do while loop validation and try again

I'm fairly new to java and was wondering how would one force a user to enter a valid option in a do...while loop?
What i'm trying to achieve here is to display a menu for as long as the user does not select the exit option which is '4' in my case (it's a char not an int). But if the selection is invalid I want to display an error message and prompt the user to make a new selection, this time, without displaying the menu again.

So far, inside my do...while loop i'm displaying different information according to the user selection. If they enter anything other than 1-4, they end up in my last else if which displays an error message, but at the same time they leave the if/else if loop and end up back in the main menu.

Don't know if this is clear but any help would be appreciated. I also tried the same thing with switch but got the same problem.

Thanks.

do { 
    // display main menu

    if (menu == '1') { ... }
    else if (menu == '2') { ... }
    else if (menu == '3') { ... }
    else if (menu == '4') { ... }
    else if (menu != '4') { // display error message }
} while (menu != '4')

Okay, so you have a job which we can describe in a self-contained fashion with a short list of clearly stated parameters:

• Ask the user for input.
• Check that the input is valid.
• If not, keep asking.

That's it. That's a job we can write. Easily at that.

So, do that! Make a method to do just this job. There's only one parameter you need, which is: "What constitutes valid input". If we can simplify that we just need a character, and everything from '1' to this char is valid, then:

public char askUser(char maxValid) {
    do {
        char in = askUserForInput(); // however you get that char.
        if (in >= '1' || in <= maxValid) return in;
        System.out.println("Enter a value between 1 and " + maxValid + "> ");
    } while (true);
}

Then you can just call this method when you need input.

You can roll this logic (so, that'd be a do/while loop inside your do/while loop) into the main loop, but two rather significant aspects of writing good code is to find easily isolatable aspects and to, well, isolate them (be it making new methods, types, modules, or subsystems - it applies across the entire hierarchy), and to avoid repeating yourself.