How-to validate user input when using scanner for input gathering

Question

Description

What I am aiming to do is request user to provide an input only to those entries which triggered an exception excluding those input requests which have been satisfactory

Example

Note: input is an instance of a Scanner

public Fluid addFluid() {
    do{
        try{
            System.out.println("Please enter fluids ID: "); 
            f.setFluidID(input.next());
            System.out.println("Please enter fluids molecular weight: ");   
            f.setMolecularWeight(input.nextDouble()); 
            System.out.println("Please enter fluids temperature: ");            
            f.setTemperature(input.nextDouble());
            error = false;
        }
        catch(InputMismatchException e){
            System.out.println("Error! Please provide the right input.");
           // if exception happens on input for setMolecularWeight(input.nextDouble()); skip print statement for fluids ID and f.setFluidID(input.next());
        }
    }
    while(error != false);
    getFluid().add(f);
    System.out.println(getFluid());
    return f;
}

Output

Please enter fluids ID: 
propane
Please enter fluids molecular weight: 
a
Error! Please provide the right input.
Please enter fluids ID:                // how to exclude this print statement
Please enter fluids molecular weight:

Would there be a way how to do this using an if / else structure. I would want to avoid doing try / catch for every input operator. Many thanks in advance.

Community · Accepted Answer · 2020-06-20 09:12:55Z

Short answer:

You'll need clear the buffer on the Scanner after the call to nextDouble(), since your InputStream will contain a newline \n.

Adding input.next(); or input.nextLine(); to the end of your catch blocks should work to skip that extra \n.

(See java.util.Scanner : why my nextDouble() does not prompt?)

Explanation:

Even though nextDouble blocks execution until the user enters a newline \n, it only returns characters from Scanner's buffer that are valid characters (characters that will parse correctly to a double). This means
if you enter input that doesn't parse to a double, those invalid characters are still sitting in the buffer. When you attempt to read the nextDouble again, it will just repeat the same process on the existing input, resulting in the infinite loop.
Therefore, you'll need to "clear" the buffer before continuing. Since there doesn't seem to be a method for this, you'll just need to advance the Scanner to the end of the buffer. In this case, input.next(); will suffice.

(Credits to user jonhopkins for the code I shamelessly grabbed from his answer)

System.out.println("Please enter fluids ID: ");
while (true) {
    try {
        f.setFluidID(input.next());
        break; // break out of the current loop
    } catch (InputMismatchException e) {
        System.out.println("Error! Please provide the right input.");
        input.next();
    }
}

System.out.println("Please enter fluids molecular weight: ");
while (true) {
    try {
        f.setMolecularWeight(input.nextDouble());
        break; // break out of the current loop
    } catch (InputMismatchException e) {
        System.out.println("Error! Please provide the right input.");
        input.next();
    }
}

System.out.println("Please enter fluids temperature: ");
while (true) {
    try {
        f.setTemperature(input.nextDouble());
        break; // break out of the current loop
    } catch (InputMismatchException e) {
        System.out.println("Error! Please provide the right input.");
        input.next();
    }
}

NOTE: You may even need an extra input.next() call before you read the 2nd and 3rd inputs (I haven't tested this).

Very nice answer. I never knew that was an issue, hence the confusion about my answer below.

Jeroen Vannevel · Accepted Answer · 2013-11-22 13:35:32Z

2

Simply place it outside your loop:

public Fluid addFluid() {
    System.out.println("Please enter fluids ID: "); 
    f.setFluidID(input.next());

    do{
        try{    
            System.out.println("Please enter fluids molecular weight: ");   
            f.setMolecularWeight(input.nextDouble()); 
            System.out.println("Please enter fluids temperature: ");            
            f.setTemperature(input.nextDouble());
            error = false;
        }
        catch(InputMismatchException e){
            System.out.println("Error! Please provide the right input.");
           // if exception happens on input for setMolecularWeight(input.nextDouble()); skip print statement for fluids ID and f.setFluidID(input.next());
        }
    }
    while(error != false);
    getFluid().add(f);
    System.out.println(getFluid());
    return f;
}

Now it will only print it once while the other input can be repeated. If you want to check for individual problems with the molecular weight and fluids temperature, you'll have to repeat the try-catch.

edited Nov 22, 2013 at 13:35

answered Nov 22, 2013 at 13:29

Jeroen Vannevel

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3 Comments

e.doroskevic Over a year ago

Thank you for the answer, however that wouldn't be sufficient enough. The reason for that is - I want the program to check for invalid input for all input fields. That applies to when setting fluidID as well.

