Return an object of the same type as method's parameter

843793

Hi,

Is it possible to do something like that?

public <T, E> T<E> method(T<E> arg) {    
    T<E> copy = arg.getClass().newInstance();
    return copy;
}

The method should create and return generic object on the basis of parameter's type. I would like to operate on Collection<E> without worrying about actual type of collection (List, Set...).

method(new ArrayList<Something>) should return ArrayList<Something>
method(new HashSet<Something>) should return HashSet<Something>
and so on.

Cheers

DrClap

Sure, that will work as long as the type in question has an accessible zero-argument constructor. So yes, in your example you could return a new ArrayList or a new HashSet. Not a new ArrayList<Thing>, though, because generics are a compile-time concept only and mean nothing at run time.

By the way it's more traditional to just pass a Class object directly, instead of a wasted object of that class:

public Class<?> method(Class<?> arg) {    
    Class<?> copy = arg.newInstance();
    return copy;
}

And to call that:

ArrayList result = method(ArrayList.class);

843793

@DrClap: shouldn't your method be:

public Object method(Class<?> arg) {    
    Object copy = arg.newInstance();
    return copy;
}

DrClap

Robert.Bossy wrote:
@DrClap: shouldn't your method be:

public Object method(Class<?> arg) {    
Object copy = arg.newInstance();
return copy;
}

Yes, you're right. Or perhaps something like

public T method(Class<? extends T> arg) {    
    T copy = arg.newInstance();
    return copy;
}

Although I have to say, this method doesn't look as useful to me as it apparently did to the OP.