I. Encapsulation

Encapsulation in Java

Encapsulation is one of the four fundamental Object-Oriented Programming (OOP) principles, along with Abstraction, Inheritance, and Polymorphism.

It refers to the concept of wrapping data (fields) and methods (functions) into a single unit: a class, while restricting direct access to some of the object’s internal components.

Overview:

• Class and Object
• Constructors
• Getter and setter
• Current object
• Other member funtions
• Access modifier

A. Class and Object

1. What is a Class?

Definition: A class is a blueprint or template from which individual objects are created. It defines:

• Fields (also known as attributes or member variables): Represent the state of the object.
• Methods (also called member functions): Define the behavior of the object.

Constructors: Special methods to initialize objects.

Syntax:

class ClassName {
    // Fields (state)
    // Constructor(s)
    // Methods (behavior)
}

2. What is an Object?

Definition: An object is a runtime instance of a class. When an object is created:

• Java allocates memory for its fields.
• The object can invoke the class’s methods.

Each object has its own copy of instance variables.

Syntax:

ClassName obj = new ClassName();

Example:

class Car {
    String brand;
    int speed;

    void drive() {
        System.out.println(brand + " is driving at " + speed + " km/h");
    }
}

public class Main {
    public static void main(String[] args) {
        Car myCar = new Car();      // Creating an object
        myCar.brand = "Toyota";     // Setting fields
        myCar.speed = 100;
        myCar.drive();              // Calling method
    }
}

• Output:

  Toyota is driving at 100 km/h
  

  3. Key Concepts:

Concept	Explanation
class	Defines the structure and behavior of objects
object	Instance of a class with its own field values
new	Allocates memory and initializes the object
. operator	Accesses fields or methods of an object

4. Important Notes:

• Multiple objects can be created from the same class:

Car car1 = new Car();
Car car2 = new Car();

Each object has its own copy of the brand and speed fields.

• Objects are reference types, meaning variables like car1 and car2 are references pointing to memory where the actual object is stored.

B. Constructors

1. What is a Constructor?

A constructor is a special method that is automatically called when a new object is created using the new keyword.

Key Features:

• Has the same name as the class.
• No return type (not even void).
• Used to initialize the fields of an object.

2. Types of Constructors

a) Default Constructor

• Takes no parameters.
• Java automatically provides a default constructor if no constructors are defined.

class Book {
    String title;

    Book() {
        title = "Untitled";
    }
}

b) Parameterized Constructor

Takes parameters to allow object initialization with custom values.

class Book {
    String title;

    Book(String t) {
        title = t;
    }
}

c) Constructor Overloading

A class can have multiple constructors with different parameter lists.

class Book {
    String title;
    int year;

    Book() {
        title = "Unknown";
        year = 0;
    }

    Book(String t) {
        title = t;
        year = 0;
    }

    Book(String t, int y) {
        title = t;
        year = y;
    }
}

3. Using Constructors to Encapsulate Initialization

Constructors help enforce encapsulation by ensuring that an object is always created with valid or required values.

Person.java

class Person {
    private String name;
    private int age;

    Person(String n, int a) {
        name = n;
        age = a;
    }

    public void introduce() {
        System.out.println("Hi, I'm " + name + " and I'm " + age + " years old.");
    }
}

Main.java

public class Main {
    public static void main(String[] args) {
        Person p = new Person("Alice", 30);
        p.introduce();
    }
}

4.Why Use Constructors?

Benefit	Explanation
Automatic Initialization	Called immediately when object is created
Encapsulation	Ensures required data is provided at creation time
Flexibility	Constructor overloading provides multiple ways to construct an object
No Return Type	Simplifies syntax and eliminates ambiguity

Things to Watch For:

• If you define any constructor, Java will not automatically create a default constructor.
• Constructors can call other constructors using this(…).

class Student {
    String name;
    int age;

    Student() {
        this("Unknown", 0); // calls another constructor
    }

    Student(String name, int age) {
        this.name = name;
        this.age = age;
    }
}

5. Parameters vs. Arguments

Term	Definition
Parameter	A placeholder variable declared in the method or constructor definition.
Argument	The actual value passed to the method or constructor when it is called.

