After type erasure, all that is known about T is that it is some subclass of Object. You need to specify some factory to create instances of T.
One approach could use a Supplier<T>:
class MyClass<T> {
private final Supplier<? extends T> ctor;
private T field;
MyClass(Supplier<? extends T> ctor) {
this.ctor = Objects.requireNonNull(ctor);
}
public void myMethod() {
field = ctor.get();
}
}
Usage might look like this:
MyClass<StringBuilder> it = new MyClass<>(StringBuilder::new);
Alternatively, you can provide a Class<T> object, and then use reflection.
class MyClass<T> {
private final Constructor<? extends T> ctor;
private T field;
MyClass(Class<? extends T> impl) throws NoSuchMethodException {
this.ctor = impl.getConstructor();
}
public void myMethod() throws Exception {
field = ctor.newInstance();
}
}
