FULL PRODUCT VERSION :
Windows :
java version "1.8.0_60"
Java(TM) SE Runtime Environment (build 1.8.0_60-b27)
Java HotSpot(TM) 64-Bit Server VM (build 25.60-b23, mixed mode)

Ubuntu : OpenJDK 8

ADDITIONAL OS VERSION INFORMATION :
Tested under Windows 7 as well as Ubuntu 16.04

A DESCRIPTION OF THE PROBLEM :
Why private access specifier is not allowed for static methods in interfaces as we can access it from other static methods of that interface ??

PFB example:

/**
* Tested under Java 8 as well as OpenJDK 8
*/

interface TestInterface
{
   /*
    * why private access specifier is not allowed for method1 as we can call
    * method1()
    * from method2() irrespective of it's access specifier
    */

   // private static void method1()
   static void method1()
   {
      System.out.println("Inside method method1()");
   }

   static void method2()
   {
      System.out.println("Inside method method2(). Calling method method1()");
      method1();
   }
}

public class Java8Interface implements TestInterface
{
   public static void main(String[] args)
   {
      TestInterface.method2();
   }
}


STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
run the program below:

/**
* Tested under Java 8 as well as OpenJDK 8
*/

interface TestInterface
{
   /*
    * why private access specifier is not allowed for method1 as we can call
    * method1()
    * from method2() irrespective of it's access specifier
    */

   // private static void method1()
   static void method1()
   {
      System.out.println("Inside method method1()");
   }

   static void method2()
   {
      System.out.println("Inside method method2(). Calling method method1()");
      method1();
   }
}

public class Java8Interface implements TestInterface
{
   public static void main(String[] args)
   {
      TestInterface.method2();
   }
}


EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
private access specifier should be allowed for method method1()
ACTUAL -
private access specifier is not allowed for method method1()

ERROR MESSAGES/STACK TRACES THAT OCCUR :
Illegal modifier for the interface method method1; only public, abstract, default, static and strictfp are permitted

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
/**
* Tested under Java 8 as well as OpenJDK 8
*/

interface TestInterface
{
   /*
    * why private access specifier is not allowed for method1 as we can call
    * method1()
    * from method2() irrespective of it's access specifier
    */

// private static void method1()
   static void method1()
   {
      System.out.println("Inside method method1()");
   }

   static void method2()
   {
      System.out.println("Inside method method2(). Calling method method1()");
      method1();
   }
}

public class Java8Interface implements TestInterface
{
   public static void main(String[] args)
   {
      TestInterface.method2();
   }
}

---------- END SOURCE ----------