OOP Fundamentals

Abstraction Definition

The ability to separate the interface from the implementation, hiding details and showing only what is necessary.

Abstraction (method example)

Putting code inside a method creates a new concept whose interface (name, inputs, outputs) is what matters.

Abstraction Levels

Methods → classes → packages represent increasing abstraction layers.

Abstraction Benefits

Higher readability, reusability, testability, maintainability, and simplification of complex systems.

Inheritance Definition

A two-way capability: (1) a child extends a parent (gains its features and adds more), and (2) common capabilities of children can be factored into a parent.

Inheritance is called an “__-_” relationship

Also called an “is-a relationship” in OOP.

Superclass

A more general class from which others inherit.

Subclass

A more specialized class that extends a superclass.

Default Parent in Java

Every class implicitly inherits from the Object class.

extends Keyword

Java keyword used to denote inheritance.

Inheritance Asymmetry

A manager is-an employee, but an employee is NOT necessarily a manager.

Method Overriding

Re-implementing an inherited method in the subclass.

Constructors and Inheritance

Constructors cannot be inherited or overridden because they are not class members.

UML Inheritance Connector

A solid line with a hollow triangle pointing to the superclass.

Visible Members That Are Inherited

All members visible according to access modifiers (public, protected, package).

super Keyword

Used to call superclass constructors and methods from a subclass.

Is Multiple Inheritance in Java supported?

Not supported for classes due to increased complexity.

What is a final Class?

A class that cannot be extended.

What is a final Method?

A method that cannot be overridden.

What is a final Attribute?

An attribute that cannot be changed after initialization.

Does Inheritance increase complexity?

Overusing inheritance increases complexity

Encapsulation Definition

Bundling attributes and behaviors in a class while controlling their visibility.

Encapsulation Abilities

Control access levels—read-only, write-only, read/write, or hidden.

Encapsulation Benefits

Organizes code, protects data, controls external access, prevents accidental misuse.

Is encapsulation just data hiding?

Encapsulation is NOT just hiding—it also includes bundling and controlled access.

Mutable Object

An object whose state can change

Interface (general meaning)

The set of visible members of a class (its API).

Java Interface

A separate construct containing abstract methods and optional default/static methods.

interface Keyword

Used to declare a Java interface.

implements Keyword

Used by a class to implement an interface.

Interface Requirements for inheriting class

A class must implement all interface methods or compilation fails.

Interface Naming

Interfaces should have abstract, generic names (e.g., AnimalInt vs DogInt).

Interface Default Methods

Methods in an interface with implementation (allowed since Java 8).

Are Interface Constants allowed?

Must be public static final (often implied automatically).

Are Interface Instance Variables allowed?

Not allowed—interfaces cannot have state.

Interface-Class Relationship

A class implementing an interface “is-an” instance of that interface.

Interface UML Connector

Dashed line with hollow triangle pointing to the interface.

Interfaces Extending Interfaces

An interface may extend multiple interfaces.

Association (UML)

A general relationship indicating dependency, aggregation, or composition.

Polymorphism

The ability to assign/pass a child object to a parent reference, enabling behavior to be chosen at runtime.

Runtime Binding

Java determines which overridden method to call at runtime based on the actual object.

Why Polymorphism Exists

Allows code to handle new types with minimal changes, solving Change and Complexity.

Method Overloading Definition

Same method name but different parameter lists in the same class.

Method Overloading Uses

Allows multiple versions of a behavior tailored to different input types.

Dependency Reduction via Polymorphism

Code depends on one interface instead of many classes.

What does (A-PIE) stand for?

Abstraction, Polymorphism, Inheritance, Encapsulation.

Abstraction (A-PIE summary)

Separating interface from implementation.

Polymorphism (A-PIE summary)

Passing a child object to a parent reference at runtime.

Inheritance (A-PIE summary)

Extending or factoring common behavior across classes.

Encapsulation (A-PIE summary)

Bundling data + behaviors and controlling their visibility.