Thanks, at last the code is working.i think somebody got angry and pushed me out from further reply.i wan't to say it that was my mistake.hope would learn the programming so that i would eat your brains less
i would thanks all of you mr.paul s,AWOL,jml.thank you.i leant lot from you,a lot yet to come and yes,its light bulb moment .
#include <SoftwareSerial.h>
SoftwareSerial BTserial(10, 11); // RX | TX
// Connect the HC-05 TX to Arduino pin 10 RX.
// Connect the HC-05 RX to Arduino pin 11 TX
void setup()
{
 Serial.begin(9600);
 Serial.println("Arduino is ready");
 BTserial.begin(9600);
 BTserial.println("Bluetooth is ready");
}
void loop()
{
 if (BTserial.available() > 0) {
  String s = BTserial.readString();
  if ( s == "hello") { // if the bluetooth read string is hello serially print hi
   Serial.println("hi");
  }
  else if ( s == "hey") { // if bluetooth read string is hey serially print yeah
   Serial.println("yeah");
  }
  else if (s != "hello" && s != "hey")
  {
   Serial.println("invalid"); //
  }
 }
}