java - Differences in type inference JDK8 javac/Eclipse Luna?

Question

I'm trying to switch a project to Java8, and encounter odd differences between Eclipse Luna and javac's type inference. With JDK 1.7.0_65 javac this code compiles just fine. JDK 1.8.0_11 complains that both toString(char[]) and toString(Throwable) match for the "toString(getKey(code, null));" line. Eclipse Luna 4.4 (I20140606-1215) compiles it happily with either JDK:

public class TypeInferenceTest {
    public static String toString(Object obj) {
        return "";
    }

    public static String toString(char[] ca) {
        return "";
    }

    public static String toString(Throwable t) {
        return "";
    }

    public static <U> U getKey(Object code, U defaultValue) {
        return defaultValue;
    }

    public static void test() {
        Object code = "test";
        toString(getKey(code, null));
    }
}

I think the only signature that could possibly match is toString(Object).

Of course I could simply add a cast to Object, but I wonder why javac can't infere the type by itself (while eclipse does), and why the heck javac considers Throwable and char[] suitable matches, but not Object.

Is this a bug in Eclipse or javac? (I mean only one compiler can be right here, either it compiles or it doesn't)

Edit: Error message from javac (JDK8):

C:XXXXWorkspaceXXXXsrc>javac -cp . TypeInferenceTest.java
TypeInferenceTest.java:22: error: reference to toString is ambiguous
                toString(getKey(code, null));
                ^
  both method toString(char[]) in TypeInferenceTest and method toString(Throwable) in TypeInferenceTest match
1 error

Answer 1

Compilers can only inspect the method signatures, not the method body, so that part is irrelevant.

This "reduces" your code to (psuedocode):

public class TypeInferenceTest {
    public static String toString(Object obj);

    public static String toString(char[] ca);

    public static String toString(Throwable t);

    public static <U> U getKey(Object code, U defaultValue);

    public static void test() {
        Object code = "test";
        toString(getKey(code, null));
    }
}

Also note that the <U> U getKey(...) really is: <U extends Object> U getKey(...).

All it knows that getKey(code, null) returns is: ? extends Object, so it returns a subtype of Object, or an Object itself.
There are three signatures that match, namely Object, char[] and Throwable, where both char[] and Throwable match equally and better than Object, because you asked for an ? extends Object.

So it cannot choose which is the correct one, because all three match the signature.

When you change it to:

public static Object getKey(Object code, Object defaultValue);

then only public static String toString(Object obj); matches, because it matches better as any other ? extends Object that is not equal to Object.

Edit, I looked over the original intent of the question: Why does it compile in Java 7, but not in Java 8?

In Java 8 type inference got greatly improved.

Whereas in Java 7 it could for example only infer that getKey returned an Object, it now in Java 8 infers that it returns an ? extends Object.

When using Java 7 there was only one match, namely Object.

To have the change visualized even better, consider this piece of code:

public class TypeInferenceTest {
    public static String toString(Object obj) { return "1"; }

    public static String toString(Throwable t) { return "2"; }

    public static <U> U getKey(Object code, U defaultValue) { return defaultValue; }

    public static void test() {
        Object code = "test";
        String result = toString(getKey(code, null));
        System.out.println(result);
    }

    public static void main(String[] args) {
        test();
    }
}

On Java 7 it prints 1, on Java 8 it prints 2, exactly because of the reasons I have outlined above.