In plain English, abstract means a concept or idea not associated with any specific instance and does not have a concrete existence. Abstraction in Object Oriented Programming refers to the ability to make a class abstract. Abstraction captures only those details about an object that are relevant to the current perspective, so that the programmer can focus on a few concepts at a time.

Java provides interfaces and abstract classes for describing abstract types.

• An interface is a contract or specification without any implementation. An interface can't have behavior or state.

• An abstract class is a class that cannot be instantiated, but has all the properties of a class including constructors. Abstract classes can have state and can be used to provide a skeletal implementation. 

Constructor of an abstract class is invoked by a subclass from its constructor using super keyword (e.g. super()).

Defining an interface

An interface can be defined using the interface keyword.

interface MyInterface{

// public static and final fields
// public and abstract methods

}

Properties of an Interface

1. An interface is a contract and they don't have state, behavior or constructors.

2. Interfaces can have only public and abstract methods.

   • The public and abstract keywords are implicit for interface methods, but can also be declared explicitly.

3. Interfaces can have static constant fields (public, static and final).

   • Fields of an interface are implicitly public, static and final; though they can be explicitly defined with these modifiers. 

4. An interface can have another class as a member

   • An inner class of an interface can be marked with public, static, final and abstract; but not final and abstract together.

5. An interface can have another interface as a member

   • An inner innerface of an interface can be marked with public, static, and abstract.

6. First concrete subclass should implement all abstract methods.

   • An abstract subclass may or may not implement any methods of an interface

7. A class can implement an interface using the keyword implements.

   • class A implements I {}

8. An interface can extend another interface using extends keyword.

   • interface I1 extends I2 {}

9. An inteface may be marked as abstract explicitely

   1. E.g. abstract interface Inter1{}

10. Top level interface can be only public or package-private (default)

Defining an abstract class

We can define an abstract class using the abstract keyword. 

An abstract class is a class marked as abstract and they cannot be instantiated.

class abstract MyAbstractClass{

  abstract void myAbstractMethod();

  void myConcreteMethod(){

    //method body

  }

}

An abstract method is a method with only signature and no body, whereas a concrete method is a method with body. 

The abstract methods are defined using the abstract keyword and omitting the body and ending the statement with a semicolon.

Properties of Abstract Class

1. Abstract classes cannot be instantiated using the new keyword. 

2. Abstract classes can have state and can be used to provide a skeletal implementation. 

3. Abstract classes have all the features of a class, including constructors (except that abstract classes cannot be instantiated using the new keyword).

   • Constructor of an abstract class is invoked by the subclass constructor using super keyword (e.g. super()).

   • Abstract classes can use this pointer.

   • Abstract classes can have private or any access level methods.

   • Abstract classes can also have synchronized methods.

4. Abstract classes can have abstract and/or concrete methods in any combination.

   • Abstract class can have only concrete methods and no abstract methods.

5. A concrete class should extend an abstract class (using keyword extends) and implement all abstract methods.

A detailed comparison of interfaces and abstract classes can be found at interface-vs-abstract-class.