Java Programming Basics with Examples

Inheritance in Java Inheritance is one of the core concepts of Object-Oriented Programming (OOP). It allows one class to inherit the state (variables) and behavior (methods) of another class.  In simple words, inheritance promotes code reuse and establishes a parent–child relationship between classes. ✅ Definition Inheritance is a mechanism where one class acquires the properties and methods of another class The class that provides data is called the Superclass (Parent class) The class that receives data is called the Subclass (Child class) In Java, inheritance is implemented using the extends keyword 🌍 Real-Life Example Family Inheritance A child inherits eye color, height, and surname from parents Parents → Superclass Child → Subclass Similarly, a subclass inherits variables and methods from its superclass. 🔹 Syntax of Inheritance class SubClass extends SuperClass {     // subclass members } 🔑 Important Points A subclass consumes the state and behavior of the supercl...

this() Statement in Java The this() statement in Java is used to call one constructor from another constructor within the same class. It helps in constructor chaining inside the same class and avoids code duplication . ✅ Correct Definition this() is used to call the current class constructor It is used inside a constructor It must be the first statement in the constructor body It is written explicitly by the programmer It does not support recursive calls A constructor body must contain either this() or super(), not both ⚠️ Important Correction this() → calls constructor of the same class super() → calls constructor of the super (parent) class 🌍 Real-Life Example Online Order System One constructor sets default values Another constructor sets custom values Instead of repeating code, one constructor calls another using this() 🔹 Syntax this(parameterList); 🔹 Example of this() Statement class Student {     Student() {         this(101, "Rahul");   ...

this Keyword in Java The this keyword in Java is a reference variable that refers to the current object of the class that is currently executing. In simple words, this represents the current memory address of the object. ✅ Definition this refers to the current object It is used inside constructors, methods, and instance blocks It is explicitly written by the programmer The compiler does not create this implicitly It helps differentiate between instance variables and local variables when they have the same name 🌍 Real-Life Example Classroom Example A teacher says: “This student should come forward” The word “this” refers to the current student being addressed Similarly, in Java, this refers to the current object. 🔹 Uses of this Keyword in Java 1. Differentiating Instance Variables and Local Variables When local variables and instance variables have the same name, this is used to avoid confusion. class Student {     int id;     Student(int id) {     ...

Constructor Chaining in Java Constructor chaining is a mechanism in Java where one constructor calls another constructor, usually from the parent (super) class, during object creation.   This process ensures that all parent class constructors are executed before child class constructors, maintaining proper object initialization. ✅ Key Requirements Constructor chaining requires inheritance It is performed using the super() statement It cannot be performed within the same class using super() The super() call must be the first statement in the constructor 🌍 Real-Life Example Building Construction First, foundation is built (Parent class constructor) Then walls and rooms are built (Child class constructor) Similarly, Java first executes the superclass constructor, then the subclass constructor. 🔹 Types of Constructor Chaining Constructor chaining can be performed in two ways: Implicit Constructor Call Explicit Constructor Call 1. Implicit Constructor Call ✅ Definition An implicit con...

Constructor Overloading in Java Constructor overloading is a feature in Java where multiple constructors are created within the same class, all having the same class name but different parameter lists. It allows objects to be initialized in different ways, depending on the values provided during object creation. ✅ Definition Constructor overloading occurs when: Constructors have the same name as the class But have different parameters (number, type, or order) Java decides which constructor to call based on the arguments passed during object creation. 🌍 Real-Life Example Mobile Phone Purchase Buy phone with only model name Buy phone with model + price Buy phone with model + price + color Each purchase method represents a different constructor . 🔹 Ways to Perform Constructor Overloading Constructor overloading can be achieved in three ways: 1. Changing the Data Type of Parameters class Sample {     Sample(int a) {         System.out.println("Integer Constru...

Constructor in Java A constructor in Java is a special type of method that is used to initialize the instance variables of a class. It is automatically called when an object is created. Unlike normal methods, constructors are mainly used to set initial values for an object. Rules of Constructor   A constructor must follow these rules: The constructor name must be the same as the class name A constructor does not have any explicit return type (not even void) It is called automatically during object creation It is used to initialize instance-level data Types of Constructors in Java     Constructors are classified into three types: Default Constructor Zero Parameter Constructor Parameterized Constructor 1. Default Constructor ✅ Definition A default constructor is a constructor that is implicitly created by the Java compiler if no constructor is defined by the programmer. It has the same name as the class It has no parameters Its body contains a super() statement It supports ...

