相同方法的不同返回类型

Question

i need to implement the newInstance method in order to make this code to work:

protected IDatabase createDatabase() throws Exception{
    return newInstance(CLASS_NAME_DATABASE);
}

protected IDataLoader createDataLoader() throws Exception{
    return newInstance(CLASS_NAME_DATA_LOADER);
}

I'm not allowed to touch this code, but when i try to implement the newInstance as Object it means i need to cast the newInstance in the code above you. Is there a way for me to implement newInstance without changing the code?

Thanks!

Answer 1

If the parameter to newInstance were a Class object, or something else with a generic type parameter, you could do the following, which would avoid needing to cast the return value, and be type-safe:

protected <T> T newInstance(Class<T> klass) throws Exception {
    return klass.newInstance();
}

(Note: klass.newInstance() is the reflection API method, not the same as the method being defined, despite the same name.)

However, you can use generics no matter what sort of parameters newInstance takes:

@SuppressWarnings("unchecked")
protected <T> T newInstance(String className) throws Exception {
    return (T)Class.forName(className).newInstance();
}

This is a bit dirty, because it completely eliminates compile-time error checking on the type of the return value. The compiler will allow IDataLoader x = newInstance(CLASS_NAME_DATA_LOADER); but will just as happily allow int x = newInstance(CLASS_NAME_DATA_LOADER); (which actually casts it to an Integer then unboxes it).

However, manually casting an Object return value would also not be checked at compile time, so using generics here is probably still an improvement since it reduces the number of times you have to write each type.