1. Objects and Classes

A class is a blueprint. An object is the actual instance created from that blueprint.
Each object has:

• State (values)

• Behavior (methods)

• Identity (reference in memory)

Java provides a default constructor if no constructor is defined.

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2. Ways to Create Objects

2.1 Using new keyword

The most common method.

Test t = new Test();

2.2 Using Reflection

Load class dynamically, then create an instance.

Class cls = Class.forName("Test");
Object obj = cls.getConstructor().newInstance();

2.3 Using clone()

Create a new object by copying an existing one.

Test t1 = new Test();
Test t2 = (Test)t1.clone();

2.4 Serialization & Deserialization

Object → File → Object. Used for deep copying or transferring objects.

2.5 Anonymous Object

Created without reference, used once.

new Test().display();

2.6 Multiple Objects via Upcasting

A single parent reference can point to different child objects.
Old child reference becomes unreachable → eligible for garbage collection.

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3. Methods in Java

Method structure:
modifier returnType methodName(parameters) { }

Types of methods

• Predefined methods (from Java libraries)

• User-defined methods

Based on binding

• Instance methods → require object

• Static methods → class-level, no object needed

Method Signature

Defined by:

• Parameter count

• Parameter types

• Parameter order

Return type is not part of the method signature.

Calling Methods

• Instance methods via object

• Static methods via class name

• Abstract methods must be implemented by child classes

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4. Constructors

Constructors initialize objects. They have no return type and share the class name.

Types

• Default constructor (provided by compiler)

• Parameterized constructor

• Copy constructor (manually created in Java)

• Private constructor (controls object creation)

Constructor Overloading

Multiple constructors with different parameter lists.

this keyword

Used to:

• Access current object's variables

• Call current class constructors

• Improve naming clarity

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5. Encapsulation

Bundling data and methods.
Achieved by:

• Private variables

• Public getters and setters

Ensures controlled access.

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6. Inheritance

A class acquires properties and behaviors of another class.

Types:

• Single

• Multilevel

• Hierarchical

• Hybrid (combination)

• Multiple inheritance is only allowed via interfaces

Inheritance represents an **"is-a" relationship".

Upcasting

Parent reference → Child object

Parent p = new Child();

Useful for polymorphism.

super keyword

Used to access:

• Parent class variables

• Parent class methods

• Parent constructor (must be first statement)

Parent class must have the constructor being called, or the compiler provides default ones accordingly.

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7. Polymorphism

Compile-time polymorphism (Method Overloading)

Same method name with different parameters.

Runtime polymorphism (Method Overriding)

Child class provides its own implementation of a parent method.
Decided at runtime based on object type.
Enabled by dynamic method dispatch.

Parent p = new Child();  
p.show(); // executes Child's show()

Return type does not matter in runtime polymorphism; method signature must match.

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8. Abstraction

Hiding implementation details and exposing only essential features.

Abstract Class

• Can have abstract and concrete methods

• Can contain variables

• Cannot be instantiated

• Extended by child classes

Represents partial abstraction.

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9. Interface

A completely abstract structure (before Java 8).
Used to achieve multiple inheritance.

Modern Interface capabilities

• Abstract methods

• Default methods

• Static methods

• Private methods (Java 9+)

• Variables are allowed but must be:

  • public static final

A class implementing an interface must implement all abstract methods unless declared abstract itself.

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