Java Puzzle: modifiers

Question

Can you, please, tell me 1) why it is possible to assign a constructor as public for a package-visible class? For example:

class TestModifiers {
    public TestModifiers() {

    }
}

This class can't be instantiated everywhere but in the same package. So that isn't it enough to use a package-visible modifier for a constructor? That is also true with any method(public) in such a class(package-visible)

2) say, in an inner class I have created two private variables. Why can I see them from the outer class? Isn't it strange?? For example:

class A {
  A(){}

  class B {
    private int b1;
    private int b2;
  }

  public static void main(String[] args) {
   new B().b1 = 1;
   new B().b2 = 2;
  }
 } 

Answer 1

There is no value in a non-public class having a public constructor since, as you rightly state, the class isn't accessible outside of that scope. It will compile - that's just the way things are - but many code analysis tools will generate a warning.

There is value in a non-public class having public methods if the class extends or implements a public class or interface , since the derived class can stand in for the base class or interface type:

package mypackage;

class MyRunnable implements Runnable {
    private final String message;

    MyRunnable(String message) {
        this.message = message;
    }

    @Override
    public void run() {
        System.out.println(this.message);
    }
}

public class Surprises {
    public static Runnable getSurprise() {
        return new MyRunnable("boo!");
    }
}

Code outside of mypackage can then obtain a MyRunnable instance (as a Runnable ) and call the public method:

Runnable r = Surprises.getSurprise();
r.run();

I'll ignore your second question since it was answered elsewhere in the comments.

Answer 2

Public modifiers are allowed on non-public classes because public methods defined in those classes may need to be overridden in public subclasses. Eg

abstract class MyBaseClass {
  public abstract void method();
}

public class MySubClass extends MyBaseClass {
  @Override public void method() { ... }
}

Rather than impose a restriction on constructors that does not appear on other members, public constructors are allowed on classes regardless of the visibility of the containing classes.

Answer 3

1. It may be useful for inheritance.
2. class B , B.b1,b2 belongs to class A, they are inside A.

Answer 4

1) why it is possible to assign a constructor as public for a package-visible class?

It's much more convenient then it could be.

Just imagine that you have a public class, and then you understand that you can make it package-private, and after that compiler tells you that there's 45 errors (on every of your public methods).

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Cannot reply on your second question. I also think that it's weird.