handle wrong input int/null

Lets say we got a program and we want someone to type either 1 or 2 in order to do something. If they type 1) any other integer or if 2) they do not type anything (and just press enter), how can i make the program not crash? I mean e.g. if they type 4 then the program to show again the phrase 'please type either 1 or 2'; and to repeat again and again the prompt until the user types 1 or 2. thank you

You can use some if-else statements to test/qualify the iuput.
For example,

Code:

---

BufferedReader reader = ...
int n = -1;
do {
    System.out.println("enter 1 or 2");
    String line = reader.readLine();
    if(line != null && line.length() > 0) {
        // User entered something so we may have something to parse.
        // You may want to check that "line" contains parsable digits
        // with something like [i]Character.isDigit[/i].
        // Or, you could use a try/catch for NumberFormatException.
        n = Integer.parseInt(line);
    }
} while(n != 1 && n != 2);

---