Resources.
Fourth, understand the price you pay when you use a hierarchy prematurely. It really can lead you down the wrong path quickly, because having the classes there, named as they are, with the methods they have, makes it very easy to assume all of that should be as it is.
Maybe that hierarchy made sense when you created it, but it might not make sense anymore. Inertia can make it resistant to change. In a nutshell, be smart about using hierarchies. Remember to refactor. Other classes implement the interface, which means that they can use any constants in that interface by name, and that they must specify behavior for the method definitions in the interface.
Any class in any hierarchy can implement a particular interface. That means that otherwise unrelated classes can implement the same interface. You can name the interface anything you want, as long as the name is valid, but by convention interface names look like class names.
You can include constants, method declarations, or both in an interface. Constants defined in an interface look like constants defined in classes.
Methods defined in an interface look different generally speaking from methods defined in classes, because methods in an interface have no implementation. Any implementer of the interface is responsible for supplying the body of the method. You can define hierarchies of interfaces just as you can define hierarchies of classes.
You do this with the extends keyword, like so: public interface interfaceName extends superinterfaceName, Just list them after extends, separated by commas.
You do that with the implements keyword: public class className extends superclassName implements interfaceName, You can implement more than one interface by listing the interface names separated by commas. We have to implement those methods with signatures that match the ones in the interface, with the addition of the public access modifier. No, because Person is an abstract class, and the abstract keyword is assumed for methods in an interface.
That means any abstract class implementing the interface can implement what it wants, and ignore the rest. If it does not implement one or more methods, it passes that responsibility on to its subclasses. In our Person class, we chose to implement move and not talk , but we could have chosen to implement neither. Some helpful constants are, and we can reference them by name in any class that implements the interface, as we did when we initialized gender.
Because an Adult is a Human once it implements that interface. You can cast an Adult as a Human, then call methods defined by the interface, just as you can cast anAdult to Person and call Person methods on it. Baby also implements Human. An Adult is not a Baby, and a Baby is not an Adult, but both can be described as type Human or as type Person in our hierarchy. As long as whatever we get back from getPerson is of type Human, we can call move on it and expect it to respond accordingly.
Why interfaces? There are three primary reasons for using interfaces: To create convenient or descriptive namespaces. To relate classes in different hierarchies. To hide underlying type details from your code. When you create an interface to collect related constants, that interface gives you a descriptive name to use to refer to those constants. For example, you could have an interface named Language to store constant string names for languages.
Then you could refer to those language names as Language. This can make your code easier to read. The Java language supports single inheritance only.
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In other words, a class can only be a subclass of a single superclass. With interfaces, you can relate classes in different hierarchies.
In essence, an interface simply specifies a set of behaviors that all implementors of the interface must support. That means that any class implementing Person would also be a Mover. Adult and Baby would qualify. But so would Cat or Vehicle. These historical download files will be available for some time but will be eventually EOLd.
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Any class that implemented Mover would have the move behavior.
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