Cast via reflection and use of Class.cast() [duplicate]

Answer 1

The return type of Class.forName will be Class<? extends Object>. You want Class<? extends String>, for instance using String.class.

String str = String.class.cast(object);

Not very useful, until you start replacing String with some kind of interface type.

In any case, reflection is generally evil.

Answer 2

Class.forName will return a Class<?> type object. The cast method will return the type parameter of Class. Thus, in this case it'll return a ? (Object) type object.

You should try:

Class<String> strClass = (Class<String>) Class.forName("java.lang.String");
String str = strClass.cast(object);

See also Class.cast()

Answer 3

You can avoid a warning e.g. with the following code:

public <T> T getObject(Class<T> type){
  Object o = getSomeObject();
  if (type.isInstance(o)){
     return type.cast(o);
  } else {
     return null;
  }
}

while the following code will raise a warning:

public <T> T getObject(){
  return (T) getSomeObject();
}

Answer 4

An example where is does work:

class Favorites {
    private Map<Class<?>, Object> map = new HashMap<Class<?>, Object>();

    public <T> T get(Class<T> clazz) {
        return clazz.cast(map.get(clazz));
    }

    public <T> void put(Class<T> clazz, T favorite) {
        map.put(clazz, favorite);
    }
}

which allows you to write:

Favorites favs = new Favorites();
favs.put(String.class, "Hello");
String favoriteString = favs.get(String.class);

The reason your code doesn't work is that Class.forName() returns a Class<?>, i.e. a class object representing an unknown type. While the compiler could possibly infer the type in your example, it can not in general. Consider:

Class.forName(new BufferedReader(System.in).readLine())

what's the type of this expression? Clearly the compiler can not know what the class name will be at runtime, so it doesn't know whether

String s = Class.forName(new BufferedReader(System.in).readLine()).cast(o);

is safe. Therefore it requests an explicit cast.