Package private types in your public API

Bar.java

// src/main/java/j/bar/Bar.java
package j.bar;

abstract class Foo {
  public abstract String value();
}

public class Bar {
  public static Foo makeFoo() {
    return new Foo() {
      public String value() {
        return "whatever";
      }
    };
  }
}

CallJava.java

// src/main/java/j/CallJava.java
package j;

public class CallJava {
  public static void main(String[] args) {
    /**
     * The following line would fail with:
     * j/qux/Qux.java:5: error: value() in Foo is defined in an inaccessible class or interface
     *     System.out.println(j.bar.Bar.makeFoo().value());
     *                                           ^
     */
    //System.out.println(j.bar.Bar.makeFoo().value());
  }
}

CallScala.java

// src/main/java/j/CallScala.java
package j;

public class CallScala {
  public static void main(String[] args) {
    System.out.println(s.bar.Bar.makeFoo().value());
  }
}

Demo.scala

// src/main/scala/Demo.scala
package s

object CallScala {
  def main(args: Array[String]): Unit = println(s.bar.Bar.makeFoo.value)
}

object CallJava {
  def main(args: Array[String]): Unit = println(j.bar.Bar.makeFoo.value)
}

package bar {
  private[bar] abstract class Foo {
    def value: String
  }

  object Bar {
    def makeFoo: Foo = new Foo {
      def value = "whatever"
    }
  }
}

> runMain j.CallScala
[info] Running j.CallScala 
whatever
[success] Total time: 0 s, completed Feb 2, 2016 12:49:56 PM
> runMain s.CallScala
[info] Running s.CallScala 
whatever
[success] Total time: 0 s, completed Feb 2, 2016 12:49:59 PM
> runMain s.CallJava
[info] Running s.CallJava 
[error] (run-main-3) java.lang.IllegalAccessError: tried to access class j.bar.Foo from class s.CallJava$
java.lang.IllegalAccessError: tried to access class j.bar.Foo from class s.CallJava$
        at s.CallJava$.main(Demo.scala:9)
        at s.CallJava.main(Demo.scala)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.lang.reflect.Method.invoke(Method.java:497)