Trying to use a For-Loop with a 'menu'

Question

Beginner here, please be as explanatory as possible!

A course question asked me to create a menu (done).

Have multiple option on the menu give different one-time result (done).

Now it wants me to implement a for, while and do...while loop (CANNOT UNDERSTAND)

I have genuinely tried all of my rudimentary knowledge, including creating and populating an array inside the for loop (which in hindsight was a stupid idea).

public void displayMenu()
{
    System.out.println("A. Option #A");
    System.out.println("B. Option #B");
    System.out.println("C. Option #C");
    System.out.println("D. Option #D");
    System.out.println("X. Exit!");
    System.out.println();
    System.out.println("Please enter your choice:");
}

public void start()
{
    displayMenu();
    Scanner console = new Scanner(System.in);
    String input = console.nextLine().toUpperCase();
    System.out.println();

    switch (input)
    {
        case "A": System.out.println("Option #A was selected"); break;
        case "B": System.out.println("Option #B was selected"); break;
        case "C": System.out.println("Option #C was selected"); break;
        case "D": System.out.println("Option #D was selected"); break;
        case "X": System.out.println("You chose to Exit"); break;
        default: System.out.println("Invalid selection made"); break;
    }

}

public void startFor()
{
  /*Each of these methods will modify the original start() method, each 
   *will add a loop of the specific type so that the menu is displayed 
   *repeatedly, until the last option is selected. When the last option 
   *is selected, exit the method (i.e. stop the loop).
   */
}

Answer 1

As you asked for an example with for in the comments.

The point of the exercise seems to be to iterate on the menu until an exit condition is met ("X".equals(input)). That means than between the three conditions in the for statement, that's the only one you need to specify. This is because the general form of a (basic) for statement is

for ( [ForInit] ; [Expression] ; [ForUpdate] )

Where none of those terms between brackets are mandatory, so we can as well get rid of [ForInit] and [ForUpdate] (but keeping the semicolons). This has the effect of not initializing anything with [ForInit] and doing nothing at the end of each iteration of the loop with [ForUpdate], leaving us only checking for the exit condition that is given by the [Expression] expression (when it's evaluated to false, the loop exits).

Notice that the console is declared outside the loop, since it would be wasteful to allocate one at each iteration. And also input, since you need it in the for statement's condition.

Scanner console = new Scanner(System.in);
String input = "";

for (;!"X".equals(input);) { // notice, the first and last part of the for loop are absent

    displayMenu();

    input = console.nextLine().toUpperCase();
    System.out.println();

    switch (input) {
        case "A": System.out.println("Option #A was selected"); break;
        case "B": System.out.println("Option #B was selected"); break;
        case "C": System.out.println("Option #C was selected"); break;
        case "D": System.out.println("Option #D was selected"); break;
        case "X": System.out.println("You chose to Exit"); break;
        default: System.out.println("Invalid selection made"); break;
    }
}

You may notice this is a bit awkward, as this is not what you usually use a for loop for.

Anyway, at this point, the while version becomes trivial (while (!"X".equals(input))) and, in this case, the do...while is equivalent as well, (do { ... } while (!"X".equals(input))) as the same condition applies both at the end of the current loop and at the beginning of the next one, and there are no side effects between them.

As an aside, you may notice that while (condition) and for (; condition ;) are functionally equivalent and you may wander why you should use one instead of the other. The answer is readability. It's a lot more clear what you want to do when you do while (condition).