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Hey, I have forgotten what a Java interface is.


Would someone be so kind as to pick my brain as to what an interface in Java is.

Many Thanks,


3 Answers

Craig Dennis
Craig Dennis
Treehouse Teacher

Think about them like a contract. Classes use the keyword implements and are then required to implement all the methods.

When you declare a variable, it is best practice to declare it using the interface that it implements because then you can later change the implementation.

That help bring it all back?

A Java interface is like a class except that it has no instance variables, no constructors, and all its methods are abstract.

Why have them? If a class implements an interface it is promising to implement (make concrete) those abstract methods.

For example, Shape couldn't be a class if it has a getArea() method because there is no area formula for a generic shape. Instead, Shape could be an interface with an abstract getArea() method, and require any implementing class, like Circle or Rectangle, to make that abstract class getArea() concrete.

Note, there are folk who would argue that Shape should be an abstract class rather than an interface, but that is another story. The example still holds.


Thank you both, this has really brought it back for me.

Many thanks,