Java OOP (Object-Oriented Programming)

Java OOP (Object-Oriented Programming)

Java is a versatile and robust programming language widely used in the software development industry. It supports object-oriented programming (OOP) principles, allowing for modular and reusable code. With its rich library and extensive community support, Java is suitable for various domains such as web development, mobile app development, and enterprise software. It promotes platform independence through the Java Virtual Machine (JVM), enabling Java programs to run on different platforms. Its key features include strong type checking, automatic memory management, and exception handling. Java's popularity stems from its reliability, scalability, and wide range of applications.

Tools you need

  1. IntelliJ for Coding and JavaFX

  2. NetBeans for Java Swing

Java Documentation

  1. Oracle

  2. net_Informations

  3. বাংলায় শিখুন জাভা প্রোগ্রামিং


  1. জাভা প্রোগ্রামিং (দ্বিতীয় সংস্করণ) আ ন ম বজলুর রহমান

  2. Head First Java

Things you should know

Why Java?

Learning Java can be beneficial for several reasons:

  1. Versatility: Java is a versatile programming language widely used in various domains, such as web development, mobile app development (Android), enterprise software, and more.

  2. Job opportunities: Java is one of the most in-demand programming languages in the job market. Learning Java can open up a wide range of career opportunities.

  3. Popularity and community support: Java has a vast and active community of developers, which means there are numerous resources, libraries, and frameworks available to support your learning and development.

  4. Object-oriented programming (OOP): Java is an object-oriented language, which helps you understand key programming concepts like classes, objects, inheritance, and polymorphism, which are widely used in many other languages.

  5. Scalability and performance: Java is known for its scalability and performance. It's used in building robust and high-performance applications, making it suitable for both small projects and large-scale enterprise systems.

How Java works

Java identifiers

Identifiers are names that are helpful in uniquely recognizing different types of content or aspects that build up language.

Here are some rules for naming Java identifiers:

  • Identifiers must start with a letter.

  • Identifiers can contain letters, digits, and underscores.

  • Identifiers cannot contain spaces.

  • Identifiers cannot start with a digit.

  • Identifiers cannot be the same as any Java keywords.

Here are some examples: myVar, my_var, myVar123, $myVar, etc. But these are invalid Java identifiers: 123myVar, my var, myVar-123, class, etc.

Data type in Java

Data types in Java are used to define the values that can be stored in variables. There are two types of data types in Java: primitive data types and reference data types.

Primitive data types

Primitive data types are the basic data types in Java. They are used to store basic values, such as numbers, characters, and Boolean values. There are eight primitive data types in Java:

  • byte

  • short

  • int

  • long

  • float

  • double

  • boolean

  • char

Reference data types

Reference data types are used to store references to objects. Objects are created from classes, and they can contain data and methods. There are many different types of reference data types in Java, including:

  • String

  • Arrays

  • Classes

  • Interfaces

Class in JAVA

Everything we write in Java is under a class. In Java, a class is a blueprint for creating objects. It defines the properties and behaviors of an object. A class can contain Fields, Methods, Constructors, Blocks, Nested classes, Interfaces, etc.

To understand more about the class: Class & Object

Print Statement without Semicolon

int i = 0;
if(System.out.printf("Sanfoundary!! ") == null) {}
for(i = 1; i < 2; System.out.println("Hello World."))
Scan One char from input (Char Input)
<a target="_blank" rel="noopener noreferrer nofollow" href="" style="pointer-events: none">scan*.next().charAt(0);
Char to string
String str = ""+ 'a';


These are only a few of the 98 platform modules. The diagram is a dependency graph. Each box indicates a module, and if a module has an arrow pointing to another module, it means that the module where the hand originates needs the module it is pointing at to perform its functions.

For Each loop

Array in Java

To define an array in Java, you need to specify the type of elements it will hold, followed by the array name and the size of the array (number of elements) in square brackets []. Here's the general syntax:

dataType[] arrayName = new dataType[arraySize];

To input values into the array, you can use a loop or assign values individually to each element. Here's an example of how you can input values into an array using a loop:

        Scanner scanner = new Scanner(;
        int[] numbers = new int[5];

        System.out.println("Enter 5 numbers:");
        for (int i = 0; i < numbers.length; i++) {
            numbers[i] = scanner.nextInt();


To print the array, you can use a loop to iterate over the elements and print each one individually. Here's an example:

        int[] numbers = {1, 2, 3, 4, 5};

        System.out.println("Array elements:");
        for (int i = 0; i < numbers.length; i++) {

List VS ArrayList

The List is an interface, and the ArrayList is a class of the Java Collection framework. The List creates a static array, and the ArrayList creates a dynamic array for storing the objects.

