Learn Java Programming : Lesson 6 - Object-oriented Programming Concepts With Classes and Object

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I. Introduction

• OOP or Object-Oriented Programming is about creating objects that contain both attributes and methods that perform operations. Aside from Java other popular OOP languages are C++ and Python.
• On the other hand, Procedural Programming is about writing procedures/routines/methods/functions that perform an operation on data. Popular examples would be PHP, C, Typescript, Javascript, etc.
• Basically, there are 4 pillars of Object-Oriented Programming namely inheritance, polymorphism, abstraction and encapsulation. But before that let’s take a look at classes and objects first.

II. Class

• Class is the template or blueprint for creating an object and everything in Java is associated with them.
• A class would normally contain a constructor (always), attributes and methods.
• To create a class, we would use the keyword “class”.

III. Object

• To instantiate a class and create an object, we will use the keyword “new”.
• A class is instantiated by calling the constructor of the class.
• A constructor with no argument is available by default.

Running the jar in the terminal.

java -cp java-programming-beginner-0.0.1-SNAPSHOT.jar com.czetsuyatech.lesson6.Account

IV ToString, HashCode, Equals

• By default, all class defined in Java extends the object class which makes several methods available such as toString, hashCode and equals.
• equals - it compares whether 2 objects instantiated from the same class are equal. This is different compared to the “==” operator when comparing reference types as it compares whether the 2 objects refer to the same object in the memory.
• hashcode - returns a hash code value (int) of the object.
• Hash code value is always the same on the same object.
• If objects a and b are equals, then they should have the same hash code.
• Invoking hashcode with the same result does not necessarily mean that they are equal. This is due to the limitation of the value available in int.
• toString - is the string representation of the object

V. Defining Methods

• A method or a function in Java is just a block of code that performs certain actions when called.
• Data can be pass to a method via parameter.
• Why define a method? For code reuse, as they said to define a method once and use it multiple times.
• The void method returns nothing.
• A variable declared inside a method is called local, it is stored in a temporary state.

VI. Pass by Value

• Everything in Java is passed by value.
• Unfortunately, when passing an object it passes the reference to that object. Thus changing the value of the object inside a method, change the value in the caller as well. This is indeed, confusing for beginners.

For example, when defining an account:

Account account; ← this statement creates a new pointer to the Account. So,

Account account = new Account(“Sarrah”);

passByValue(account); ← means we are passing the pointer to the account object. See my previous video on how objects are stored in the memory.

VII. Class vs Instance

• A class variable is another name for a static field.
• Example static int x = 10;
• Instance variable is another name for non-static field.
• Example int x = 10;

 

Static/Class variable	Instance variable
Declared with a static keyword inside a class but outside a method, constructor or any block.	Declared inside a class but outside a method, constructor or any block.
Created when the program starts and destroyed on termination.	Created when a new object is created via “new” keyword.
It can be accessed by calling the class name. No need to initialize a class.	It can be accessed by calling the object declaration name.
Only one copy of each class variable per class.	Can hold multiple values per class. Different values per object.