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