Equal method Override

Overriding the equals() method in Java is important because it allows us to define custom equality for objects. The default implementation of equals() compares object references, which may not reflect the desired equality criteria for certain classes. By overriding equals(), we can compare the internal state of objects and determine if they are equal based on our own criteria, providing more meaningful and accurate comparisons when needed.

Object Class in JAVA

In Java, the Object class is the root class for all other classes. It serves as the base class from which all other classes are derived. The Object class provides fundamental methods such as toString(), equals(), and hashCode(), which can be overridden by subclasses to customize their behavior. It also acts as a reference type for objects of any class, allowing them to be treated uniformly and enabling features like polymorphism in Java.

Access Modifiers in JAVA

Garbage Collection in JAVA

Java has no destructor because Java has Garbage Collection.


Constructors in Java are special methods used to initialize objects of a class. They have the same name as the class and are called automatically when an object is created. Constructors can be used to set initial values for object attributes and perform any necessary setup tasks.

a special method responsible for initializing objects

Method VS Constructor

Exception Handling (Try-Catch Finally Block)

Method Overloading

Method overloading in Java allows the creation of multiple methods with the same name but different parameters within a class. It enables the flexibility of using the same method name for similar operations with varying input types or quantities. Method overloading is resolved at compile-time based on the arguments used during method invocation.

Method Overloading | Geekboots | Java programming tutorials, Computer  science programming, Java



Encapsulation in Java is the principle of bundling data and methods within a class, allowing for data hiding and abstraction. It ensures that the internal state of an object is protected and can only be accessed through defined methods, enhancing security and maintainability of the code.


Polymorphism in Java refers to the ability of an object to take on many forms. It allows a single method or class to have different implementations, depending on the context in which it is used. Polymorphism enables code reusability, flexibility, and extensibility in object-oriented programming.

  • polymorphism in Java allows us to perform the same action in many different ways.

  • means more than one form


Inheritance in Java allows for the creation of new classes based on existing classes. It enables the reuse of code and the establishment of hierarchical relationships between classes, where the derived class inherits the properties and methods of the parent class. Inheritance promotes code reusability, modularity, and abstraction.

Hybrid Inheritance in Java with Example


Abstraction is one of the fundamental concepts of object-oriented programming. It is a way of hiding the implementation details of a class and only exposing the essential features to the user. This makes the code more modular and reusable, and it also makes it easier to understand and maintain.

Abstract Class in Java

An abstract class in Java serves as a blueprint for other classes. It cannot be instantiated and may contain both abstract and non-abstract methods. It provides a way to define common methods and attributes that can be shared among multiple subclasses while enforcing implementation requirements.

Abstraction in Java - Java Abstract class and methods - Java Abstraction


An interface in Java is a contract that defines a set of methods that a class implementing the interface must implement. It allows for multiple inheritances and supports the concept of loose coupling. Interfaces are used to achieve abstraction and provide a way to achieve polymorphism in Java.

Java Swing

Everything about swing


JavaFX GUI Tutorials

Practice Java:
CodingBat Java
Hacker Rank

Some Youtube Channels to learn JAVA

  • APNA College

  • Code with harry.

  • Abdul Bari

  • Zulkarnine Mahmud

  • Code with MOSH

  • NESO Accademy

  • Great learning

  • Telusko

  • Coding with John

  • Derek Banas

Useful Shortcuts for Intellij

Ctrl + SideArrows(→) To move Word by word

Ctrl + up and down key work as muse scroll

Alt+ SideArrows(→) To move another class

Ctrl + Shift + Side arrows to select word by word

Alt + Shift + UpDown Keys to move the line up or down

Ctrl + W to select code block by block

Shift + ctrl + enter to end line with a semicolon

Alt+ ctrl + enter to index code

ctrl + click to see the prebuild methods body

ctrl + / for comment

ctrl+ alt +L formatting lines

Shift + F10 Run code

Java interview questions you need to know: Link

Java Project Ideas

  • Bank Management Software.

  • Temperature Converter.

  • Electricity Billing System.

  • Supermarket Billing Software.

  • Memory Game.

  • Link Shortener.

  • Chatting Application.

  • Digital Clock.

Note: Some used pictures are taken form google and some websites.