OOPS

• Classes
  • Blueprint/template for the objects
  • has
    • fields/properties
    • methods/behaviour
    • constructor
      • By default, JVM injects a default constructor(no-args constructor)
• Objects
  • Instances of the class
• Constructor
  • used to initialize the internal state
  • [DEFAULT] No-Argument Constructor, the compiler adds a default, no-argument constructor
    • default value for primitives types are 0 or 0.0 or false
    • default value for reference types(objects) are null
  • Parameterized constructor
  • Copy Constructor
  • Chained Constructor this(arg1, arg2); or super(arg1, arg2)
    • this or super expression MUST be the first statement to use in the constructor
  • Value Objects
    • Don't change its instance state after they are created
    • Fields are marked as final i.e immutable instances
  • If too many constructor are there then, may be we can use the Creational Design Pattern for better object creation
    • Singleton
    • Factory
    • Abstract Factory
    • Builder etc
• Access Modifiers
  • [default] package private - all members are visible within the same package accessible
  • public - we’re making it available to the whole world
  • private - accessible from the same class only
  • protected - same package (as with package-private access level), as well as from all subclasses of its class
• Concrete Class
  • can create instance of using the new keyword (can be instantiated)
  • Eg: HashMap, HashSet, ArrayList, and LinkedList
• Encapsulation
  • responsible for bundling methods and data, while information hiding is a consequence of encapsulation — it hides an object’s internal representation.
  • information hiding
  • achieved using
    • access modifiers like private
• Abstraction
  • Flexibility to think in more general terms about the domain we are trying to model
  • achieved using
    • interface
      • blueprint for a class
      • Eg: Map, List, and Set
    • abstract classes
      • class that has unimplemented methods
      • Eg: AbstractMap and AbstractList.
• Inheritence
  • enables us to reuse existing code or extend an existing type
  • subclass class inherits the non-static protected and public members from the superclass class
  • private and static members of a class are not inherited.
  • this and super keywords are extensively used to access the current and parent class members
  • Inheritence is a IS-A relation
  • Composition is a HAS-A kind of relation
• Association
  • Aggregation - Have independent life-times for the dependent objects and can exist independently to the parent object (Eg: student-departement)
  • Composition (HAS-A relation) - child cannot exist independent (Eg: company-departement)