Programming Forums > Application Development > Java

namsu · Jan 7th, 2008, 5:34 PM

Hi, I am very confused with interfaces. I understand how to use them and when to use them. I understand the difference between the abstract class and interfaces. Can someone explain to me the real importance of using an interface, because according to me it seems as if we declare an interface so all other classes can use the same name for a particular method. Why don't people just declare the same name in each class rather than having to go through something just to use the name?

Sane · Jan 7th, 2008, 6:12 PM

I am even more confused over how you're managing to skip from learning about "For Loops" and "Conditional Operators", to learning about Object Oriented Programming with Interfaces in Java.

Either you're going too quickly, or you're trying to understand things you don't yet need to. Regardless, maybe you need to spend more time on the square before this one, to gain further insight into programming before you ask these questions. I can promise the answers will make more sense that way, and you may not even need to ask this question when you get to that point.

namsu · Jan 7th, 2008, 6:15 PM

Hi Sane I appreciate your help. I don't think I am going to fast, I just need an explanation, do you mind giving me one.

titaniumdecoy · Jan 7th, 2008, 6:34 PM

Quote:

Originally Posted by namsu

Why don't people just declare the same name in each class rather than having to go through something just to use the name?

For the same reason you need to declare the relevant variables and methods in the base class when using polymorphism. Perhaps an example will help clear things up:

 Java Syntax (Toggle Plain Text) 
// FirstClass and SecondClass both implement MyInterface
MyInterface obj1 = new FirstClass();
MyInterface obj2 = new SecondClass();
 
// MyInterface defines the doSomething() instance method
System.out.println(obj1.doSomething());
System.out.println(obj2.doSomething());
// FirstClass and SecondClass both implement MyInterface
MyInterface obj1 = new FirstClass();
MyInterface obj2 = new SecondClass();

// MyInterface defines the doSomething() instance method
System.out.println(obj1.doSomething());
System.out.println(obj2.doSomething());

It does not matter what type of object obj1 and obj2 are; however, since they implement MyInterface you know that they both implement the doSomething method.

In Java, you can implement any number of interfaces but extend only a single class, which is another reason interfaces are used (other languages such as C++ allow for multiple inheritance).

To put it simply, you should only extend a class when the class you are creating has an "is a" relationship with the class you are extending; for example, a GraduateStudent "is a" Student. In contrast, a Student might be able to fly, and would implement the Flyable interface. (Clearly, the Student does not have an "is a" relationship with his/her ability to fly.)

null_ptr0 · Jan 7th, 2008, 6:59 PM

Used like callbacks and function pointer replacements.

titaniumdecoy · Jan 7th, 2008, 9:07 PM

Quote:

Originally Posted by null_ptr0

Used like callbacks and function pointer replacements.

Care to explain?

null_ptr0 · Jan 7th, 2008, 9:29 PM

Quote:

Originally Posted by titaniumdecoy

Care to explain?

When you have a callback, in c++ event handling, you pass a function ptr so it calls it each time when an event occurs.
In java, there are no function ptrs.
So, we use interfaces with a method like eventCallback(Event e) that each sub/class has to implement and just call that method.

Jan 7th, 2008, 5:34 PM	#1
namsu Newbie Join Date: Jan 2008 Posts: 15 Rep Power: 0	Interfaces Hi, I am very confused with interfaces. I understand how to use them and when to use them. I understand the difference between the abstract class and interfaces. Can someone explain to me the real importance of using an interface, because according to me it seems as if we declare an interface so all other classes can use the same name for a particular method. Why don't people just declare the same name in each class rather than having to go through something just to use the name?

Jan 7th, 2008, 6:12 PM	#2
Sane Programming Guru Join Date: Apr 2005 Location: Waterloo, Ontario Posts: 1,885 Rep Power: 5	Re: Interfaces I am even more confused over how you're managing to skip from learning about "For Loops" and "Conditional Operators", to learning about Object Oriented Programming with Interfaces in Java. Either you're going too quickly, or you're trying to understand things you don't yet need to. Regardless, maybe you need to spend more time on the square before this one, to gain further insight into programming before you ask these questions. I can promise the answers will make more sense that way, and you may not even need to ask this question when you get to that point.

Jan 7th, 2008, 6:15 PM	#3
namsu Newbie Join Date: Jan 2008 Posts: 15 Rep Power: 0	Re: Interfaces Hi Sane I appreciate your help. I don't think I am going to fast, I just need an explanation, do you mind giving me one.

Jan 7th, 2008, 6:59 PM	#5
null_ptr0 12 years old Join Date: Nov 2007 Posts: 94 Rep Power: 1	Re: Interfaces Used like callbacks and function pointer replacements.

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