Inheritance from library classes

Option 1

If I understand correctly, then interfaces can help you:

There is an abstract class:

public abstract class Abstr { public abstract void method(); } 

And there is an interface: in which you can create a method with a default implementation and methods that you need to implement.

 public interface Interface { //Поле, которое будет присутствовать во всех имплементациях public int field = 0; //"Пустой" метод который необходимо определить в имплементации public void anotherMethod(); //Метод с реализацией по умолчанию, будет доступен в имплементации default int retInt(){ return 5; }; } 

Implementations:

 public class RealizationAbstr extends Abstr implements Interface { @Override public void method() { System.out.println("Hello from first Realization"); } @Override public void anotherMethod() { System.out.println(1); } } public class RealizationAbstr2 extends Abstr implements Interface { @Override public void method() { System.out.println("Hello from second Realization"); } @Override public void anotherMethod() { System.out.println(2); } } 

However, with this approach there is a limitation - the interface fields are essentially constants and there is no possibility to change them.

Option 2

Another option would be to embed an intermediate class. That is, move from this model of Inheritance:

 Abstract <- 3x(Realization) 

To this:

 Abstract <- AbstractWithFieldsAndMethods <-3x(Realization) 

The method is simple and suitable if you can edit classes that implement an abstract class.

You can use defaulted implementations of interface methods .

You make an heir and you specify an interface with it that contains the default implementation:

 class Base { public int functionFromBase() { return 0; } } class A extends Base { public int functionFromA() { return 1; } } class B extends Base { public int functionFromB() { return 2; } } interface Common { default int functionFromCommon() { return 7; } } class AA extends A implements Common {} class BB extends B implements Common {} 

 AA a = new AA(); BB b = new BB(); System.out.println(a.functionFromBase() + " " + b.functionFromBase() ); System.out.println(a.functionFromA() + " " + b.functionFromB() ); System.out.println(a.functionFromCommon() + " " + b.functionFromCommon()); 

Full example: http://ideone.com/9adriG

PS: In C #, extension methods play a similar role. In some other languages ​​(for example, js) the notion of mixins can be used.