Example:

class Book {
    // title is a **parameter**
    Book(String title) {
        System.out.println("Book title: " + title);
    }
}

public class Main {
    public static void main(String[] args) {
        // "Java Basics" is an **argument**
        Book myBook = new Book("Java Basics");
    }
}

Rule of Thumb:

• Parameter = variable in the method/constructor definition
• Argument = value passed during method/constructor call

C. Getters and Setters

1. What Are Getters and Setters?

• Getters are methods used to read (get) the value of a private field.
• Setters are methods used to write (set) the value of a private field.

This is a core part of encapsulation — where class fields are kept private and accessed via public methods.

2. Why Use Getters and Setters?

Benefit	Description
Data Protection	Keeps fields private to prevent unauthorized access
Controlled Access	Allows validation before setting values
Abstraction	Hides internal implementation from external users
Flexibility	You can change field logic without affecting outside code

3. Example

Person.java

class Person {
    private String name;      // private field
    private int age;

    // Getter for name
    public String getName() {
        return name;
    }

    // Setter for name
    public void setName(String name) {
        this.name = name;
    }

    // Getter for age
    public int getAge() {
        return age;
    }

    // Setter for age
    public void setAge(int age) {
        if (age >= 0) {
            this.age = age;
        }
    }
}

Main.java

public class Main {
    public static void main(String[] args) {
        Person p = new Person();

        p.setName("Alice");           // Set value using setter
        p.setAge(25);

        System.out.println(p.getName()); // Get value using getter
        System.out.println(p.getAge());
    }
}

4. Benefits of Encapsulation via Getters and Setters

• Prevents direct access to class fields.
• Adds logic or validation (e.g. prevent setting age < 0).
• Allows future changes (e.g. converting age to birth year) without changing method names.

Common Mistakes | Mistake | Fix | | —————————— | ———————————————– | | Making fields public | Use private + getter/setter | | Not validating input in setter | Add logic inside the setter | | Forgetting this in setter | Use this.field = parameter to avoid ambiguity |

Tip: In NetBeans can automatically generate getters and setters for all fields — saving time and reducing typos. (Right Mouse/Insert code…/Geter and setter)

---

D. Current Object

Use Case	Syntax Example	Purpose
Disambiguate field/parameter	this.name = name;	Clarifies which variable is which
Constructor chaining	this("John", 25);	Reuse constructor logic
Method chaining	return this;	Enable fluent interfaces
Pass current object	someMethod(this);	Send current object as an argument

1. What is this in Java?

The this keyword refers to the current object — the instance of the class whose method or constructor is being executed.

It is used:

• To distinguish instance variables from parameters or local variables.
• To call other constructors in the same class.
• To return the current object from a method.

2. Why is this Useful?

Use Case	Description
Disambiguation	Resolves naming conflict between fields and parameters
Constructor chaining	Calls another constructor in the same class (this(...))
Return current object	Useful for method chaining (return this)
Passing current object	You can pass this as a reference to another method or constructor

3. Disambiguating Fields

class Student {
    private String name;

    public void setName(String name) {
        this.name = name;  // 'this.name' = instance variable, 'name' = parameter
    }
}

Without this, name = name would just assign the parameter to itself. this.name ensures you’re referring to the field.

4. Calling Another Constructor (this(…))

class Student {
    String name;
    int age;

    Student() {
        this("Unknown", 0); // Calls the other constructor
    }

    Student(String name, int age) {
        this.name = name;
        this.age = age;
    }
}

Rules:

• Must be the first line in the constructor.
• Used to reuse constructor logic and reduce redundancy.

5. Returning Current Object

This enables method chaining:

class User {
    private String name;

    public User setName(String name) {
        this.name = name;
        return this;
    }

    public User sayHello() {
        System.out.println("Hello, " + name);
        return this;
    }
}

Usage:

new User().setName("Anna").sayHello();

6. Passing this to Another Method

class Printer {
    void printUser(User user) {
        System.out.println("User: " + user.name);
    }
}

class User {
    String name = "Bob";

    void printYourself() {
        Printer p = new Printer();
        p.printUser(this); // Passing current object
    }
}

Tip: Always use this when there’s naming overlap, or when you’re building flexible, chainable APIs.

