Introduction to Object-Oriented Programming and Java Basics

This lesson introduces object-oriented programming (OOP), the Java platform, the structure of a basic Java program, variables, data types, and operators.

By the end of the lesson, a learner should be able to explain common OOP features, describe how Java source code becomes executable bytecode, write valid variable declarations, choose appropriate primitive data types, and use basic Java operators.

Procedural programming organizes a program as a sequence of instructions. The program often moves step by step through statements, decisions, loops, and procedure calls.

Object-oriented programming organizes a program around objects. An object combines data and behavior in one unit. In Java, objects are created from classes. This style is useful for larger programs because it lets developers model real-world entities, reuse code, and keep related logic together.

• Encapsulation keeps data and the methods that work on that data together. It also helps protect data from direct outside access.
• Inheritance allows one class to reuse and extend the features of another class.
• Polymorphism allows the same operation or method name to behave differently depending on the object or parameters involved.
• Abstraction focuses on important details while hiding unnecessary implementation details.
• Message passing happens when objects communicate by calling methods and passing values as arguments.

Java is both a programming language and a platform. Java source code is written in .java files, compiled into bytecode, and then executed by the Java Virtual Machine (JVM).

The main idea is portability: the same compiled bytecode can run on any system that has a compatible JVM. This is often described as write once, run anywhere.

The Java platform includes:

A basic Java application normally contains a class and a main method. The main method is the starting point of execution.

Important parts of a simple Java program:

Create a file named HelloJava.java and run it.

public class HelloJava {
    public static void main(String[] args) {
        System.out.println("Hello, Java learner!");
    }
}

Expected output:

Hello, Java learner!

Comments are notes for human readers. The Java compiler ignores them.

A variable is a named storage location. In Java, every variable must have a data type.

Example:

int age = 20;
double price = 99.75;
char grade = 'A';
boolean isPassed = true;

Common primitive data types:

A Java variable name must be a valid identifier. It cannot be a reserved keyword, and it cannot duplicate another variable name in the same scope.

Common naming conventions:

• Variable names usually start with a lowercase letter: score, totalAmount.
• Class names usually start with an uppercase letter: StudentRecord.
• Use camelCase for multi-word variable names: isVisible, studentAge.

A variable may be given a value when it is declared.

int count = 0;

A final variable can be assigned only once.

final double TAX_RATE = 0.15;

After a final variable receives a value, attempting to assign a new value will cause a compile-time error.

Arithmetic operators perform numeric calculations.

Relational operators compare values and produce a boolean result.

Conditional operators combine or reverse boolean expressions.

Assignment shortcuts combine calculation and assignment.

x += 2;  // same as x = x + 2
x -= 1;  // same as x = x - 1
x *= 3;  // same as x = x * 3
x /= 2;  // same as x = x / 2
x %= 5;  // same as x = x % 5

An expression is a combination of values, variables, operators, and method calls that evaluates to one result.

Examples:

int total = price * quantity;
boolean isAdult = age >= 18;
int larger = (a > b) ? a : b;

Run the code below. Try changing the variable values and add your own.