Jeroen Vannevel Over a year ago

@E.Doroskevic: in that case you will need multiple while loops.

e.doroskevic Over a year ago

Agree, it looks like there is no better way.

bary · Accepted Answer · 2013-11-22 13:35:55Z

1

Not pretty but should work:

public Fluid addFluid() {
    boolean alreadyAnswered1 = false;
    boolean alreadyAnswered2 = false;
    boolean alreadyAnswered3 = false;
    do{
        try{
            if (!alreadyAnswered1) {
                System.out.println("Please enter fluids ID: ");
                 f.setFluidID(input.next());
                 alreadyAnswered1 = true;
            }
            if (!alreadyAnswered2) {
                System.out.println("Please enter fluids molecular weight: ");
                f.setMolecularWeight(input.nextDouble());
                alreadyAnswered2 = true;
            }
            if (!alreadyAnswered3) {
                System.out.println("Please enter fluids temperature: ");
                f.setTemperature(input.nextDouble());
                alreadyAnswered3 = true;
            }
            error = false;
        }
        catch(InputMismatchException e){
            System.out.println("Error! Please provide the right input.");
           // if exception happens on input for setMolecularWeight(input.nextDouble()); skip print statement for fluids ID and f.setFluidID(input.next());
        }
    }
    while(error != false);
    getFluid().add(f);
    System.out.println(getFluid());
    return f;
}

answered Nov 22, 2013 at 13:35

bary

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2 Comments

Linus Over a year ago

Because it will print out the required inputs to the console.

e.doroskevic Over a year ago

@bary thank you for your answer. It works this way as well, you are correct.

jonhopkins · Accepted Answer · 2013-11-22 13:43:51Z

1

It looks to me like you want to repeat the request for input for whichever ones where an exception is thrown. I would just use a few smaller loops, each with their own separate try-catch blocks.

System.out.println("Please enter fluids ID: ");
while (true) {
    try {
        f.setFluidID(input.next());
        break; // break out of the current loop
    } catch (InputMismatchException e) {
        System.out.println("Error! Please provide the right input.");
    }
}

System.out.println("Please enter fluids molecular weight: ");
while (true) {
    try {
        f.setMolecularWeight(input.nextDouble());
        break; // break out of the current loop
    } catch (InputMismatchException e) {
        System.out.println("Error! Please provide the right input.");
    }
}

System.out.println("Please enter fluids temperature: ");
while (true) {
    try {
        f.setTemperature(input.nextDouble());
        break; // break out of the current loop
    } catch (InputMismatchException e) {
        System.out.println("Error! Please provide the right input.");
    }
}

This will cause the program to loop for each input until no exception is thrown for that input. It also removes the need for an outer loop that keeps going until error == false. If you want the input prompts to be displayed again after the error message, simply put the prompts inside their respective while loops.

edited Nov 22, 2013 at 13:43

answered Nov 22, 2013 at 13:38

jonhopkins

3,8523 gold badges30 silver badges39 bronze badges

6 Comments

e.doroskevic Over a year ago

This was the way I had in mind, although I was hoping I could avoid doing try/catch for individual fields. Thank you for your answer, if nothing else will come up I will choose it as the answer for this question.

jonhopkins Over a year ago

I can understand wanting to avoid it. I'm not entirely sure if it is possible though, aside from something like @bary's answer, so I'd be interested to see if a better answer comes up too.

e.doroskevic Over a year ago

I've looked up on-line for an answer, but wasn't successful with regards to my question. Jon the code sample you've provided ends up in an infinite loop.

jonhopkins Over a year ago

An infinite loop? Is the break leaving the try instead of the while? If that's the case, I would just use a flag that is set after the input to get out of the while loop.

jonhopkins Over a year ago

I moved this code out of the original do { } while(error != false); by the way. I don't know if you left it in there, which could be a cause of the problem.

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