Possible Duplicate:
Java generics constraint require default constructor like C#
I want to set a constraint to type T that it must have a constructor without parameters. In C# it will be like:
public interface Interface<T> where T : new() { }
Is this feature available in Java?
You cannot define interface for constructors in Java. Nor you cann put any other constraints to type parameters, other than type variance.
Answering your updated question: only class can have a constructor in Java, T - is a type literal, it is not necessary has to be a class. During runtime using reflection you can check if your class is a ParameterizedType has a parameter that is actually a class and if it has an empty constructor.
You cannot do this at compile time in Java. I think that the best you can do is to try to verify this at runtime, by using reflection to do something like:
public static <T> boolean hasDefaultConstructor(Class<T> cls) {
Constructor[] constructors = cls.getConstructors();
for (Constructor constructor : constructors) {
if (constructor.getParameterTypes().length == 0) {
return true;
}
}
return false;
}
Then you can invoke this function by doing the following:
hasDefaultConstructor(String.class) => true;
hasDefaultConstructor(Integer.class) => false;
If this function returns false, then you know the class will have no default constructor, and you can throw an exception or whatever is appropriate for your application.
You could make T extend a class with the constructor you want
public interface Interface<T extends Constructable>{}
public abstract class Constructable{
public Constructable(params..){
}
}
