Adapters-shoud i still use "implements or extends" keyword
hi all...especially Mr. Glen McCluskey...
i've just read the TechTips using Adapters-9 May 2000...
there was an example about using adapters...there wasn't keyword "extends or implements" in that example ...
so the question is:
if i use adapters, should i still extend or implements my derived class from base class? or this formula is just enough, for example:
frame.addWindowListener(new WindowAdapter(){................});
the example available at: developer.java.sun.com/developer/TechTips/index.html the date is 9 May 2000 writer:Glen McCluskey ...
thank you, all...
I don't understand why you say "there wasn't keyword "extends or implements" in that example ", I read the article and see:abstract class A_ADAPTER implements A {
public void f1() {}
public void f2() {}
public void f3() {}
public void f4() {}
public void f5() {}
} which clearly implements interface A, and the class that uses this adapter must extend the adapter. So yes, your adapter must implement the interface it is adapting, and the class that is using this adapter must extend the adapter to "use" it.
Lee
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void method2();
void method3();
public class Test {
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Ok, easy question...About implements and extends
Im sure this will be simple to all of you, but I am still new to the Java language. I want to know the difference between using "extends" and "implements" in the way that its used in these examples:
With the "extends" keyword...
class PrimeThread extends Thread {
long minPrime;
PrimeThread(long minPrime) {
this.minPrime = minPrime;
public void run() {
// compute primes larger than minPrime
}And then with the "implements" keyword...
Using the interface "Runnable"
class PrimeRun implements Runnable {
long minPrime;
PrimeRun(long minPrime) {
this.minPrime = minPrime;
public void run() {
// compute primes larger than minPrime
}Can someone please explain when you would want to use the "extends" version and when you would want to use the "implements" version? Sorry for practically insulting your intelligence...I'm just trying to learn this language without a book as an experiment to see (how much/how fast) I can learn purely using the web resources...Thanks to anyone who wishes to explain this to me.That's pretty much it.
An interface may extend zero or more other interfaces.
A class (except Object) always extends exactly one other class--either the class explicitly named with "extends", or Object if "extends" is not present.
A class may implement zero or more interfaces.
"Extends" implies a parent-child relationship. Interfaces extend other interfaces, and classes extend other classes.
"Implements" implies a contract stating that the class will provide implementations for all the methods declared in the interface (or the class will be declared abstract). If a class implements A, and A extends B, then that class must implement all methods declared in A and all methods declared in B.
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What is the difference between implements and extends?
For example, public class Banner extends Applet implements Runnable =>
If you extend a Class, then you can use the members of the superclass(which is Applet in this situation). And if you implement a Class then you can also use the members of the class. And as far as i know the Class youre implementing is called an interface. I know that an interface is not implemented yet, and it contains a lot of empty methods like public interface Series{ int getNext(); void reset(); void setStart(int x); } .
Then if you want to implement this interface you must use another class. In this class you define the methods and also write a couple of variables, whom you will be putting in the not implemented methods, like this=>
class ByTwos(extends superclass) implements Series { int start; int val; ByTwos(){start=0; val=0;}
public in getNext(){val +- 2} public void reset(){start=0; val=0;}
public void setStart(int x) {start=x; val=x;} }.
Then you create another Class with a main method and an object of type ByTwos, wherein you USE THE MEMBERS OF THE INTERFACE CLASS.
Are the things i wrote right? What is the difference then?Shelby wrote:
jverd wrote:
If all the methods defining a type are abstract, you can make the type an interface. An
interface is very much like a class with all abstract methods. Users of that interface
don't care which implmentation they get. All they need to know is that they'll be able to
call the declared methods.Dense people really annoy me, and right now I'm being REALLY dense about the advantage of using
an interface. I can understand what you say about using an interface to define a type of something
like an animal. But since all of the variables in an interface are final, and all of the methods are only
signatures, it seems to me it would just be easier to create classes that use their own methods that
actually perform a task.You're only thinking of writing the concrete classes. Think about the code that uses the type.
Animal animal = goGetSomeRandomAnimalFromSomewhere();
animal.speak();
animal.feed();I define the Animal type, as an interface, and every user of Animal knows that any Animal (meaning an instance of any class that implements Animal) will be able to speak(), feed(), etc.
