How do I use do/while with a case statement?

Question

I want to generate a unique term that is one letter followed by two numbers, and then I want to check that it is not in a database. I have tried to use a do, followed by a while, but I cannot get it to work. I get an java: illegal start of expression error. Can you see what I am doing wrong? Alternatively, is there a better way to do this?

Here is my code:

    public String setNumber(ReportType type) {

        Random rand = new Random();


  
        String number = String.format("%02d", rand.nextInt(100));

     
        String prefix = "";

        do {
            switch (type) {
                case CHILD:
                    prefix = "A";
                    break;
                case ELDER:
                    prefix = "B";
                    break;
                case BOOMER:
                    prefix = "C";
                    break;
                
            }
            String fullNumber = prefix + number;
            while (dataBase.findInDataBase(fullNumber)) throws FileNotFoundException);

        }


        return fullNumber;
    }

I don't think you can use `objects` other than `Integer` and `String` with a `switch` — nehacharya, Mar 28 '21 at 08:10
Also your `do` is not paired up with your `while`. And `while (dataBase.findInDataBase(fullNumber)) throws FileNotFoundException)` does not make sense. And has mismatched parentheses. — khelwood, Mar 28 '21 at 08:26
There are max. 100 potential numbers you can choose from. Make a list of all those numbers (A00 to A99 in case of CHILD), get all numbers from the database and remove them from the list and choose randomly one of the remaining numbers in the list. I think that would be faster because you only need to ask the database once. — mayamar, Mar 28 '21 at 08:34
same ? https://stackoverflow.com/q/66804027/15244370 at least code is similar and indentation/structure errors are same - the `'{}'` are important — , Mar 28 '21 at 08:47
@neha you forgot `enum` and all integral types: *"The type of the selector expression must be char, byte, short, int, Character, Byte, Short, Integer, String, or an enum type"* — , Mar 28 '21 at 08:52

Ole V.V. · Answer 1 · 2021-03-28T12:36:23.997

There are two syntax errors in your code:

The curly brace starting after do must end before while, not after it.
throws FileNotFoundException does not belong where it stands. You may have copied it from the declaration of the findInDataBase method or elsewhere. Just delete it.

Edit:

findInDataBase throws a FileNotFoundException exception when an item is not found in the database. It does not return a boolean. When nothing is found, I want to return the confirmed unique term. How do I complete the while statement?

Use a try-catch construct to see if the exception is thrown. And a boolean variable to control the loop.

public String setNumber(ReportType type) {

    Random rand = new Random();

    String number = String.format("%02d", rand.nextInt(100));

    String prefix = "";

    String fullNumber;
    boolean found;
    do {
        switch (type) {
            case CHILD:
                prefix = "A";
                break;
            case ELDER:
                prefix = "B";
                break;
            case BOOMER:
                prefix = "C";
                break;
        }
        fullNumber = prefix + number;
        try {
            dataBase.findInDataBase(fullNumber);
            found = true;
        } catch (FileNotFoundException fnfe) {
            found = false;
        }
    } while (found);

    return fullNumber;
}

Alternative: The following might start an intense discussion since some find that it’s bad style. It simplifies a few things since it relieves us of having the two uninitialized variables declared before the loop.

public String setNumber(ReportType type) {

    Random rand = new Random();

    String number = String.format("%02d", rand.nextInt(100));

    String prefix = "";

    // This loop will be terminated by a return statement inside of it.
    while (true) {
        switch (type) {
            case CHILD:
                prefix = "A";
                break;
            case ELDER:
                prefix = "B";
                break;
            case BOOMER:
                prefix = "C";
                break;
        }
        String fullNumber = prefix + number;
        try {
            dataBase.findInDataBase(fullNumber);
        } catch (FileNotFoundException fnfe) {
            // Nothing is found; return the confirmed unique term:
            return fullNumber;
        }
    }
}

You are not out of problems yet. You will need to draw a new random number from rand every time through the loop. Otherwise if the first number drawn is already in the database, you are in an infinite loop. If all 100 numbers from 00 through 99 are in use, you will be anyway.

Note: Since Java SE 14^*, you can use the switch expression instead of switch statement.

do {
    prefix = switch (type) {
    case CHILD -> "A";
    case ELDER -> "B";
    case BOOMER -> "C";
    };
    fullNumber = prefix + number;
} while (dataBase.findInDataBase(fullNumber));

^{* The switch expression was introduced with Java SE 12. However, it remained as a Preview feature in Java SE 12 and 13 and finally got standardized with Java SE 14. You can check this Q/A to learn more about it.}

Thanks, `findInDataBase` throws and a `fileNotFoundException` exception, when an item is not found in the database. Not a boolean. When nothing is found, I want to return the confirmed unique term. How do I complete the `while` statement? — Gabriel, Mar 28 '21 at 12:05

How do I use do/while with a case statement?

1 Answers1