java - Trying to test Enum's method -

im trying manually test enum rotation method next() it's returning me nothing, null. when assign variable tester = rotation.cw0 , call method next(), method should return cw90 returning nothing. please smb take i've done wrong in code?

public class tester {      public static void main(string[] args)     {         rotation tester = rotation.cw0;         tester.next();     } } public enum rotation {      cw0, cw90, cw180, cw270;      // calling rot.next() return next enumeration element     // representing next 90 degree clock-wise rotation after rot.     public rotation next()     {         if(this == cw0)             return cw90;         else if(this == cw90)             return cw180;         else if(this == cw180)             return cw270;         else if(this == cw270)             return cw0;          return null;     } 

public class tester {      public static void main(string[] args) {         rotation tester = rotation.cw0;         system.out.println(tester.next());     } }  public enum rotation {      cw0, cw90, cw180, cw270;      public rotation next() {         switch (this) {         case cw0:            return cw90;         case cw90:            return cw180;         case cw180:            return cw270;         case cw270:            return cw0;         }         return null;     } }