Skip to main content

2. OO and Java Programming Refresher (2)

10/10/21

Java collections framework​

  • A collection is an object that represents a group of objects
  • Java Collections Framework contains data structures (Arrays, lists, maps etc)
  • Java Collection Framework contains algorithmic operations (searching, sorting)
  • Collection: Something that holds a dynamic collection of objects (Represents a group of objects/elements)
  • Map: Defines mapping between keys and objects (Maps keys to values; no duplication)
  • Iterable: Collections are able to return an iterator object that can scan over the contents of a collection one object at a time. (Represents an iterator object)
  • Queue: Represents FIFO queues or LIFO stacks
  • Deque: Represents a double ended queue
  • Set: A collection that cannot contain duplicate elements
  • List: An ordered sequence of elements that allows duplicate elements.

Classes that implement the collection interfaces typically have names in the form of <Implementation style><Interface>

  • Non typesafe collections (DO NOT USE) - Collection constructors are not able to specify the type of objects the collection is intended to contain
  • Typesafe collections with 'Generics' - Classes support generics by allowing a type variable to be included in their declaration; type are declared for the reference and constructor. Cannot type a collection using a primitive type (int -> Integer)

TreeSet Class​

Provides an implementation of the Set interface that uses a tree for storage. Objects are stored in sorted, ascending order.

HashMap Class​

HashMap is a Hash table implementation of the Map interface. This implementation provides all of the optional map operations, and permits null values and the null key

Implementation of object oriented concepts in java​

Aggregation​

An object of class B is part of an object of class A (semantically) but the object of class B can be shared and if the object of class A is deleted, the object of class B is not deleted.

Composition​

An object of class A owns an object of class B and the object of class B cannot be shared and if the object of class A is deleted, the object of class B is also deleted

Inheritance​

Forming new classes based on existing ones Superclass: Parent class being extended Subclass: Child class that inherits behaviour from the parent class. "Is-A" relationship

Polymorphism​

  • Polymorphism is an object oriented concept
  • Method overloading and method overriding are two forms of polymorphism
  • Method overloading: Methods with the same name co-exists in the same class but they must have different method signatures. Resolved during compile time
  • Method overriding: Methods with the same name is declared in super and sub class. Resolved during runtime

Abstract Methods and Classes​

Abstract classes cannot be used to instantiate objects but references to abstract classes are legal Classes can have instance methods that implement a default behaviour. May contain non-final variables

Interfaces​

  • Methods are implicitly abstract and most cannot have implementations. Variables declared are by default static and final
  • Interface is an abstract type that is used to describe a behaviour that classes must implement. May only contain method signature and constant declarations
  • Cannot be instantiated, but rather are implemented
  • A class that implements an interface must implement all of the non-default methods described in the interface, or be an abstract class.
  • They are less restrictive when it comes to inheritance