PassGen.java

package passgen;
import java.util.Random;
import java.util.Scanner;

/**
 * Exercise 37 from "Exercises for programmers".
 * @author Piotr Likus
 */
public class PassGen {

    static Random rnd = new Random();
    
    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.println("--- Password generator ---");
        
        int passLen = readInt(sc, "What's the minimum length?");
        int specCount = readInt(sc, "How many special characters?");
        int numCount = readInt(sc, "How many numbers?");
        
        int cnt = 5;
        while(cnt-- > 0) {
          String passwd = generatePassword(passLen, specCount, numCount);
          System.out.println("Your password is:"+passwd);
        }
    }

    private static int readInt(Scanner sc, String prompt) {
        System.out.println(prompt);
        return sc.nextInt();
    }

    private static String generatePassword(int passLen, int specCount, int numCount) {
        final String specialChars = "!@#$%^&*()-=_+\\|";
        final String digits = "0123456789";
        final String alpha = "abcdefghijklmnopqrstvuwyzABCDEFGHIJKLMNOPQRSTVUWXYZ";
        
        StringBuilder res = new StringBuilder();
        
        int specAdded = 0;
        int numAdded = 0;
        
        while(res.length() < passLen) {
          CharType ct = PassGen.randomCharType();
          
          switch(ct) {
              case SPECIAL: {
                  if (specAdded < specCount) {
                      res.append(randomChar(specialChars));
                      specAdded++;
                      break;
                  }
              }
              
              case DIGIT: {
                  if (numAdded < numCount) {
                      res.append(randomChar(digits));
                      numAdded++;
                      break;
                  }
              }
              
              default: {
                  res.append(randomChar(alpha));
              }
          }
        }
        
        return res.toString();
    }

    private static CharType randomCharType() {
        int idx = rnd.nextInt(3);
        switch(idx) {
            case 0: return CharType.SPECIAL;
            case 1: return CharType.DIGIT;
            default:
                return CharType.ALPHA;
        }
    }

    private static char randomChar(String text) {
        int idx = rnd.nextInt(text.length());
        return text.charAt(idx);
    }

    private enum CharType {
       SPECIAL,
       DIGIT,
       ALPHA
    }
    
}