CODEWITHSUNDEEP
javagenericsreflection
Cedric Reichenbach
Cedric Reichenbach

Reputation: 9319

Access generic types of an object at runtime

I know how to access generic types of fields using reflection: Just check if field.getGenericType() instanceof ParameterizedType.

But how can one check the same for an arbitrary object, not knowing where it was declared?

Upvotes: 2

Views: 216

Answers (2)

Saintali
Saintali

Reputation: 4571

Objects do not store generic information per se, so the bytecode generated for new ArrayList<Integer>() is exactly the same as the one for new ArrayList<String>(). I mean exactly. This is what is called type erasure of Java generics. They are just desugared to new ArrayList().

But, in almost all other situations, type parameters are retained, like field/parameter/return type declarations. One of the cases that is retained and not erased is the super class of a class. So if you create a class that extends ArrayList<String> you can access that information at runtime.

But this seems overkill, doesn't it? A new class that extends ArrayList<String> and another one that extends ArrayList<String>, etc. seems impractical. Anonymous inner classes can make this much easier. So, if you want to keep the generic information, you just do new ArrayList<String>() {} instead of new ArrayList<String>(). You can call getClass().getGenericSuperclass() on the created object to get the generic info.

Upvotes: 2

Ian Roberts
Ian Roberts

Reputation: 122364

Generics apply to variable declarations, method return types, etc., not objects per-se. You can determine whether the Class of a particular object uses generics via myObj.getClass().getTypeParameters() but you can't determine what values of those type parameters the specific object instance was created with.

Upvotes: 2

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