@s0bc

RandomChars.java

import java.util.Random;
 
public class RandomChars
{
  private final static String answer = "あいうえ";
  public static int count = 0;
 
  public static void main(String[] args)
  {
    char[] chars = answer.toCharArray();
    String strs = "";
 
    while(!strs.equals(answer))
    {
      count++;
      if(strs.length() >= answer.length()) {
        strs = strs.substring(1, answer.length());
      }

      char c = chars[(int)(Math.random() * chars.length)];
      strs += c;
      System.out.print(c);
    }
 
    System.out.println("\ncount = " + count);
  }
}