JAVA - define a method in interface

Question

I would like to define a method in JAVA interface. The reason is that every time I implement interface the method is the same, but I need to implement two interfaces for particular classes. Example:

interface A
method A()

interface B
method B()

class first implements A,B

class second implements A

method A() has same body everywhere.

You can do that in Java 8 with the default keyword. You can't do it in earlier versions. docs.oracle.com/javase/tutorial/java/IandI/defaultmethods.html — khelwood
– khelwood, Commented Oct 1, 2014 at 12:40
As long as it's only interface A that needs implementations, you can get away in java <8 by making it an abstract class and keeping B an interface — OhleC
– OhleC, Commented Oct 1, 2014 at 12:43
I was thinking about abstract classes but there will be more interfaces with defined methods that I would like to implement. Thanks for an answer khelwood, your comment was very helpful for me. — jakub
– jakub, Commented Oct 1, 2014 at 12:45

khelwood · Accepted Answer · 2014-10-01 12:45:24Z

4

As of Java 8, you can put method implementations into interfaces. http://docs.oracle.com/javase/tutorial/java/IandI/defaultmethods.html

interface A {
    default void aMethod() {
        // method body
    }
}

In earlier versions, you would have to make A a class instead of an interface. An abstract class if that suits your model better.

abstract class A {
     public void aMethod() {
          // method body
     }
}

class first extends A implements B {
    ...
}

answered Oct 1, 2014 at 12:45

khelwood

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