How to Generate Unique Random Numbers?

import com.sun.crypto.provider.RSACipher;
import java.util.Random;

public class RandomRange {

    static  int START = 1;
    static int END;
   static  Random random = new Random();
 
    private static void showRandomInteger(int aStart, int aEnd, Random aRandom){
            if ( aStart > aEnd ) {
                throw new IllegalArgumentException("Start cannot exceed End.");
            }
            //get the range, casting to long to avoid overflow problems
            long range = (long)aEnd - (long)aStart + 1;
            // compute a fraction of the range, 0 <= frac < range
            long fraction = (long)(range * aRandom.nextDouble());
            int randomNumber =  (int)(fraction + aStart);                     
            //  System.out.println("Hi this is from senthil == >"+ j );
            log("Generated : " + randomNumber);
    }
   
    private static void log(String aMessage){
        System.out.println(aMessage);
    }   
   
    public static void main(String aArgs[]){
        int countrec = 9;
        log("Generating random integers in the range "+START+"...." + countrec);
        END = countrec - 1;
      for (int idx = START; idx <= 5; ++idx){
            showRandomInteger(START, END, random);
        }
        log("Done.");
       
       
    }
}