Tech-Notes

Object oriented programming

• Introduction
• Class
• Constructor
• Object
• Methods
• Encapsulation
• Inheritance
• Polymorphism
• Abstraction
• Other Topics

Introduction

Oops is a programming paradigm(style), based on concept of object & class

Adv of oops: easy to extend/improve, code reusability, readability

Example:

• to understand class object. take shapes as class and box is shape model
  Attribute are characteristics of Box ie:length, breadth, height
  Methods are abilities/functionalities of Box. eg: calculateVolume(){lbh}

Class

template or blueprint for object (defines variable and methods) ie:noun & verbs

Types of Classes:

1. Regular class
2. Nested class : OuterClass.InnerClass obj = outerObject.new InnerClass();
3. Nested static: OuterClass.NestedStaticClass obj = new OuterClass.NestedStaticClass();
4. Anonymous : A obj = new A(){//class body};

Steps to create immutable class:

1. Make class as final
2. Declare all fields as private and final
3. Use parameterized constructor to initialize variables.
4. Dont provide setters method
5. Return a new instance of objects in the clone() method.
6. If using mutable objects, return defensive copies return new ArrayList<>(hobbies);

Common class Exceptions:

1. NoClassDefFoundError: Class present at compiletime but missing at runtime.
2. ClassNotFoundException: Occurs during runtime, Class.forName() or loadClass(

Constructor

special type of method, been called while intialize object

Note: it has no return type, cannot be final.

1. Default Constructor.
2. Parameterized Constructor

super() must call first line of child class constructor
this() will call current class constructor

Object

instant of a class

ClassName obj = new ClassName();
LHS (heap) reference = RHS (stack) instance

Anonymous object

created and used only omce without assigning

new ClassName().functionName();

Methods of Object class:

1. toString() - String representation of an Object
2. hashCode() - JVM generates a unique number, used for hash based DS.
3. equals(Object obj) - Compares the given object to “this” object
4. getClass() - Returns the class object of “this” object Class c = obj.getClass(); c.getClassName();
5. finalize() - called just before an object is garbage collected
6. clone() - returns a new object
7. wait(), notify() notifyAll() - related to Concurrency

4 ways to create object

1. New keyword
2. clone()
3. class.forName(“classname”).newInstance();
4. Deserialization while readObject()

Garbage collection

Automatic memory management that destroys objects that are no longer referenced.

• uses Mark and Sweep algorithm
• Garbage first (G1)collector used in java8
• System.gc(); invokes garbage collection.

  Ways Garbage Collection is Triggered:

  1. while nullfying the object rajesh obj1=new rajesh(); obj1=null;
  2. while assigning reference to other obj1=obj2
  3. while using anonymous object
     Finalize() Method : Called before Garbage Collector
     Note: finalize method deprecated in java9 & removed in java18. use AutoCloseable and try-with-resources
     cause OutOfMemoryError

Methods

block of code that perform specific task. it has methodName, arguments, returnType & returnValue

Varargs : Allows passing a variable number of arguments to a method
void methodname(int ...a){//use as a[];}

Encapsulation:

hiding data

Access Modifiers:

1. Public- Any class any package
2. Default- Specific package - default if no specifier mentioned
3. private - Specified Class
4. protected –subsiding class(inheritance)

Non Access Modifiers:

1. final - cannot be overrided/modified/extended
2. abstract - does not have method body, cannot create object directly
3. static - belong to class rather object
4. transient - attribute&methods skipped while serialization
5. synchronized - can access by one thread at a time
6. volatile - value not cached for thread level rather in Main memory level

Note: we cannot use private, protected in class, but we can use in innerclass

Inheritances

one class acquires the properties of another class.

Types of Inheritance

1. Single inheritance
2. Multilevel inheritance
3. Hierarchical inheritance

Super keyword:

1. super.variablename;
2. super.methodname();
3. super(); - explicitly call a specific constructor of the parent class
   Note: When child constructor is called, parent constructor is executed first automatically without super().

Polymorphism

ability to define a method(&constructor) in many forms

1. Method overriding : same method name, same no. of arguements, same type ie.static/compiletime
2. Method overloading : same method name, diff no. of arguements, diff type ie.dynamic/runtime

Method overriding Scenarios:

1. Covariant Return Type - allowed
   ParentClass getValue(){} -> ChildClass getValue(){}
2. Subclass or without Exceptions: allowed
   int getValue() throws IOException{} -> int getValue() throws FileNotFoundException{}
int getValue() throws IOException{} -> int getValue(){}
3. Access Modifiers - same or narrow access allowed protected void show()-> public void show()
4. Static method: method hiding will happen not overriding static void show(){} -> static void show(){}
5. Synchroniezed method: allowed, no effect
   ` synchronized void execute() {} -> void execute() {} `
6. Varargs Methods: allowed
   void log(String... messages) {} -> void log(String... messages) {}

Abstraction

Hiding the implementation

1. Abstract(0 to 100% )
2. Interface (100%) - variables are public static final by default, methods are public abstract
   a) Normal interface
   b) Marker interface - empty body eg:cloneable, serializable
   c) Functional interface - have only one method declaration
   d) defualt access modfier, method which has body

Other Topic

Relationship

1. IS-A relationship. also called as inheritance
   eg: Circle(child class) IS A Shape(parent class)
2. HAS-A reltionship. also called as association.
   eg Circle(class) HAS A radius(attribute)
   eg: Circle(class) HAS A CalculationUtil(class)

Forms of Association:

1. Aggregation - week bond
   eg: Car HAS A MusicPlayer
2. Composition - strong bond
   eg: Car HAS A Engine