How can I create a new instance of a generic class

Question

I create a function like this

public <T extends ClientCoporate> void myFunction(List<T> clients){
    T client = new T(); // issue
    ....
}

IDE error Cannot instantiate the type T

Simple question, how can I create a new instance of whatever class extends ClientCoporate here? I try to search but no hope, if you can find any duplicate question feel free to put a flag for me. Thanks.

`T` is Generics Type Variable you can't say `new` on any Generics Type Variable. If you want to use `client` then just write `T client;`. — , Jan 09 '19 at 09:39
Runtime you need the class. You should either pass it to the function or make sure the specified list is not empty. In the latter case, you could try `clients.get(0).getClass().newInstance()` — Robert Kock, Jan 09 '19 at 09:42

score 9 · Answer 1 · edited Jan 09 '19 at 09:50

9

You can't.

Because generics are erased in run-time and compiler does not know what type of object to create. T is simply a placeholder, so It is illegal to attempt to instantiate it.

Usually this problem is solved by

public <T extends ClientCoporate> void myFunction(Class<T> clazz, List<T> clients){
    T client = clazz.getDeclaredConstructor().newInstance();        ....
}

edited Jan 09 '19 at 09:50

answered Jan 09 '19 at 09:40

talex

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And the List clients can add that `client` into it, right? – Dang Nguyen Jan 09 '19 at 09:45
Yes, it can. Why not? – talex Jan 09 '19 at 09:46
Thanks you so much for the effort, but for me, the accepted answer is a little bit better. – Dang Nguyen Jan 09 '19 at 09:53

Michael · Accepted Answer · 2019-01-09T09:54:54.400

One of the reasons you can't do this - though there are probably at least a couple more - is because the concrete type of T is not known at the point you are trying to construct it. We cannot say for sure that the concrete type of T has a no-argument constructor. The only thing we do know is that is has the methods in ClientCoporate.

Consider the following class:

class MyClientCoporate extends ClientCoporate // or implements, it doesn't matter
{
    private MyClientCoporate() {}
}

We couldn't use new T() on this class because the constructor is not public. That's one reason for the compiler to disallow it.

If you can change the signature of the method then I'd do something like this:

public <T extends ClientCoporate> void myFunction(List<T> clients, Supplier<T> constructor){
    T client = constructor.get();
    ...
}

Sample usage:

myFunction(aList, SomeClientCoporateClass::new);

brilliant idea! the code will be more readable by using this than the answer from talex. — Dang Nguyen, Jan 09 '19 at 09:48

How can I create a new instance of a generic class

2 Answers2