I recently saw this constructor in a class:
public MyClass(){ }
There were no other constructors.
Is there a reason for this? Java automatically creates a default constructor, so why would you declare one explicitly? Or is this considered good practice in the same way as using braces for single-statement if statements - in case other constructors are added later and you forget that you don't have a default...?
A couple minor points that aren't likely to be why you saw it in this case.
As far as "in case other constructors are added later and you forget that you don't have a default" - that might be a reason, I suppose. But if a non-default constructor were added, any code that used the default constructor would fail to compile, so the guy adding the new constrcutor would generally need to also add a defintion for the default ctor as well.
Then again, I can't think of any particular harm in having the empty ctor defined (though now that I've typed that, I get a feeling that someone might point out some corner of C++ where it could bite you).
Because you believe in the second sentence of the zen of python :
Explicit is better than implicit.
Correct me if I'm wrong (I haven't done Java in a while), but doesn't this prevent a call to the parent constructor? In which case the reasoning is obviously that the parent is going to do something automatically that you don't want to happen in this class.
Here is what Java Language Specification says:
If a class contains no constructor declarations, then a default constructor that takes no parameters is automatically provided:
- If the class being declared is the primordial class Object, then the default constructor has an empty body.
- Otherwise, the default constructor takes no parameters and simply invokes the superclass constructor with no arguments.
So default constructor will be created anyway. There is no sense to write it if you don't. have to.
IMO you should only do it if you want to change the visibility level of your constructor, i.e. make it private or package protected
I was taught that a public default constructor was required by the java bean specification.. The wikipedia page http://en.wikipedia.org/wiki/Java_Bean also lists it as a requirement.
On the other hand, I looked at the Sun/Oracle site and couldn't find mention of it. See http://java.sun.com/developer/onlineTraining/Beans/Beans1/simple-definition.html
Although nothing is gained by adding the constructor explicitly at the technical level, there are potentially reasons to do it. One would be if the class is instantiated via reflection, you may want to put some documentation on the default constructor to indicate that it is required even though adding a new constructor would not cause a compilation error.
Another is that some coding standards prefer it in order to explicitly indicate you thought about what kind of constructor this class is supposed to have.