从对象创建实例的通用方法

Question

Is this a safe way to create an instance (assuming T has a accessible default constructor) ?

<T>
public T defaultObj(T obj) throws Exception {
    return (T) obj.getClass().newInstance();
}

Because of the type erasure, the above codes will generate an unchecked warning. Is it possible to get rid of this warning other than @SuppressWarnings?

Many thanks in advance!

Edit: Yes, I know if I could pass a class<T> type which would be much better. But let's just assume it is out of question for now.

Answer 1

您可以将签名稍微更改为<T> T defaultObj(Class<T> obj) ，然后newInstance()将返回适当的对象。

Answer 2

Is it possible to get rid of this warning other than @SuppressWarnings?

You could constrain T to extend some interface that provides a factory method.

interface Newable<T>
{
    T getNewInstance();
}

<T extends Newable<T>> T defaultObj(T obj)
{
    return obj.getNewInstance();
}

example implementation:

class Foo implements Newable<Foo>
{
    @Override
    public Foo getNewInstance() {
        return new Foo();
    }
}

Answer 3

That's exactly what @SuppressWarnings is supposed to do: this code is safe (because calling getClass() on an object of type T actually yields Class<? extends T> ), but compiler cannot understand it (return value of getClass() is declared as Class<?> due to limitations of generics) and generates warning.

It's absolutely legal to use @SuppressWarnings in this case, and you shouldn't try to avoid using it.