OOP Concepts

OOP Concepts

The Object Oriented Programming (OOP) concepts are encapsulation, inheritance and polymorphism.

1. Encapsulation

The main principles of the encapsulation:

  • The instance variables should be kept protected (with an access modifier, usually private).
  • The instance variable should be accessed only by public accessor methods.
  • The accessor methods should follow the JavaBeans naming convention.

2. Inheritance

The inheritance relationships are implemented in Java by extending a class. Java supports only single inheritance - a class cannot extend more than one class, but a class can have multiple ancestors. The two most general reasons to use inheritance are:

  • Promoting code reuse means that methods with generic functionality don't have to be reimplemented.
  • Using polymorphism

2.1 IS-A Relationship

The IS-A relationship is based on class inheritance or interface implementation. In Java it is expressed using the keywords extends and implements.

2.2 HAS-A Relationship

The HAS-A relationship is based on usage. A class A HAS-A B if code in class A has a reference to an instance of class B.

3. Polymorphism

Polymorphism means "many forms". If a Java object can pass more than one IS-A test, it can be considered polymorphic.

  • A reference variable can be of only one type and that type cannot be changed (but the object it references can change).
  • A reference is a variable, so it can be reassigned to other objects.
  • A reference variable's type determines the methods that can be called on the object the variable is referencing.
  • A reference variable can refer to any object of the same type as the declared reference, or to any subtype of the declared type.
  • A reference variable can be declared as a class type or an interface type. If the variable is declared as an interface type, it can reference any object of any class that implements the interface.
  • Polymorphic method invocations apply only to instance methods, not to static methods or variables.


Sources:  SCJP Sun Certified Programmer for Java 6 Exam 310-065.

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