Java generics with unbounded wildcard?

Question

I have an interface to convert object to string:

public interface Converter {
    String asString(T object);
}

And a map to store

Answer 1

First, you should encapsulate the map inside a helper class like this, whose operations preserve the invariant (that Class maps to Converter):

public class ConverterMap {
    Map, Converter> converterMap = new HashMap, Converter>();
    public  void addConverter(Class clazz, Converter converter) {
        converterMap.put(clazz, converter);
    }
    @SuppressWarnings("unchecked")
    public  Converter getConverter(Class clazz) {
        return (Converter)converterMap.get(clazz);
    }
}

Now, to break down the task, let's take the small step of writing a function that takes any object and converts it based on the converter map (assuming the object's class is in the converter map):

ConverterMap cm = new ConverterMap;
private static String convert(Object x);

This seems simple, but is harder than it looks, because you will run into a special case of the Java type system in how it types .getClass(). You will have the problem of convincing the compiler that x is an instance of the parameter of x.getClass(). The best way to solve this is:

@SuppressWarnings("unchecked")
private static  String convert2(Class clazz, Object x) {
    return cm.getConverter(clazz).asString((T)x);
    // you can alternately do clazz.cast(x) instead of the unchecked cast (T)x
}
private static String convert(Object x) {
    return convert2(x.getClass(), x);
}

And then you can solve the rest of the problem:

for (Object datum : data) {
    stringData.add(convert(datum));
}