Abstract classes和Interfaces到底差在哪?

在比較他們之前，先來了解他們到底是什麼?

本篇是John影片的筆記

Abstract Classes

定義

該class不能被instantiate，他就像是繼承他的class的藍圖，提供相同的功能與特質。(An abstract class in Java is a class marked with the abstract keyword that cannot be instantiated. It serves as a blueprint for other classes to inherit and provides common functionality or characteristics.)

目的

用來定義屬性和方法好讓subclass繼承，並同時允許subclass實作自己的abstract methods。(They are used to define common attributes or methods that subclasses can inherit while allowing individual implementations of abstract methods.)

特點

允許abstract或是已實作的method同時存在
Abstract methods沒有實作的話，subclass一定要override並實作
被subclasses繼承的instance variables不能直接被存取，而是透過getter和setter
一個class只能extends一個abstact class

使用情境

Ideal for defining shared characteristics among subclasses.
Useful when certain methods should have a common structure but different implementations in subclasses.

例如

import java.sql.Date;

public abstract class Animal {
    private String species;
    private String type;
    private String habitat;
    private Date dateOfBirth;

    public abstract void makeSound();

    public String getSpecies() {
        return species;
    }

    public void setSpecies(String species) {
        this.species = species;
    }

    public String getType() {
        return type;
    }

    public void setType(String type) {
        this.type = type;
    }

    public String getHabitat() {
        return habitat;
    }

    public void setHabitat(String habitat) {
        this.habitat = habitat;
    }

    public Date getDateOfBirth() {
        return dateOfBirth;
    }

    public void setDateOfBirth(Date dateOfBirth) {
        this.dateOfBirth = dateOfBirth;
    }

    public Animal(String species, String type, String habitat, Date dateOfBirth) {
        this.species = species;
        this.type = type;
        this.habitat = habitat;
        this.dateOfBirth = dateOfBirth;
    }


}

Interfaces

定義

用來定義abstract methods，就像是跟實作(implement)了這個interface的classes簽了契約，Methods必須被實作。(An interface in Java declares a set of methods without providing implementations. It’s a contract that specifies a behavior that implementing classes must define.)

目的

為了多重實作，一個class可以implement無限個interface

特點

只有method signatures，沒有method bodies
支援多重繼承，一個class可以implement無限個interface
Fields declared in interfaces are by default static and final (constants).

使用情境

適合為不相關的classes定義common behaviour，以供他們實作
Useful when enforcing a certain method signature across different classes

例子

public interface AnimalStuff {
    // 無須寫上abstract，因為interface只能默認都是abstract classes
    public void poop();
}