What are the differences between abstract classes and interfaces in Java 8?

Solution 1:

Interfaces cannot have state associated with them.

Abstract classes can have state associated with them.

Furthermore, default methods in interfaces need not be implemented. So in this way, it will not break already existing code, as while the interface does receive an update, the implementing class does not need to implement it.
As a result you may get suboptimal code, but if you want to have more optimal code, then your job is to override the default implementation.

And lastly, in case a diamond problem occurs, then the compiler will warn you, and you will need to choose which interface you want to implement.

To show more about the diamond problem, consider the following code:

interface A {
    void method();
}

interface B extends A {
    @Override
    default void method() {
        System.out.println("B");
    }
}

interface C extends A { 
    @Override
    default void method() {
        System.out.println("C");
    }
}

interface D extends B, C {

}

Here I get the compiler error on interface D extends B, C, that:

interface D inherits unrelated defaults for method() form types B and C

The fix is:

interface D extends B, C {
    @Override
    default void method() {
        B.super.method();
    }
}

In case I wanted to inherit the method() from B.
The same holds for if D were a class.

To show even more about the difference between interfaces and abstract classes in Java 8, consider the following Team:

interface Player {

}

interface Team {
    void addPlayer(Player player);
}

You can in theory provide a default implementation of addPlayer such that you can add players to for example a list of players.
But wait...?
How do I store the list of players?
The answer is that you cannot do that in an interface, even if you have default implementations available.

Solution 2:

There have been some very detailed answers, but they seem to be missing one point that I at least consider as one of the very few justifications to have abstract classes at all:

Abstract classes can have protected members (and members with default visibility). Methods in interfaces are implicitly public.