Problems calling private static methods

class Temp {
  private static void foo(int arg) { System.out.println("arg=" + arg);}

  static void callFooFromStaticMethod() {
    Temp.foo(0);          // ok: arg=0
    foo(1);               // ok: arg=1
//    this.foo(2);        // non-static variable this cannot be referenced from a static context
    new Temp().foo(3);    // ok: arg=3
  }

  void callFooFromInstanceMethod() {
    Temp.foo(4);         // ok: arg=4
    foo(5);              // ok: arg=5
    this.foo(6);         // ok: arg=6
    new Temp().foo(7);   // ok: arg=7
  }

  public static void main(String[] args){
    Temp.callFooFromStaticMethod();
    new Temp().callFooFromInstanceMethod();
  }
}

class Test {
  // all errors go away if method is declared non-private
  private static foo(arg) { println "arg=" + arg }

  static callFooFromStaticMethod() {
    Test.foo(0)        // ok: arg=0
    foo(1)             // ok: arg=1
    this.foo(2)        // Non-static variable 'this' cannot be referenced from the static method callFooFromStaticMethod.
    new Test().foo(3)  // java.lang.IncompatibleClassChangeError
  }

  def callFooFromInstanceMethod() {
    Test.foo(4)        // ok: arg=4
    foo(5)             // ok: arg=5
    this.foo(6)        // ok: arg=6
    new Test().foo(7)  // java.lang.IncompatibleClassChangeError
  }
}

Test.callFooFromStaticMethod()
new Test().callFooFromInstanceMethod()