Compilers behave differently with a null parameter of a generic method

The problem is due to a JLS specification that mandates that otherwise uninferrable type arguments must be inferred as Object, even if it doesn’t satisfy the bounds (and would consequently trigger a compilation error).

The following is an excerpt from the “bug” report (which has been further annotated for clarity):

“Bug” ID 6299211 – method type variable: inference broken for null

This program does not compile:

public class Try {
    void m() {
        java.util.Collections.max(null);
    }
}

State: CLOSED, NOT A DEFECT.

Evaluation: THIS IS NOT A BUG. The inference algorithm cannot gather any information from the argument (null) and the method is not called in a place where there are any expectations on the returned value. In such cases the compiler must infer java.lang.Object for the type variable.

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JLS 15.12.2.8 Inferring Unresolved Type Arguments

Any remaining type variables that have not yet been inferred are then inferred to have type Object

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However, Object is not a subtype of Comparable<? super Object> and thus not within the bounds of the type variable in the declaration of Collections.max:

<T extendsObject & Comparable<? super T>> T max(Collection<? extends T>)

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Further explorations

Using explicit type parameters “fixes” the problem:

HowBizarre.<Number,Integer>doIt(null); // compiles fine in javac

To show that this has less to do with a null argument and more to do with the absolute lack of information for type inferrence, you can try e.g. either of the following declarations:

<T,U extends Comparable<T>> void doIt()

<T extends Number,U extends T> void doIt()

In either case, an invocation doIt(); doesn’t compile in javac, as it must infer U to be Object as per 15.12.2.8, even if doing so would trigger a compilation error.

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Note on Eclipse

While none of the snippets above compile in some version of javac, they all do in some version of Eclipse. This would suggest a bug on Eclipse’s part. It’s been known that there are disagreements between the different compilers.

Related questions

• Generics compiles and runs in Eclipse, but doesn’t compile in javac