Question

I need to put test values in a class, somehow like in this simplified example:

import java.util.List;
import java.util.Map;
import com.google.common.collect.Lists;
import com.google.common.collect.Maps;
public class TestVal {
  private static Map<Class<?>, Object> dummyValues = Maps.newHashMap();
  static {
    dummyValues.put(String.class, "Banana");
    dummyValues.put(Integer.class, 42);
  }
  @SuppressWarnings("unchecked")
  static final <T> T ofType(Class<T> type) {
    return (T) dummyValues.get(type);
  }
  public static void main(String[] args) {
    List<? extends Object> list =
        Lists.newArrayList(ofType(String.class), ofType(Integer.class));
    for (Object object : list) {
      System.out.printf("%s: %s%n", object, object.getClass().getName());
    }
  }
}

which, once run, gives me:

Banana: java.lang.String
42: java.lang.Integer

so it’s fine as a “basic” system.

I’d like more type-safety though.

I was trying to study Guava’s ImmutableClassToInstanceMap but… I find it a little difficult, take a look at this method signature:

public static
    <B, S extends B>
        ImmutableClassToInstanceMap<B>
            copyOf(
                Map<? extends Class<? extends S>,
                ? extends S> map
            )

brutal, isn’t it? How would I use it for “any” type? Did not see an example on Google’s Wiki..

Can I move “one step up” without “going all the way” like the thing above? Am I missing another obvious way to do the same?

Answer 1

Eh? All you need to do with ImmutableClassToInstanceMap is something like

ImmutableClassToInstanceMap<Object> map = ImmutableClassToInstanceMap
  .builder()
  .put(String.class, "foo")
  .put(Integer.class, 42)
  .build();

and then you just call map.getInstance(String.class) to get out a String.

The generics on copyOf() are complicated but necessary, but for this case you’ll almost certainly want to just use the builder instead.