Methods in Java A method in Java is a block of statements that is used to perform a specific task and may return a result. Methods help improve code reusability, readability, and modular programming. Java methods are mainly classified into two types: Static Method Instance Method 1. Static Method ✅ Definition A static method is a method that is declared using the static keyword inside a class. It belongs to the class, not to any specific object. Static methods are generally used to perform common operations that are the same for all users. 🔹 Syntax static returnType methodName() {     // method body } 🌍 Real-Life Example ATM Bank Interest Rate Interest calculation formula is the same for all customers So it can be implemented as a static method 🧠 Example Code class Calculator {     static int add(int a, int b) {         return a + b;     } } public class Main {     public static void main(String[] args) {     ...

Object-Oriented Programming (OOP) in Java Object-Oriented Programming (OOP) is a programming approach that organizes software design around objects rather than functions or logic. Java is a pure object-oriented programming language, which means everything revolves around objects and classes. 1. What Is an Object?      An object is a real-world entity that has: State (properties or characteristics) Behavior (actions or functionality) In Java: State is represented by instance variables Behavior is represented by instance methods Java also supports a special member called an instance initializer block, which is used to initialize instance-level data. This block executes when an object is created and does not initialize static members of the class. Real-Life Example of an Object Car Object State (Properties): color, speed, brand, fuelType Behavior (Functions): start(), accelerate(), brake(), stop() Car  ├─ State → color, speed, brand  └─ Behavior → start(), drive(),...

Loop Statements in Java Loop statements in Java are used to execute a block of code repeatedly as long as a given condition is true. They help reduce code duplication and make programs efficient. Java provides three types of loop statements: While Loop Do-While Loop For Loop 1. While Loop ✅ Definition The while loop executes a block of code only if the condition is true. The condition is checked before executing the loop body. 🔹 Syntax        while (condition) {                // loop body       } 🌍 Real-Life Example       Checking mobile battery level       → While battery is not full, keep charging. 📊 Flowchart (While Loop)             Start                  |                 v       Check Condition              (tr...

Conditional Statements in Java Conditional statements are used to control the flow of execution in a Java program based on certain conditions. They allow the program to make decisions and execute different blocks of code. Java provides the following conditional statements: if statement if-else statement if-else-if-else statement switch statement Real-Life Examples of Conditional Statements in Java 1. if Statement – Real-Life Example 📌 Example: Checking age for voting Real life: If a person’s age is 18 or more, they are eligible to vote. int age = 20; if (age >= 18) {     System.out.println("Eligible to vote"); } 👉 If the condition is true, the message is printed. 👉 If false, nothing happens. 2. if-else Statement – Real-Life Example 📌 Example: Pass or Fail result Real life: If marks are greater than or equal to 40, the student passes; otherwise, they fail. int marks = 35; if (marks >= 40) {     System.out.println("Pass"); } else {     System.out.pr...

Operators in Java Operators are special symbols used to perform operations on variables and values in Java. Java provides several types of operators to perform arithmetic, logical, relational, and other operations. 1. Arithmetic Operators       Used to perform basic mathematical operations. + Addition - Subtraction * Multiplication / Division % Modulus (remainder) 2. Relational Operators      Used to compare two values and return a boolean result (true or false). > Greater than < Less than >= Greater than or equal to <= Less than or equal to == Equal to != Not equal to 3. Compound Assignment Operators       Used to perform an operation and assign the result to the same variable. += Add and assign -= Subtract and assign *= Multiply and assign /= Divide and assign %= Modulus and assign 4. Unary Operators      Used to operate on a single operand. ++ Increment -- Decrement ! Logical negation Increment Operators Post-inc...

