Abstract class or an interface : JAVA? [closed]

Question

I have one common method which i need to use in several classes with just a single call from the calling class. So what i see is i can call it in two ways.

public abstract class TestAbstractClass {
    void commonMethod(){
        System.out.println("Calling common method : TestAbstractClass");
    }
}

calling class:

public class RunApplication extends TestAbstractClass{

    public void testMethod(){
        commonMethod();
    }
}

[OR]

Using Java 8 feature of default method in interface.

public interface TestInterface {
    default void commonMethod(){
        System.out.println("Calling common method : TestInterface");
    }
}

calling class:

public class RunApplication implements TestInterface{

    public void testMethod(){
        commonMethod();
    }
}

They both works fine for me, But what is better approach, an abstract class with non-abstract method OR Interface with default method.

Answer 1

Abstract class or an interface

If I were you I will judge the appropriateness by checking whether all those classes are related or not.

Generally, I will use interface if the classes which implements it have little or not relation at all and they just happened to share certain behaviour which can be exposed by the method from the interface.

I will use abstraction when the classes are a subset of the other.

Example:

Using bird and plane as an example. Birds and planes both flies. But other than that, they isn't related at all. You will naturally want to implement an interface instead of using abstract class because neither planes are birds and neither birds are planes.

class Bird implements Flyable
{
    @Override
    public void fly(){

    }
}

class Plane implements Flyable
{
    @Override
    public void fly(){

    }
}

---

Another issue to consider is multiple inheritance. In Java, multiple inheritance of a class is not allowed. Hence if you use abstraction, your sub-class won't be able to extends to another class.

If your class implements an interface, it can always implements another interface or extends to one other class.