java 7 heterogeneous containers assignability

Question

The following Java 7 code compiles fine without type safety warnings:

@SuppressWarnings("javadoc")
public class Foo {
    public static class Bar<T> {
        public final Class<T> type;

        public Bar(Class<T> type) {
            this.type = type;
        }
    }

    @SuppressWarnings("unused")
    public static void main(String[] args) {
        new Foo(
                Arrays.asList(
                        new Bar<>(Object.class),
                        new Bar<>(String.class)));
    }

    public final List<Bar<?>> list;

    public Foo(List<Bar<?>> list) {
        this.list = list;
    }
}

However, changing

new Bar<>(Object.class),

to

new Bar<>(String.class),

fails to compile with following error:

The constructor Foo(List<Foo.Bar<String>>) is undefined

Compiling with Java 8 succeeds. According to previous posts on StackOverflow

there was some improvement in Java 8 which now allows this to compile.

The question is now what could be the best workaround for Java 7.

Adrian.

@PSM please post an answer rather than dumping unreadable code in comments. — Andy Turner, Mar 30 '16 at 07:08
There is no answer as code compile fine.I put the same code for ref. purpose. I just change new Bar<>(Object.class) to new Bar<>(String.class). — P S M, Mar 30 '16 at 07:09
I copied your code into Eclipse. The Eclipse compiler does not show any error or warning. Additionally, I compiled it then using "javac" (JDK 1.8.0_77). Again it compiles without any error or warning. So, please show us also how you compile this and with which Java version. — Seelenvirtuose, Mar 30 '16 at 07:11
Then write "the code compiles for me" rather than dumping code in a comment. It is unclear what you are saying with the current comment. — Andy Turner, Mar 30 '16 at 07:11
yes It is true @ Seelenvirtuose.No compile time error or warning. — P S M, Mar 30 '16 at 07:12
Ok @ Andy Turner.But Why you told that "you'd get a StackOverflowError if this did compile". — P S M, Mar 30 '16 at 07:13
I tried it also now with Java 7 (JDK 1.7.0_80). There I get the mentioned error, in Eclipse and with "javac". So it is about the Java version. — Seelenvirtuose, Mar 30 '16 at 07:15
The StackOverflowError is not the issue... Replaced the instance initializer block with a static main method; same compilation issue. — Adrian Herscu, Mar 30 '16 at 07:30
Tried now with Java 8 and it works; seems like a limitation/defect in Java 7. — Adrian Herscu, Mar 30 '16 at 07:33
Answered here: http://stackoverflow.com/questions/24188824/difference-of-assignability-with-nested-wildcards-in-java-7-8-generics — Adrian Herscu, Mar 30 '16 at 07:41
Also http://stackoverflow.com/questions/26269055/why-does-this-compile-in-java7-and-does-not-in-java8 — Adrian Herscu, Mar 30 '16 at 07:42
Question is now... Is there an workaround? I will re-edit the question shortly. — Adrian Herscu, Mar 30 '16 at 07:44
@AdrianHerscu Workaround is to declare a `List extends Bar>> list` (if possible). — Seelenvirtuose, Mar 30 '16 at 09:30

java 7 heterogeneous containers assignability

0 Answers0