Does anyone have any suggested or established best practices and naming conventions for copy constructors / factory methods etc in Java? In particular, say I have a class Thing
and I want a method somewhere that returns a new Thing
with the same value as a Thing
passed in (or as the instance if it's an instance method). Would you have this as constructor or a static factory method or instance method? What would you call it?
As per the title, I want to avoid clone()
and Cloneable
.
Effective Java recommends either of the following:
A copy constructor (as noted by others):
public Item(Item item)
A copy factory method:
public static Item newInstance(Item item)
(Also, no copying for immutables)
The primary difference is that with #1 you choose the actual class of the result, and with #2 the implementer can return a subclass. The semantics of the class may guide you into which one is best.
I would call it a copy method or a copy constructor (as the case may be). If it was a static method, then I would call it a factory.
In terms of what to do, the most flexible and long living option is a copy constructor. This gives subclasses the ability to copy themselves just like the parent.
I'd do a constructor
...
public Thing(Thing copyFrom)
{
attr1 = copyFrom.attr1;
attr2 = copyFrom.attr2;
//etc...
}
then when you want to clone it
Thing copy = new Thing(copy from me);
You can overwrite the clone() -method, if you want. Another used practice is a constructor, that takes an object of this type, ie new ArrayList(anotherList) .
您有一些选择,实现Cloneable
,添加一个复制构造函数,但是我的首选方式是使用一个名称(描述静态操作)的方法(静态或实例)-是深层还是浅层复制等
Use immutable data structures . The only reason you feel that you need clone()
is that you're mutating your objects in place. Stop doing that. Think about how you can:
For example, here's a "setter" for an immutable 3D vector object:
public Vector3D setX(double x) {
return new Vector3D(x, this.y, this.z);
}
So I guess what I'm saying is... I use copy constructors instead of mutation, and I just name them according to the attribute that I want to modify.
Another option is to implement the copying method in the source object, eg:
interface Has3DCoords {
void setLocation(double x, double y, double z);
void copyCoordsTo(Has3DCoords dest);
}
You would then implement copying with a piece of code like:
class Thing implements Has3DCoords {
private Point3D loc;
// ...
void setLocation(double x, double y, double z) {
loc.setLocation(x, y, z);
// or: loc = new Point3D(x, y, z);
}
void copyCoordsTo(Has3DCoords dest) {
loc.copyCoordsTo(dest);
// or: dest.setLocation(loc.getX(), loc.getY(), loc.getZ());
}
OtherThing createOtherThing() {
OtherThing result = new OtherThing();
this.copyCoordsTo(result);
return result;
}
}
This can be useful if:
loc
as a property of Thing
This is not the nicest approach to copying objects but the following is sometimes useful if you wish to perform a deep copy of a Serializable object. This avoids having to write copy constuctors, implement Cloneable or writing factory classes.
ByteArrayOutputStream bos = new ByteArrayOutputStream();
ObjectOutputStream oos = new ObjectOutputStream(bos);
//Serializes the input object
oos.writeObject(input);
ByteArrayInputStream bais = new ByteArrayInputStream(bos.toByteArray());
ObjectInputStream ois = new ObjectInputStream(bais);
//Copy of the input object
Object output = ois.readObject();
Don't forget to handle the exceptions and to close the streams nicely.
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