public static void main(String[] args) {
    

    
    List<String> list = new ArrayList<String>();
    String str = "abcdefghijklmnopqrstuvwxyz0123456789";
    StringBuffer stringbuffer = new StringBuffer();
    for(
    int j1 = 0;j1<str.length();j1++){
    for (int j2 = 0; j2 < str.length(); j2++) {
    for (int j3 = 0; j3 < str.length(); j3++) { 
    for (int j4 = 0; j4 < str.length(); j4++) {
    for (int j5 = 0; j5 < str.length(); j5++) { 
    for (int j6 = 0; j6 < str.length(); j6++) {
    String ss = String.valueOf(str.charAt(j1)) +
            String.valueOf(str.charAt(j2)) + 
            String.valueOf(str.charAt(j3)) + 
            String.valueOf(str.charAt(j4))+
            String.valueOf(str.charAt(j5)) + 
            String.valueOf(str.charAt(j6));
    System.out.println(ss);
    }
    }
    }

}
    }}