Types isAssignable and isSubtype misunderstanding

Question

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?

Ian Roberts · Accepted Answer · 2012-10-05T16:28:46.727

9

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.

edited Oct 05 '12 at 16:28

answered Oct 05 '12 at 16:18

Ian Roberts

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You know, I thought that, and I thought since the generic letters matched up that the types would match up, but I guess not. Thanks! – Patrick Oct 06 '12 at 04:54
2

This entire approach and @Patrick 's implementation of it are far too complex. Just use `types.isAssignable(type, types.erasure(baseGenerifiedType))` (e.g. check if type is assignable to raw `HashMap` instead of `HashMap, ?>`) – user1643723 Mar 20 '16 at 02:45

score 2 · Answer 2 · edited May 23 '17 at 12:18

Update (March 2016): Based on a comment by @user1643723 , there appears to be a types.erasure(TypeMirror) library function that I wasn't aware of in 2012.

Based on Ian's Answer, I'm now using the following method to match base types like I described for Map in the question.

TypeElement COLLECTION = elementUtils.getTypeElement("java.util.Collection");
TypeElement MAP = elementUtils.getTypeElement("java.util.Map");
TypeElement VOID = elementUtils.getTypeElement("java.lang.Void");
WildcardType WILDCARD_TYPE_NULL = typeUtils.getWildcardType(null, null);
Map<String,DeclaredType> cachedParentTypes = new HashMap<String, DeclaredType>();

...

public static boolean isA(TypeMirror type, TypeElement typeElement) {

    // Have we used this type before?
    DeclaredType parentType = cachedParentTypes.get(typeElement.getQualifiedName().toString());
    if (parentType == null) {
        // How many generic type parameters does this typeElement require?
        int genericsCount = typeElement.getTypeParameters().size();

        // Fill the right number of types with nulls
        TypeMirror[] types = new TypeMirror[genericsCount];
        for (int i = 0; i < genericsCount; i++) {
            types[i] = WILDCARD_TYPE_NULL;
        }

        // Locate the correct DeclaredType to match with the type
        parentType = typeUtils.getDeclaredType(typeElement, types);

        // Remember this DeclaredType
        cachedParentTypes.put(typeElement.getQualifiedName().toString(), parentType);
    }

    // Is the given type able to be assigned as the typeElement?
    return typeUtils.isAssignable(type, parentType);
}

which I invoke like

if (isA(elType, VOID)) {
    isVoid = true;
} else if (isA(elType, COLLECTION) || elType.getKind() == TypeKind.ARRAY) {
    isCollectionOrArray = true;
} else if (isA(elType, MAP)){
    isMap = true;
}

Types isAssignable and isSubtype misunderstanding

2 Answers2