Question

I’d like to remove all ‘unchecked’ warnings from this general utility method (part of a larger class with a number of similar methods). In a pinch, I can use @SuppressWarnings(‘unchecked’) but I’m wondering if I can use generics properly to avoid the warning.

The method is intended to be allow callers to compare two objects by passing through to compareTo, with the exception that if the object is a strings it does it in a case insensitive manner.

public static int compareObject(Comparable o1, Comparable o2) {     if ((o1 instanceof String) && (o2 instanceof String))         return ((String) o1).toUpperCase().compareTo(((String) o2).toUpperCase());     else         return o1.compareTo(o2); } 

This was my first (incorrect) attempt at a solution. The parameters work fine, but the line o1.compareTo(o2) has a compile error ‘The method compareTo(capture#15-of ?) in the type Comparable is not applicable for the arguments (Comparable’.

public static int compareObject(Comparable<?> o1, Comparable<?> o2) {     if ((o1 instanceof String) && (o2 instanceof String))         return ((String) o1).toUpperCase().compareTo(((String) o2).toUpperCase());     else         return o1.compareTo(o2); } 

Any suggestions?

Answer 1

I just tried this:

public static <T extends Comparable> int compareObject(T o1, T o2) {     if ((o1 instanceof String) && (o2 instanceof String))         return ((String) o1).toUpperCase().compareTo(((String) o2).toUpperCase());     else         return o1.compareTo(o2); } 

It compiles, but gives a unchecked cast warning on the compareTo() call.
I tried changing it to

public static <T extends Comparable<T>> int compareObject(T o1, T o2) { 

and the String checks failed to compile ("inconvertible types: found: T, required: String"). I think this must be close, though.

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EDIT: As pointed out in the comments, this is a bug in javac. The second form is indeed correct, but will not compile currently. Crazy as it may look, this is the code that works with no warnings:

public static <T extends Comparable<T>> int compareObject(T o1, T o2) {     if (((Object) o1 instanceof String) && ((Object) o2 instanceof String))         return ((String) (Object)o1).toUpperCase().compareTo(((String) (Object)o2).toUpperCase());     else         return o1.compareTo(o2); } 

As you can see, the only difference is all the redundant casts to Object.