I need help on this exercise. Can anyone help out a beginner?

/*  So the age old knock knock joke goes like this:

    Person A:  Knock Knock.
    Person B:  Who's there?
    Person A:  Banana
    Person B:  Banana who?
    ...as long as Person A has answered Banana the above repeats endlessly
    ...assuming the person answers Orange we'd see
    Person B:  Orange who?
    ...and then the punchline.
    Person A:  Orange you glad I didn't say Banana again?
    (It's a really bad joke that makes it sound like "Aren't you glad I didn't say Banana again?")

    Let's just assume the only two words passed in from the console from Person B are either banana or orange.
*/

// ====BEGIN PROMPTING CODE====
String PersonA = console.printf("Knock Knock.\n");
String who;
boolean isInvalidWord;
do {
  who = console.readLine("Who's there?      ");
  isInvalidWord = (who.equalsIgnoreCase("banana")||
                  who.equalsIgnoreCase("BANANA"));
  if (isInvalidWord) {
    console.printf("Try again.\n\n");
  }
} while (isInvalidWord);
String PersonB = console.printf("%S who?\n", who);

// ==== END PROMPTING CODE ====