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在Java面试过程中，我们通常会被问到作为一门OOP的语言，最主要的特点有哪些? 或许很多同学在实际的应用中都能够慢慢的总结出OOP这种语言在实际工作中所带来的便利，有些时候我们却发现这些OOP的基本特点我们虽然知道但是我们却很难系统地，准确地表达给面试官。在这里，包子帮助各位同学进行总结，让大家能有深刻的了解，感觉萌萌哒~
Q: What are Encapsulation, Inheritance and Polymorphism?
上面这道题是面试中常常会被问道的。有些时候面试官很直接地想面试者说出这三个代表着OOP的关键词，就是想考察面试者是否能够了解这三词的在OOP中的含义。然而有些时候，面试官并没有直接指出这三个关键词，这时候就是想考察面试者是否有足够的基本知识能够知道OOP的特性。
作为一个通用的解答，每个关键词都可以用简短的一句话来进行详细的补充:
Encapsulation: is the mechanism that binds together code and data it manipulates and keeps both safe from outside interference and misuse.
Inheritance: is the process by which one object acquires the properties of another object
Polymorphism: is the feature that allows one interface to be used for general class actions.
知道这些词的描述只能帮助我们给面试官开始留下一个较好的印象，但是对于那些要求比较高面试，面试官想考察面试者是否能够真正的了解这些特性。
下面我们包子就会详细地为大家解读隐藏在这些关键词背后信息
- Encapsulation：
Encapsulation 作为OOP fundamental concepts之一是经常被考察的对象。 我们常说OOP 的coding要考虑到maintainability, flexibility and extensibility，换句话说， Encapsulation的好坏直接影响着code本身的质量。Open for extension, Closed for modification.
从下面的例子中我们可以发现Encapsulation的作用：
/* File name : EncapTest.java */
public class EncapTest{
private String name;
private String idNum;
private int age;
public int getAge(){
return age;
}
public String getName(){
return name;
}
public String getIdNum(){
return idNum;
}
public void setAge( int newAge){
age = newAge;
}
public void setName(String newName){
name = newName;
}
public void setIdNum( String newId){
idNum = newId;
}
}
在上述的例子里我们可以看出Class EncapTest有两个private的member, 分别是 name 和 idNum. 但是我们向外部提供了public的接口允许我们去做set跟get的action. 或许有人会问上述的例子中，getter跟setter其实跟直接获取attributes或者给attributes赋值没有任何区别，但是为什么我们还是需要提供getter或者setter呢？
其实上述的例子只是一个最为基本的概念，在实际应用中，如果我们将attributes变成public, 那么用户就可以随意的set其中的value, 这在工业上是一种相当危险的行为，因此大多数的setter内部都会调用相应的的validation作为检查，这样customer的input只有在通过检查，符合的规定之后才能后对里面的attribute进行赋值。当然，想getter和setter这种boiler plate的代码都可以自动生成，或者直接用attribute定义，比如Java的Lombok, etc
Benefits of Encapsulation:
- The fields of a class can be made read-only or write-only.
- A class can have total control over what is stored in its fields.
- The users of a class do not know how the class stores its data. A class can change the data type of a field and users of the class do not need to change any of their code.
- Inheritance：
在Java的面试过程中，有时候面试官会问面试者什么是 IS-A relationship? 其实IS-A relationship就指的是Inheritance。
下面的例子很好的阐述了Is-A relationship:
public class Animal{
}
public class Mammal extends Animal{
}
public class Reptile extends Animal{
}
public class Dog extends Mammal{
}
我们有一个基本类叫做Animal, 然后我们定义了如下IS-A relationship. Mammal 是一种 Animal, Reptile 是另外一种 Animal. Mammal跟Reptile是相互并行的关系，他们的共同点就是他们都是一种 Animal. 而最后的 Dog 被明确的指出就是一种 Mammal 同时，我们也能说Dog 是一种 Animal.
Now, if we consider the IS-A relationship, we can say:
Mammal IS-A Animal
Reptile IS-A Animal
Dog IS-A Mammal
Hence : Dog IS-A Animal as well
下面的例子更好的诠释了IS-A relationship的含义：
public class Dog extends Mammal{
public static void main(String args[]){
Animal a = new Animal();
Mammal m = new Mammal();
Dog d = new Dog();
System.out.println(m instanceof Animal);
System.out.println(d instanceof Mammal);
System.out.println(d instanceof Animal);
}
}
This would produce the following result:
true
true
true
- Polymorphism：
从字面上理解，多态就是一种类型表现出多种状态。多态在Java用有两种实现方式， 我们通常称为静态绑定跟动态绑定。
静态绑定就是说在程序运行之前进行绑定，由编译器和连接程序实现。比如overloading。
动态绑定在运行时根据对象的类型进行绑定，由方法调用机制实现。比如overriding
如何理解多态，请看下面的例子：
/* File name : Employee.java */
public class Employee
{
private String name;
private String address;
private int number;
public Employee(String name, String address, int number)
{
System.out.println("Constructing an Employee");
this.name = name;
this.address = address;
this.number = number;
}
public void mailCheck()
{
System.out.println("Mailing a check to " + this.name
+ " " + this.address);
}
public String toString()
{
return name + " " + address + " " + number;
}
public String getName()
{
return name;
}
public String getAddress()
{
return address;
}
public void setAddress(String newAddress)
{
address = newAddress;
}
public int getNumber()
{
return number;
}
}
/* File name : Salary.java */
public class Salary extends Employee
{
private double salary; //Annual salary
public Salary(String name, String address, int number, double
salary)
{
super(name, address, number);
setSalary(salary);
}
public void mailCheck()
{
System.out.println("Within mailCheck of Salary class ");
System.out.println("Mailing check to " + getName()
+ " with salary " + salary);
}
public double getSalary()
{
return salary;
}
public void setSalary(double newSalary)
{
if(newSalary >= 0.0)
{
salary = newSalary;
}
}
public double computePay()
{
System.out.println("Computing salary pay for " + getName());
return salary/52;
}
}
/* File name : VirtualDemo.java */
public class VirtualDemo
{
public static void main(String [] args)
{
Salary s = new Salary("Mohd Mohtashim", "Ambehta, UP", 3, 3600.00);
Employee e = new Salary("John Adams", "Boston, MA", 2, 2400.00);
System.out.println("Call mailCheck using Salary reference --");
s.mailCheck();
System.out.println("\n Call mailCheck using Employee reference--");
e.mailCheck();
}
}
This would produce the following result:
Constructing an Employee
Constructing an Employee
Call mailCheck using Salary reference --
Within mailCheck of Salary class
Mailing check to Mohd Mohtashim with salary 3600.0
Call mailCheck using Employee reference--
Within mailCheck of Salary class
Mailing check to John Adams with salary 2400.0
- REFERENCE：
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