掌握 Java 物件導向程式設計：綜合指南 ☕♨

Object Oriented Programming

Object-Oriented Programming (OOP) is a programming paradigm that models real-world entities as objects. These objects have properties (attributes) and behaviors (methods). OOP is based on the concepts of encapsulation, inheritance, polymorphism, and abstraction.

Java is a computer programming language that is concurrent, class-based and object-oriented. The advantages of object oriented
software development are shown below:

• Modular development: This makes code easier to maintain and modify.
• Code reusability: This reduces the need to write the same code multiple times.
• Improved code reliability and flexibility: This makes it easier to create robust and adaptable software.
• Increased code understanding: This improves the readability and maintainability of code.

Encapsulation

Encapsulation in Java is a fundamental object-oriented programming concept that involves bundling data (attributes) and methods (behaviors) within an object. It provides data hiding and access control, ensuring that data is protected and only accessed through defined methods.

class Person {
    private String name;
    private int age;

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

    public String getName() {
        return name;
    }

    public int getAge() {
        return age;
    }
}

public class Main {
    public static void main(String[] args) {
        Person p = new Person("Sam", 21);
        System.out.println("Person Name: "+ p.getName());
        System.out.println("Person Name: "+ p.getAge());
        /* 
         *  p.name = "Anderson";  -> We couldn't modify the varibale value directly. It's Error ❗.
         *  p.age = 20;
        */
    }
}

Imagine a box. Inside the box are your personal belongings. You can see the box and know what's inside, but you can't directly touch or change the items without opening the box. This is similar to encapsulation in Java.

Polymorphism

Polymorphism, in Java, is the ability of objects of different classes to respond to the same method call in different ways. It's a fundamental concept in object-oriented programming that allows for flexibility and code reusability. There are two types of Polymorphism Compile-Time Polymorphism and Run-Time Polymorphism.

Example

Imagine you have a remote control. You can press the "play" button, and it will play something. But what it plays depends on the device it's controlling: a TV, a DVD player, or a music player.

This is like polymorphism in Java. The "play" button is the same method, but the behavior (what it plays) is different depending on the object (TV, DVD player, music player).

So, polymorphism means that the same thing (a method) can have different behaviors depending on the object that calls it.

interface Playable {
    void play();
}

class TV implements Playable {
    public void play() {
        System.out.println("Playing TV show");
    }
}

class DVDPlayer implements Playable {
    public void play() {
        System.out.println("Playing DVD");
    }
}

class MusicPlayer implements Playable {
    public void play() {
        System.out.println("Playing music");
    }
}

public class PolymorphismExample {
    public static void main(String[] args) {
        Playable[] devices = {new TV(), new DVDPlayer(), new MusicPlayer()};
        for (Playable device : devices) {
            device.play();
        }
    }
}

Inheritance

Inheritance concept that allows classes to inherit attributes, properties and methods from a parent class. This promotes code reusability, modularity, and the creation of hierarchical relationships between classes.

Inheritance in Java is like a family tree. A child class can inherit traits from a parent class. There are a few different ways to do this:

• Single Inheritance: One child, one parent.
• Multilevel Inheritance: Child inherits from parent, who is also a child.
• Hierarchical Inheritance: Multiple children from one parent.

Java doesn't directly support multiple inheritance, but you can use interfaces to get a similar result.

class Animal {
    void makeSound() {
        System.out.println("Generic animal sound");
    }

    void makeSound(int numberOfTimes) {
        for (int i = 0; i < numberOfTimes; i++) {
            System.out.println("Generic animal sound");
        }
    }
}

class Dog extends Animal {
    @Override
    void makeSound() {
        System.out.println("Woof!");
    }

    @Override
    void makeSound(int numberOfTimes) {
        for (int i = 0; i < numberOfTimes; i++) {
            System.out.println("Woof!");
        }
    }
}

class Cat extends Animal {
    @Override
    void makeSound() {
        System.out.println("Meow!");
    }

    @Override
    void makeSound(int numberOfTimes) {
        for (int i = 0; i < numberOfTimes; i++) {
            System.out.println("Meow!");
        }
    }
}

public class PolymorphismExample {
    public static void main(String[] args) {
        Animal[] animals = {new Dog(), new Cat()};

        // Method overloading:
        animals[0].makeSound();
        animals[1].makeSound(3);

        // Method overriding:
        for (Animal animal : animals) {
            animal.makeSound();
        }
    }
}

Abstraction

Abstraction is the process of separating ideas from specific instances and thus, develop classes in terms of their own functionality, instead of their implementation details. Java supports the creation and existence of abstract classes that expose interfaces, without including the actual implementation of all methods. The abstraction technique aims to separate the implementation details of a class from its behavior.

abstract class Shape {
    abstract double getArea();
}

class Circle extends Shape {
    private double radius;

    public Circle(double radius) {
        this.radius = radius;
    }

    @Override
    public double getArea() {
        return Math.PI * radius * radius;
    }
}

class Rectangle extends Shape {
    private double length;
    private double width;

    public Rectangle(double length, double width) {
        this.length = length;
        this.width = width;
    }

    @Override
    public double getArea() {
        return length * width;
    }
}

public class ShapeExample {
    public static void main(String[] args) {
        Shape circle = new Circle(5.0);
        Shape rectangle = new Rectangle(4.0, 3.0);

        System.out.println("Circle area: " + circle.getArea());
        System.out.println("Rectangle area: " + rectangle.getArea());
    }
}

Imagine you have a remote control for a car, a bike, and a plane. You can use the same buttons on the remote to start, stop, and move each vehicle, even though they are very different. This is like abstraction in programming.

Differences between Abstraction and Encapsulation

Abstraction

• Focus: Hides the underlying complexity of an object, revealing only the essential features.
• Purpose: Simplifies code by focusing on what an object does rather than how it does it.
• Mechanism: Achieved through abstract classes and interfaces.
• Example: A Vehicle interface defining methods like start(), stop(), and move(), without revealing the specific implementation for each vehicle type (car, bike, etc.).

Encapsulation

• 重點： 保護物件的資料免於未經授權的存取或修改。
• 目的：增強程式碼安全性、模組化和可維護性。
• 機制：透過將資料成員設為私有並提供存取或修改它們的公共方法來實現。
• 範例： 一個 Person 類，具有姓名和年齡等私有字段，以及用於存取或修改這些字段的公共方法（如 getName() 和 setAge()）。

主要差異

Feature	Abstraction	Encapsulation
Focus	Essential features	Data protection
Purpose	Simplification	Security, modularity
Mechanism	Abstract classes, interfaces	Private fields, public methods
Example	Vehicle interface	Person class with private fields and public methods

本質上：

• 抽象是關於物件的作用。
• 封裝是關於物件如何實現它的。

將 OOP 想像成用樂高積木建造。每塊磚都是一個具有自己形狀和屬性的物體。您可以組合磚塊來創建更大、更複雜的結構。透過理解這些概念，您可以創建更有組織、靈活且高效的程式碼。

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