英文:
Instantiating an object within another object in Java (Composition vs inheritance)
问题
我正在尝试理解Java中的组合工作原理，但遇到了困难。我有一个程序，它实例化一个Customer对象，该对象具有Computer对象作为实例变量。
在我的程序中，有两种类型的Computer对象，一个是Desktop子类，另一个是Laptop子类，所以我让我的Desktop类扩展Computer，我的Laptop类也扩展Computer。我将Computer类设置为抽象类，但我知道无法实例化抽象对象。我不确定如何在Customer类中实例化Computer对象。任何见解都将不胜感激！
示例代码：
public class Customer
{
private String name;
private Computer computer;
public Customer(String n)
{
setName(n);
this.computer = new Computer(); //无法这样做，因为Computer是抽象的
}
//为姓名设置器和访问器
public Computer getComputer()
{
return new Computer(this.computer);
}
public void setComputer(Computer computer)
{
this.computer = new Computer(computer);
}
}
public abstract class Computer{
}
public class Desktop extends Computer{
}
public class Laptop extends Computer{
}
英文:
I'm trying to understand how composition in Java works and I'm getting stuck. I have a program that instantiates a Customer object who has a Computer object as an instance variable.
There are two types of Computer objects in my program, a Desktop and a Laptop subclass, so I have my Desktop class extending Computer and my Laptop class also extending Computer. I have my Computer class as an abstract class but I know that you can't instantiate an abstract object. I'm not sure how to instantiate the Computer object in my Customer class. Any insight is appreciated !
Example code:
public class Customer
{
private String name;
private Computer computer;
public Customer(String n)
{
setName(n);
this.computer = new Computer(); //Can't do this b/c Computer is abstract
}
//Setters and getters for name
public Computer getComputer()
{
return new Computer(this.computer);
}
public void setComputer(Computer computer)
{
this.computer = new Computer(computer);
}
}
public abstract class Computer{
}
public class Desktop extends Computer{
}
public class Laptop extends Computer{
}
答案1
得分: 0
是的，你说得对。你不能实例化抽象类Computer()。
相反，你可以这样做：
this.computer = new Laptop()
this.computer = new Desktop()
取决于它们具有哪些特点。
英文:
Yes you are right. You cannot instantiate the abstract class Computer().
Instead, you can do
this.computer = new Laptop()
this.computer = new Desktop()
depending on which they have.
答案2
得分: 0
解决这个问题的方式是，你的构造函数还应该接受一个计算机参数。
public Customer(String n, Computer c)
{
setName(n);
this.computer = c;
}
但是有许多理由更喜欢只具有空构造函数的POJO（普通的旧Java对象），而不是在构造类型时要求组合元素的类。更多信息请参见：https://stackoverflow.com/questions/14172621/whats-the-advantage-of-pojo
英文:
The way you solve this problem is that your Constructor should also take a Computer argument
public Customer(String n, Computer c)
{
setName(n);
this.computer = c;
}
But there are a lot of reasons to prefer POJOs (Plain Old Java Objects) with only a null constructor over classes that demand the composition elements at construct-type. For more information, see: https://stackoverflow.com/questions/14172621/whats-the-advantage-of-pojo
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