Java Object-Oriented Programming Concepts

This article is a part of the Java Programming Language article series.

Object-Oriented

• A programming language or a technique that supports classes and objects
• Java is an object-oriented programming language

Class

• Is a blueprint
• Has properties, methods, and nested classes

Object

• Instance that is created from class
• Instance is an executable copy of the class
• Models the real-world objects
• Two characteristic: state and behavior
• State: name, color, speed, on, off, volume
• Behavior: changing gear, turn on, turn off, increase volume, decrease volume
• state = fields
• behavior = methods

Class Types

• Nested Classes
• Anonymous Classes

Nested Classes

• Types: Static Nested Classes, Inner Classes
• Logically grouping classes that are only used in one place
• Increases encapsulation

Anonymous Classes

• Declare and instantiate a class at the same time

Enum Types

• A set of predefined constants
• Compass directions, the days of the week

Object-Oriented Programming Features

• Abstraction
• Encapsulation
• Inheritance
• Polymorphism

Abstraction

• Provided by interface and abstract class

Interface

• Interaction with the outside world through the methods
• Forms a contract between the class and the outside world
• All methods must be implemented in the class
• public, static, final properties (fields)
• public methods

Abstract Class

• Abstract classes cannot be instantiated, but they can be subclassed
• Not static and final properties (fields) can be defined
• public, protected, private methods can be defined

Abstract Method

• An abstract method is a method that is declared without an implementation
• If a class includes abstract methods, then the class itself must be declared abstract

Abstract Classes vs Interfaces

Abstract classes

• Share code with related classes
• Have many common methods or fields

Interfaces

• Implementation with unrelated classes
• Multiple inheritance

When an Abstract Class Implements an Interface

• An abstract class does not need to implement all of the interface’s methods

Encapsulation

• Hiding properties and methods

Access level modifiers

• public: Full access
• protected: Subclass access
• no modifier: Package access
• private: Class access

Data Encapsulation

• Hiding internal state and interact through methods

Inheritance

• Using common states and behaviors from other classes
• All classes inherits from the java.lang.Object class
• Interface is used for multiple inheritance
• superclass: base, parent class
• subclass: derived, extended, child class

Polymorphism

• A subclass can define its own unique behaviors while inheriting superclass features
• Override parent class methods with different definitions in subclasses

The this Keyword

• Within an instance method or a constructor, this is a reference to the current object

The final keyword

• The methods declared final cannot be overridden by subclasses
• Methods called from constructors should generally be declared final to avoid surprising or undesirable results
• A class that is declared final cannot be subclassed

Static Method

Method Overloading

• Declaring methods with different method signatures

Overriding and Hiding Methods

Instance Methods

• A subclass method with the same signature and return type overrides the superclass method

Static Methods

• A static subclass method with the same signature hides the static superclass method

Packages

• Namespace that organizes a set of related classes and interfaces
• Similar to folders on a computer