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So I looked online through a number of resources to understand Java interfaces. I believe I have a good general understanding of them but when programming them I am a bit confused...

I created an interface called A and have the following inside...

 public interface A { public int sum(int first, int second); }

I then created a class called B.

public class B implements A { public static void main(String [] args){ int result = sum(3, 5); } public int sum(int first, int second) { int total = first + second; return total; } }

Now what I am trying to figure out is how can I properly call / use the method "sum". In Eclipse I'm getting an error for the line "int result = sum(3, 5);" and it is telling me to make the method static. If I make it static, then the method needs to match it in the interface. However, I am not able to use static methods in an interface?

Any help is appreciated and thank you for your time to read about my problems.

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    It is not the best example: sum is fundamentally static... Commented Jan 29, 2013 at 23:50

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The issue you have is not of interface but of static method.

main is a static method. That means it is not linked to an object/instance, but to the class itself.

Since you want to use sum, which is an instance method, you need first to create an object to call its method.

A a = new B(); int result = a.sum(5, 6);

Usually, instance methods are more linked to the object status, while static methods are more like "procedures" in non OO languages. In the case of sum, your method would make more sense as static. But, if you use B to wrap a value (status), and use sum to add to your internal status, this would end (in a more OO friendly way).

A a = new B(5); a.sum(6); int result = a.getValue();

Note that both approachs are valid and both compile in Java, it is just a matter of selecting the modifiers that make more sense in each occasion.

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You cannot call sum() from your main method because sum is an instance method, not static. It needs to be called from an instance of a class.

You need to instantiate your class:

public static void main(String [] args) { B b = new B(); int result = b.sum(3, 5); }
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public class B implements A { public static void main(String [] args){ int result = new B.sum(3, 5); // Create an instance of B so you can access // the non-static method from a static reference // Or if you want to see the power of the interface... A a = new B(); int result = a.sum(3, 5); } public int sum(int first, int second) { int total = first + second; return total; } }
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By creating an instance of B like this:

A a = new B(); int result = a.sum(3, 5);
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It's the issue of static, not interface. You can not call non-static method from static method. You can call sum method by creating its object.

like,

int result = new B.sum(3, 5);

inside static method.

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I will try to make this clear by giving another example:

Make a classed named Animal.

public interface Animal { String speak(); }

Now make a class named Cat

public class Cat implements Animal { public String speak() { return "meow" } }

And a class named Dog

public class Dog implements Animal { public String speak() { return "woof" } }

Now you can do this

public String speak(Animal a) { System.out.printf("%s\n", a.speak()); } Cat cat = new Animal(); Dog dog = new Animal(); speak(cat); //prints meow speak(dog); //prints woof

That means Cat is an Animal and Dog is is an Animal too, so you can pass a Cat or Dog object to a function that takes Animal arguments.

It's like Inheritance, but since in Java a class can only inherit from one other class, you can use Interfaces to get around that. You only declare methods in the Interface; you have to define them in the class that implements it.

It can also be used for things like ActionListeners or MouseListeners. You can have a GUI class that implements it, then have a function that handles all of your ActionEvents like button clicking and mouse clicking.

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The problem seems to be very simple , either make your sum method static or create an instance of class B and use it to call the sum method:

B b=new B(); int result=b.sum(3,5);

or

just write static before your sum method

for eg:

 public static int sum(int first, int second) { int total = first + second; return total; }
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