In writing an Annotation Processor using the Java 6 API, I came across a need to handle all Maps in a particular fashion, but I'm clearly misunderstanding what the API is intended to do or how to invoke it. Here's the code that's making me unhappy:
import javax.lang.model.element.Element;
import javax.lang.model.type.TypeMirror;
import javax.lang.model.util.Elements;
import javax.lang.model.util.Types;
import javax.annotation.processing.ProcessingEnvironment;
...
public String doThing(Element el, ProcessingEnvironment processingEnv) {
// Utilities from the ProcessingEnvironment
Types typeUtils = processingEnv.getTypeUtils();
Elements elementUtils = processingEnv.getElementUtils();
// The type of the element I'm handling
TypeMirror elType = el.asType();
// Compare the element's type to Map
TypeMirror mapType = elementUtils.getTypeElement("java.util.Map").asType();
System.out.println(elType + " > " + mapType + " = " + typeUtils.isSubtype(elType, mapType));
System.out.println(mapType + " > " + elType + " = " + typeUtils.isSubtype(mapType, elType));
System.out.println(elType + " > " + mapType + " = " + typeUtils.isAssignable(elType, mapType));
System.out.println(mapType + " > " + elType + " = " + typeUtils.isAssignable(mapType, elType));
// Compare the element's type to HashMap
TypeMirror hashmapType = elementUtils.getTypeElement("java.util.HashMap").asType();
System.out.println(elType + " > " + hashmapType + " = " + typeUtils.isSubtype(elType, hashmapType));
System.out.println(hashmapType + " > " + elType + " = " + typeUtils.isSubtype(hashmapType, elType));
System.out.println(elType + " > " + hashmapType + " = " + typeUtils.isAssignable(elType, hashmapType));
System.out.println(hashmapType + " > " + elType + " = " + typeUtils.isAssignable(hashmapType, elType));
// Compare the element's type to Object
TypeMirror objectType = elementUtils.getTypeElement("java.lang.Object").asType();
System.out.println(elType + " > " + objectType + " = " + typeUtils.isSubtype(elType, objectType));
System.out.println(objectType + " > " + elType + " = " + typeUtils.isSubtype(objectType, elType));
System.out.println(elType + " > " + objectType + " = " + typeUtils.isAssignable(elType, objectType));
System.out.println(objectType + " > " + elType + " = " + typeUtils.isAssignable(objectType, elType));
}
Given that, here's the output of it:
java.util.HashMap<K,V> > java.util.Map<K,V> = false
java.util.Map<K,V> > java.util.HashMap<K,V> = false
java.util.HashMap<K,V> > java.util.Map<K,V> = false
java.util.Map<K,V> > java.util.HashMap<K,V> = false
java.util.HashMap<K,V> > java.util.HashMap<K,V> = true
java.util.HashMap<K,V> > java.util.HashMap<K,V> = true
java.util.HashMap<K,V> > java.util.HashMap<K,V> = true
java.util.HashMap<K,V> > java.util.HashMap<K,V> = true
java.util.HashMap<K,V> > java.lang.Object = true
java.lang.Object > java.util.HashMap<K,V> = false
java.util.HashMap<K,V> > java.lang.Object = true
java.lang.Object > java.util.HashMap<K,V> = false
This makes perfect sense to me except for the first block where I'd expect a HashMap element to be assignable to Map, and I'd expect HashMap to be a subtype of Map.
What am I missing here?
I suspect it's because of the type variables. HashMap<String, String>
is assignable to Map<String, String>
but without the concrete instantiation of the type variables you can't be sure that an arbitrary HashMap<A,B>
is assignable to Map<X,Y>
.
If you instantiate the variables with wildcards then you should get the result you expect
DeclaredType wildcardMap = typeUtils.getDeclaredType(
elementUtils.getTypeElement("java.util.Map"),
typeUtils.getWildcardType(null, null),
typeUtils.getWildcardType(null, null));
This will give you the type mirror for Map<?,?>
, which all HashMap
instantiations are assignable to.
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