LambdaConversionException when mixing method reference and generics

Question

The code below compiles fine but throws an exception at runtime. Is this the expected behaviour and why?

Code:

public static void main(String[] args) {
  A<Integer> a = new A<> ();
  System.out.println(a.min()); //prints null as expected
  System.out.println(a.max()); //throws exception
}

static class A<T extends Number & Comparable<? super T>> {
  Stream<T> s = Stream.empty();
  public T min() { return s.min((t1, t2) -> t1.compareTo(t2)).orElse(null); }
  public T max() { return s.max(T::compareTo).orElse(null); }
}

Output:

null
Exception in thread "main" java.lang.BootstrapMethodError: call site initialization exception
    at java.lang.invoke.CallSite.makeSite(CallSite.java:341)
    at java.lang.invoke.MethodHandleNatives.linkCallSiteImpl(MethodHandleNatives.java:307)
    at java.lang.invoke.MethodHandleNatives.linkCallSite(MethodHandleNatives.java:297)
    at abc$A.max(abc.java:19)
    at abc.main(abc.java:8)
Caused by: java.lang.invoke.LambdaConversionException: Invalid receiver type class java.lang.Number; not a subtype of implementation type interface java.lang.Comparable
    at java.lang.invoke.AbstractValidatingLambdaMetafactory.validateMetafactoryArgs(AbstractValidatingLambdaMetafactory.java:233)
    at java.lang.invoke.LambdaMetafactory.metafactory(LambdaMetafactory.java:303)
    at java.lang.invoke.CallSite.makeSite(CallSite.java:302)
    ... 4 more

Hmm, JDK 1.8.0_60 Windows 10 x64, I get the same behaviour as the OP, i.e. LambdaConversionException is thrown... I do have IllegalStateException if I change max argument to (t1, t2) -> t1.compareTo(t2) instead of T::compareTo. — Tunaki
– Tunaki, Commented Nov 3, 2015 at 19:47
This looks similar but I can reproduce that one even with javac 1.8.0_60, which I cannot do with the example in this case. — Yosef Weiner
– Yosef Weiner, Commented Nov 3, 2015 at 20:28
Of course, you could just use Comparator.naturalOrder() instead of T::compareTo but it’s a pity that there are still problems with multiple bounds… — Holger
– Holger, Commented Nov 3, 2015 at 20:38
@SkinnyJ: I can still reproduce the problem of that question with jdk1.8.0_60 — Holger
– Holger, Commented Nov 3, 2015 at 20:48
Keep in mind that the compiler is involved. When using an older compiler, like the one integrated into Netbeans, you get the problem, even if the runtime is uptodate. — Holger
– Holger, Commented Nov 3, 2015 at 21:01

Sleiman Jneidi · Accepted Answer · 2015-11-05 11:33:58Z

Your code wouldn't work even if you use lambdas instead of method references because the stream is already exhausted

 System.out.println(a.min()); 
 System.out.println(a.max()); // exhausted

Streams are one off. But lets leave this apart. When you use the method reference version, it is capturing Number as a type parameter and not Comparable where Number has no compareTo maybe because Number is more specific here. If you just use Comparable it will work fine

  static class A<T extends Comparable<? super T>> {
    Stream<T> s = Stream.empty();
    public T min() { return s.min((t1, t2) -> t1.compareTo(t2)).orElse(null); }
    public T max() {
        T t = s.max(T::compareTo).orElse(null);
        return t; }
 }

System.out.println(a.max()); //null

IMO (just to be cautious) : I believe it is a bug.

What I actually believe: It is definitely a bug.

Edit: It turns out that this was actually a bug and it was fixed as confirmed by Brian Goetz. https://bugs.openjdk.java.net/browse/JDK-8058112. According to the bug database, this was fixed in 8u40

Yosef Weiner · Accepted Answer · 2015-11-04 17:02:27Z

4

The call site initialization problem is addressed through JDK-8058112 available in JDK 8u40 b17 or later.

edited Nov 4, 2015 at 17:02

Yosef Weiner

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answered Nov 4, 2015 at 3:54

Srikanth

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Comments

hahn · Accepted Answer · 2017-07-25 15:59:07Z

3

For those who are facing this issue in 2017 with java8 1.8.0_141, there's another filed bug report JDK-8142476 and the fix version is java9 only.

answered Jul 25, 2017 at 15:59

hahn

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Comments

assylias · Accepted Answer · 2015-11-04 07:42:10Z

2

It seems to be a Netbeans issue and I can't reproduce the problem when using javac from the command line. I have filed a bug report.

edited Nov 4, 2015 at 7:42

answered Nov 4, 2015 at 7:27

assylias

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3 Comments

Sleiman Jneidi Over a year ago

I don't think it is Netbeans fault, check my answer, it was reported as a bug in the complier

assylias Over a year ago

@SleimanJneidi I don't think that's the case. That bug was fixed in 8u40 and I have the problem with 8u60 + the code compiles and runs fine with javac (but not in Netbeans).

Yosef Weiner Over a year ago

If you turn off "compile on save", the problem disappears. Perhaps that turns off the Netbeans compiler and switches to javac?

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