javajruby

JRuby can not call Java method

I tried to create a small example, where I invoke a method on a Java object, but I JRuby can't find the method.

I have the following file:

File t.java with content

class T {
  public T() {}
  public String f() {
    return "abc";
  }
}

I then do (inside the directory of t.java):

javac t.java # Produces a file T.class
jruby -e 'java_import "T"; r=T.new; puts(r.f)'

this produces the error message

NoMethodError: undefined method `f' for #Java::Default::T:0x38cee291

Interestingly, the constructor (T.new) seems to have succeeded. Why can't I call the instance method f?

Platform:

MacOS 12.6.5

jruby 9.4.3.0 (3.1.4) 2023-06-07 3086960792 Java HotSpot(TM) 64-Bit Server VM 25.341-b10 on 1.8.0_341-b10 +jit [x86_64-darwin]

Solution

  • I found, that the problem is on the Java side:

    First, the class T needs to be declared public. I don't know why I can create an instance of a non-public class, but can't call instance methods of it. This seems to be a Java-restriction.

    Second, after renaming, the Java compiler insists that I also rename the file from t.java to T.java. The file name must match the class name.

    After these changes, the example works.