Now I'm making some kind of game with animals wandering around in it. I want to draw some random animal on the screen, have it make its noise and then go eat. At the point where my code is doing that (above), I don't know what specific Animal implementation I'll get, only that it will be an Animal. So I have to declare the animal variable as a supertype of all the classes that could come back.
The Animal type serves that purpose. It could be an abstract class, rather than an interface, but an abstract class can carry implementation, and my Animal class has no implementation. How does an Animal speak()? How does it feed()? Animal is simply a pure type. Interfaces are better suited for pure type defintions than abstract classes. That's their purpose.
In addition, my implementing classes can implement other interfaces besides Animal, such as, for instance, Serializable or Comparable. Java doesn't support multiple inheritance of implementation, but it does support multiple inheritance of type--via being able to implement multiple interfaces.
Remember--inheritance is primarily about type, NOT about code sharing.
As another example, consider the java.util.List interface. You can add() to a list, remove() from it, see if it contains() a particular element, etc. There are multiple ways you could implement a List, and there's not really any common implementation you'd want to assume. So we define a List interface--the pure type--and then we implement it in very different ways via ArrayList and LinkedList. (Actually, there's a layer or two of abstract classes between List and those implementations. The abstract classes define implementations of some of the methods that do have reasonable defaults--for instance if they only rely on other methods in the List interface.)
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public void sort(List list) {
// I can sort ANY List here
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Can extends keyword be used with respect to interfaces?
can extends keyword be used with respect to interfaces?
srinu269 wrote:
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interface A {
public void a();
interface B extends A {
public void b();
class X implements B {
public void b() {
public void a() {
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I had a question about interfaces. I know how they work but I dont see the point in having them...?
If I put methods into an Interface, then each method in the interface must appear in that class.
abstract interface Drive {
public void start();
public void accel();
public void decel();
public void dropTop();
public class Car implements Drive {
}and if I have a class that extends Car, then it also implements Drive. but I dont see the benefit of having Drive at all.
help?interfaces are useful when it comes to multiple inheritance, since you can't extends more than one class, but you can implement as much interface as you want
imagine for example that your "Car" class is supposed to extends "Vehicle", but it also has to be an Observer (for the Observable/Observer pattern)
you can make it both a Vehicle and an Observer using:
class Car extends Vehicle implements Observer {}and if you want your car to be a vehicle, an observer, and an oil consumer:
class Car extends Vehicle implements Observer, OilConsumer {}if you only had two classes OilConsumer and Vehicle, you would have to choose which one car will extend ; with interfaces, you can use both at one time
interfaces can also be used as simple markers
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I'm an inexperienced programmer.
After reading a number of articles I've been convinced that its generally better to use interfaces and implements, as opposed to classes and extended subclasses. However, I'm not sure how I should do that given my current situation?
I have a base class with a number of functions and variables. There is one function that I need to implement differently for a number of situations. At the moment I have something like...
class base {
int a;
int b;
void functionA() { does whatever; }
void functionB() { does whatever; }
class baseA extends base {
void functionC() { does whatever; }
class baseB extends base {
void functionC() { does whatever; }
class baseZ extends base {
void functionC() { does whatever; }
If I use implements instead of extends, doesn't that mean I have to include a copy of functionA and functionB in all of those subclasses? Isn't that inconvenient? Or is there a better way of doing this?
Thanks for any help!I'm an inexperienced programmer.
After reading a number of articles I've been convinced
that its generally better to use interfaces and
implements, as opposed to classes and extended
subclasses. However, I'm not sure how I should do
that given my current situation?
I have a base class with a number of functions and
variables. There is one function that I need to
implement differently for a number of situations. At
the moment I have something like...
class base {
int a;
int b;
void functionA() { does whatever; }
void functionB() { does whatever; }
class baseA extends base {
void functionC() { does whatever; }
class baseB extends base {
void functionC() { does whatever; }
class baseZ extends base {
void functionC() { does whatever; }
If I use implements instead of extends, doesn't that
mean I have to include a copy of functionA and
functionB in all of those subclasses? Isn't that
inconvenient? Or is there a better way of doing this?
Thanks for any help!Intefaces aren't always the better choice, and I would say that the situation you descibe is one of those cases where they aren't the better choice.
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