Instantiating object of type parameter

问题:

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?

回答1:

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();
  }

}

回答2:

Another non-reflective approach is to use a hybrid Builder / Abstract Factory pattern.

In Effective Java, Joshua Bloch goes over the Builder pattern in detail, and advocates a generic Builder interface:

public interface Builder<T> {
  public T build();
}

Concrete builders can implement this interface, and outside classes can use the concrete builder to configure the Builder as required. The builder can be passed to MyClass as a Builder<T>.

Using this pattern, you can get new instances of T, even if T has constructor parameters or requires additional configuration. Of course, you'll need some way to pass the Builder into MyClass. If you can't pass anything into MyClass, then Builder and Abstract Factory are out.

回答3:

This may be more heavyweight than what you're looking for, but it will also work. Note that if you take this approach, it would make more sense to inject the factory into MyClass when it is constructed instead of passing it into your method each time it is called.

interface MyFactory<T> 
{
    T newObject();
}

class MyClass<T> 
{
    T field;
    public void myMethod(MyFactory<T> factory)
    {
       field = factory.newObject()
    }
}

回答4:

If you're willing to subclass you can avoid erasure as well, check out http://www.artima.com/weblogs/viewpost.jsp?thread=208860

回答5:

Class classOfT

        try {
            t = classOfT.newInstance();//new T(); NOTE: type parameter T cannot be instantiated directly
        } catch (Exception e) {
            e.printStackTrace();
        }