![]() ![]() Looks like an instance variable, doesn't it? Int i=10 /*IS THIS AN INSTANCE VARIABLE? SURE LOOKS LIKE IT */ It might just tackle the source of your confusion with regards to Java interfaces and instance variables.Ĭuriously, the following code will compile : I can have all the opinions in the world, but at the end of the day, they must all be in line with what the compiler spits out. The final word always comes down to the compiler. Of course, I can spout off all day about Java, and Interfaces, and Abstract classes. An abstract class is that bridge between a highly abstract interface, and a concrete class that provides a full implementation. ![]() If you do wish to declare instance variables in a component that also declares abstract classes, you can declare an abstract class. I do like declaring constants in an interface, but even then, there is something known as the "constants interface ANTI-pattern' which new developers can fall into if they take the practice too far. A static final variable is really just associated once with the JVM that is running, and it is not tied to any particular instance. But this is different from an instance variable. What you CAN do in an interface is declare a constant, or in Java-speak, you can declare a static final variable. Using a variable declared in an interface as an instance variable will trigger a compile time error. In a world where every programmer has their own idea of the best way to implement something, Java interfaces provide implementation flexibility, appealing to the ego of all of those narcissistic programmers out there.īut an interface indeed does not allow you to declare any instance variables. If a concrete implementing class doesn't need any instance variables to provide a concrete implementation, then it doesn't need to declare any instance variables. If a concrete implementing class needs ten instance variables to implement an interface method, it can declare ten instance variables. In fact, the methods we declare in an interface are said to be implicitly abstract, although they don't necessarily need to have the abstract keyword - doesn't hurt to put it in though.īut interfaces are flexible because they allow the implementing classes to deliver a code body in any way that makes sense. So, in Java interfaces, we simply declare methods for which implementing classes will provide an implementation. Interfaces express a common behavior, and in Java, behavior translates into methods. Applied effectively, they can become one of the most important parts of an application design. When used effectively, interfaces become extremely powerful plugin points that make your design incredibly flexible.
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