method() in x.y.z is defined in an inaccessible class or interface

Question

Super.java

package x.y.z;

public abstract class Super {
  public CustomClass a() {
    return new CustomClass();
  }

  public abstract String getName();
  public abstract String getDescription();
}

Sub.java

package x.y.z;

public abstract class Sub extends Super {
  public String getDescription() {
    return "Is a Sub";
  }
}

User.java

package x.y.z;

public class User {
  private class UseCase extends Sub {
    public String getName() {
      return "UseCase";
    }
  }

  public UseCase use() {
    return new UseCase();
  }
}

In another part of my app I try to access new User().use().a() , and I think this causes the error (it's a compile-time error though).

---

Trying to compile the above errors:

a() in x.y.z.Super is defined in an inaccessible class or interface

What's causing this error and how do I fix it?

---

New question

This makes the error disappear for me:

User.java

package x.y.z;

public class User {
  private class UseCase extends Sub {
    public String getName() {
      return "UseCase";
    }
  }

  public Super use() {
    return new UseCase();
  }
}

Changing the type of User.use() to Super "fixes" the error.

Is this a problematic "fix", or will this work fine without any hiccups?

Answer 1

The problem here is that the type that you return from User#use (ie UseCase ) is private to the User class, prohibiting it from being accessed from anywhere outside User . Modifying UseCase to be public instead of private should fix your issue.

public class UseCase extends Sub {
    @Override
    public String getName() {
        return "UseCase";
    }
}

Answer 2

a() in x.y.z.Super is defined in an inaccessible class or interface

Because a UseCase class, that you are trying to return, is private. Consider the following example, that compiles without errors:

User user = new User();
Sub sub = user.use(); // because of implicit up-casting to Sub (and Sub is public)
sub.a();

If you want to fit these 3 lines into a single expression, you need an explicit cast:

CustomClass custom = ((Sub) new User().use()).a();

Also, as you've already pointed out, you can change the use(...) method return type to a Sub or a Super class, so the following code will work without additional casts:

CustomClass custom = new User().use().a();