Input keyboard - Null pointer Exception

Question

The program reads input from keyboard in a loop and I put the characters in an array. But I get null pointer exception when I try to press control Z. Although the program should exit from the loop.
I think I have error here. But I am not sure why is it happening .
char [] dst1 = new char[str_len1];
String str = "";
    BufferedReader br = new BufferedReader(new InputStreamReader (System.in));
        System.out.println("Enter the characters press ^Z to exit\n");
    StringBuffer buf = new StringBuffer();
do {
    str = br.readLine();
    str_len1 = str.length();
      char [] dst1 = new char[str_len1];
   str.getChars(0,str_len1, dst1, 0);
}

Answer 1

I tried using
if (str.length() != 0) {
then execute the code }
But it still shows me Null Pointer ExceptionIf str is null, str.length() will also result in a NullPointerException. Read the first reply to your post and add a check accordingly.