http://www.nurcin.co/generate-a-random-password-in-java/

Password.java

import java.util.random;

public class Password {
	public static String GeneratePassword(int size) {
		String password = "";
		
		Random r = new Random();
		
		char[] symbols = {'!', '?', '_', '#', '-'};
		
		char[] lower = new char[26];
		char[] upper = new char[26];
		
		for (int i = 0; i < 26; i++) {
			lower[i] = (char)('a' + i);
			upper[i] = (char)('a' + i);
		}
		
		for(int i = 0; i < size; i++)
		{
			/*
			 * digits <-0
			 * symbols <-1
			 * lower <-2
			 * upper <-3
			 */
			
			switch(r.nextInt(4))
			{
			case 0: password += r.nextInt(10); break;
			case 1: password += symbols[r.nextInt(symbols.length)]; break;
			case 2: password += lower[r.nextInt(lower.length)]; break;
			case 3: password += upper[r.nextInt(upper.length)]; break;
			}
		}
		
		return password;
		
	}
}