Java – Instantiating object of type parameter

genericsjavareflection

I have got a template class as follows:

class MyClass<T>
{
    T field;
    public void myMethod()
    {
       field = new T(); // gives compiler error
    }
}

How do I create a new instance of T in my class?

Best Solution

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();
  }

}
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