Java: Instanceof and Generics

Question

Before I look through my generic data structure for a value's index, I'd like to see if it is even an instance of the type this has been parametrized to. But Eclipse complains when I do this:

@Override
public int indexOf(Object arg0) {
 if (!(arg0 instanceof E)) {
  return -1;
 }

What is the better way to do it?

This is the error message

Cannot perform instanceof check against type parameter E. Use instead its erasure Object since generic type information will be erased at runtime

Answer 1

arg0 instanceof this.class()

Maybe that would do it.

Answer 2

The error message says it all. At runtime, the type is gone, there is no way to check for it.

You could catch it by making a factory for your object like this:

 public static <T> MyObject<T> createMyObject(Class<T> type) {
    return new MyObject<T>(type);
 }

And then in the object's constructor store that type, so variable so that your method could look like this:

        if (arg0 != null && !(this.type.isAssignableFrom(arg0.getClass()))
        {
            return -1;
        }

Answer 3

Technically you shouldn't have to, that's the point of generics, so you can do compile-type checking:

public int indexOf(E arg0) {
   ...
}

but then the @Override may be a problem if you have a class hierarchy. Otherwise see Yishai's answer.

Answer 4

The runtime type of the object is a relatively arbitrary condition to filter on. I suggest keeping such muckiness away from your collection. This is simply achieved by having your collection delegate to a filter passed in a construction.

public interface FilterObject {
     boolean isAllowed(Object obj);
}

public class FilterOptimizedList<E> implements List<E> {
     private final FilterObject filter;
     ...
     public FilterOptimizedList(FilterObject filter) {
         if (filter == null) {
             throw NullPointerException();
         }
         this.filter = filter;
     }
     ...
     public int indexOf(Object obj) {
         if (!filter.isAllows(obj)) {
              return -1;
         }
         ...
     }
     ...
}

     final List<String> longStrs = new FilterOptimizedList<String>(
         new FilterObject() { public boolean isAllowed(Object obj) {
             if (obj == null) {
                 return true;
             } else if (obj instanceof String) {
                 String str = (String)str;
                 return str.length() > = 4;
             } else {
                 return false;
             }
         }}
     );