  Final Keyword in Java The final keyword in Java is used to make a variable’s value constant, meaning the value cannot be changed once assigned. Final Variables When a variable is declared as final: The variable must be initialized once Reinitialization is not allowed Any attempt to change the value results in a compile-time error Example : final int x = 10; // x = 20; // Compile-time error Final with Static and Instance Variables When a static variable or instance variable is declared as final, it does not support default values Such variables must be declared and initialized together The value must be assigned only once Valid syntax: class Example {     static final int A = 100;     final int B = 50; } Purpose of Final Keyword The final keyword is used to: Create constant values Prevent accidental modification of variables Improve code readability and safety

Variables in Java A variable is a symbolic name associated with a value. The value of a variable can change during program execution. In Java, variables are classified into three types: Local Variable Static Variable Instance Variable 1. Local Variable A local variable is declared inside a method, constructor, or block. Key points: Also called a method-level variable Accessible only within the block {} where it is declared Does not support default values Must be initialized before use If declared but not initialized and used, it causes a compile-time error If declared but not used, no error is generated Example : void display() {     int x = 10; // local variable     System.out.println(x); } 2. Static Variable A static variable is declared inside a class but outside any method, using the static keyword. Key points: Also called a class-level variable Supports default values Memory is allocated once and shared among all objects Can be accessed:     By variabl...

Primitive Data Types in Java Primitive data types are basic data types in Java that determine the type and amount of memory required to store a value. Java provides a fixed set of primitive data types for efficient memory usage. There are two main categories of primitive data types: Numeric Non-Numeric 1. Numeric Data Types Numeric data types are used to store numerical values. They are further divided into two types: a) Integer Data Types Integer data types store whole numbers (without decimal points). byte- 8 bits short- 16 bits int- 32 bits long- 64 bits b) Floating-Point Data Types Floating-point data types store decimal numbers. float- 32 bits double- 64 bits 2. Non-Numeric Data Types Non-numeric data types are used to store non-numerical values. char – Stores a single character and occupies 16 bits of memory boolean – Stores true or false (Memory size is JVM-dependent) Default Values in Java A default value is a value automatically assigned by the compiler when a data type is d...

Main Method in Java The main method is the entry point of a Java program. During program execution, the JVM (Java Virtual Machine) searches for the main method to start the execution. The JVM uses an inbuilt mechanism to locate the main method, which must be written in lowercase exactly as main. Java is case-sensitive, so any change in the method name will cause a runtime error. If the main method is present, the JVM: Locates the bytecode of the main method Converts the bytecode into machine-specific code Passes the machine code to the operating system Produces the output of the Java program If the main method is not found, the JVM generates a runtime error. Syntax of Main Method: public static void main(String[] args) {     // program execution starts here } println Method in Java The println() method is used to display data on the console output. Key points: It prints the given data on the console After printing, it moves the cursor to the next line The println() method bel...

Creation of Java Class A Java class is a blueprint or template used to create Java objects. It defines the properties (variables) and behaviors (methods) that the objects of the class will have. The content defined inside a Java class is represented inside the Java object created from that class. How to Create a Java Class A Java class is created using the class keyword, followed by a valid identifier (class name). Java Class Syntax: class ClassName {     // variables     // methods     // constructors } Important Points About Java Class A Java class can contain: Variables Methods Constructors Blocks and nested classes A Java class must be saved with the .java file extension. The file name should match the class name when the class is declared as public. Main Method in Java For execution, a Java program must contain the main method. The main method acts as the entry point of a Java program. If the main method is not present in the class, the program will c...

JDK (Java Development Kit) The JDK (Java Development Kit) is a software package required to develop, compile, and run Java applications. It contains all the tools needed by developers and users to work with Java programs. The JDK includes: Java Compiler (javac) Java Runtime Environment (JRE) Java Virtual Machine (JVM) Java libraries and development tools End Users of JDK Software 1. Programmer A programmer needs the JDK to perform the following tasks: Writing Java programs (coding) Compiling Java source code Executing Java applications For these tasks, the programmer must install JDK software, which provides: Java Compiler – used for compilation Java Libraries – used during coding and execution JVM (Java Virtual Machine) – used for program execution Without JDK, a programmer cannot develop Java applications. 2. Client A client (end user) is required to only execute Java applications, not write or compile them. For execution purposes, the client needs: Java Libraries JVM (Java Virtual M...