Inheritance, Interface, Composition and Multiple Inheritance In Java

Inheritance : 

• Core OOPS concept.
• Is-a relationship with objects
• "Extends" is used
• Transitive in nature A-->B B-->C  A-->C

Interface:

• 'interface' word used to define interface
• can't instantiate interface so constructors not allowed in interface
• methods can be defined but not implemented in interface i.e. can't have a body. by default menthods are public and abstract
• by default any attribute of interface is public, static and final even if no access modifier is defined
• can't extend a class but can extend 1/many interfaces. extends is the keyword used
• implements keyword is used to implement an interface.
• Once a class implements all methods should be implemented. Implemented class can define its own methods

Benefits of interface:

• Implementation class can't remove the methods defined in interface so it acts as a contract
• can act as Type and helpful in defining top level hierarchy
• class can implement multiple interfaces, so interface acts as better super class

Composition:

• Has-a relationship

Multiple Inheritance:

• Not supported in Java Classes as it can lead to diamond problem. Superclass A is abstract with method test(). A is inherited by B and C hence test() is defined. Now, if D extends both B and C and test() method is invoked in D, the compiler gets confused which is known as diamond problem.
• Supported in Java Interfaces. A single interface can 'extends' multiple interfaces
• Java class can 'implements' multiple interfaces
• when overriding any superclass method or implementing any interface method use @Override notation
• public class InterfaceImpl implements InterfaceA, InterfaceB --> valid
• public interface extends InterfaceA, InterfaceA --> Valid
• If methods from different classes need to be utilized, we can use composition by defining the class object and using the method.

Problems with Inheritance:

• Methods need to be the same can't change return type etc.
• all superclass methods are exposed to client which could lead to security issues if super class not designed properly
• Unit testing a bit difficult and we need to test all methods of super class