---

E. Other Member Functions (Methods in a Class)

Element	Purpose
Instance Methods	Operate on object data
Static Methods	Utility logic shared by all
Getters/Setters	Encapsulation helpers
Custom Methods	Add unique behaviors to objects
Overloaded Methods	Provide method flexibility

1. What Are Member Functions?

Member functions (commonly known as methods) are functions defined inside a class that operate on objects’ data.

They represent the behavior of an object.

2. General Structure

class ClassName {
    // Method definition
    returnType methodName(parameters) {
        // method body
        return value;
    }
}

Example:

class Calculator {
    int a, b;

    void setValues(int x, int y) {
        a = x;
        b = y;
    }

    int add() {
        return a + b;
    }

    int multiply() {
        return a * b;
    }
}

Usage:

public class Main {
    public static void main(String[] args) {
        Calculator calc = new Calculator();
        calc.setValues(3, 4);
        System.out.println("Sum: " + calc.add());
        System.out.println("Product: " + calc.multiply());
    }
}

3. Types of Methods

Type	Description
Setter methods	Set (assign) values to fields (void setName(String name))
Getter methods	Retrieve field values (String getName())
Business logic	Custom methods like calculateSalary(), drive(), etc.
Utility methods	Helper functions (e.g., formatDate(), isValidEmail())

4. Instance vs. Static Methods

Instance Method	Static Method
Called on an object	Called on the class itself
Can access instance variables	Can only access static variables
Needs object to use: obj.method()	Called as ClassName.method()

class MathTool {
    static int square(int n) {
        return n * n;
    }

    int cube(int n) {
        return n * n * n;
    }
}

Usage:

MathTool tool = new MathTool();
System.out.println(tool.cube(3));       // instance method
System.out.println(MathTool.square(4)); // static method

5. Void vs Returning Methods

Type	Use When	Example
void	No value needs to be returned	void printName()
Returning value	Result is needed for further processing	int calculateTotal()

6. Method Overloading

You can define multiple methods with the same name but different parameters.

class Printer {
    void print(String msg) {
        System.out.println(msg);
    }

    void print(int number) {
        System.out.println(number);
    }
}

---

F. Access Level in Java

1. What Are Access Modifiers?

Access modifiers define the visibility (or access level) of classes, methods, and variables from other classes or packages.

They are crucial for:

• Implementing encapsulation
• Protecting internal data
• Controlling who can access or modify parts of your code

2. Types of Access Modifiers

Modifier	Class	Package	Subclass	World
public	✅	✅	✅	✅
protected	✅	✅	✅	❌
default (no modifier)	✅	✅	❌	❌
private	✅	❌	❌	❌

✅ = accessible, ❌ = not accessible

3. Modifier Details

a) public

• Accessible from anywhere
• Often used for main classes and APIs

public class User {
    public String name;
}

b) private

• Accessible only within the same class
• Most secure — ideal for fields that should not be accessed directly

public class Account {
    private double balance;

    public double getBalance() {
        return balance;
    }

    public void deposit(double amount) {
        if (amount > 0) balance += amount;
    }
}

c) protected

• Accessible within the same package and in subclasses (even in different packages)
• Common in inheritance

class Person {
    protected String name;
}

d) default (package-private)

• No keyword is specified
• Accessible only within the same package

class Helper {
    void log(String message) {
        System.out.println(message);
    }
}

4. Applying Access Level

Target	Usable Access Level
Class	public, default
Field	public, private, protected, default
Method	public, private, protected, default
Constructor	Same as methods

5. Access Level in Encapsulation

Encapsulation typically involves:

• private fields
• public getters and setters
• Keeping implementation hidden and exposing only what’s needed

Example:

public class Product {
    private double price;

    public double getPrice() {
        return price;
    }

    public void setPrice(double p) {
        if (p >= 0) price = p;
    }
}

Tips:

• Always keep fields private.
• Use public only when necessary.
• Avoid protected unless you’re designing for inheritance.
• Default access is fine for package-scoped utilities.