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Editorial Team
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Asked: 2026-05-27T01:43:04+00:00

interface Pong<T> {} class Ping<T> implements Pong<Pong<? super Ping<Ping<T>>>> { static void Ping() {

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interface Pong<T> {}
class Ping<T> implements Pong<Pong<? super Ping<Ping<T>>>> {
    static void Ping() {
        Pong<? super Ping<Long>> Ping = new Ping<Long>();
    }
}

Trying to compile this gives the error:

The system is out of resources.
Consult the following stack trace for details.
java.lang.StackOverflowError
    at com.sun.tools.javac.code.Types$23.visitClassType(Types.java:2579)
    at com.sun.tools.javac.code.Type$ClassType.accept(Type.java:554)
    at com.sun.tools.javac.code.Types$UnaryVisitor.visit(Types.java:3260)
    at com.sun.tools.javac.code.Types$23.visitClassType(Types.java:2592)
    at com.sun.tools.javac.code.Types$23.visitClassType(Types.java:2579)
    at com.sun.tools.javac.code.Type$ClassType.accept(Type.java:554)
    ...

Code courtesy of etorreborre on github.

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1 Answer

  1. Editorial Team

    Clearly, it is a bug in the Java compiler. The compiler shouldn’t crash, especially on a program so small.

    It could even be a hole in the Java Language Specification; i.e. an obscure edge case in generics that the JLS authors haven’t considered.

    But (IMO) this is nothing more than a curiosity, unless you can come up with an example that isn’t so obviously contrived to break the compiler. I mean, this example code isn’t exactly meaningful …

    Someone with a deep understanding of the Java compiler’s implementation could probably figure out why this causes a stack overflow. But it is hardly relevant unless that person is also going to fix the bug. And unless someone can come up with a meaningful example that triggers the same problem, I can’t see any value in fixing it.

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