[FIXED] How to check in Servlet if request.getParameterValues has empty data

Issue

My form:

<form action="/parvaz.aero/InsertUserCategories" method="post">
    <input type="text" id="fname" name="category" placeholder="">
    <input type="text" id="fname" name="category" placeholder="">
    <input type="text" id="fname" name="category" placeholder="">
    <input type="text" id="fname" name="category" placeholder="">
    <input type="text" id="fname" name="category" placeholder="">
    <input type="submit">
</form>

My Servlet is getting values either its null, empty or data.

String[] category = request.getParameterValues("category");

Checking in Servlet as:

if (null != category && category.length > 0) {
    try {
        //insert in DB
    } catch (SQLException e) {
        
    }           
} else {
    out.println("There is no data to insert in DB";
}

When write: out.print(category.length); by sending the html form empty its giving result as:

5

Why the servlet assuming empty form length as 5 whereas it's received without any data?

How do I know it's empty or has data?

Solution

It looks like that you expected that a form submit with 5 empty fields of same name would give you a null or empty array. This is indeed not true. This will just send an array of 5 empty strings.

Only the complete absence of input field will give you null.

<form>
    <input type="submit" />
</form>

String input = request.getParameter("input");
System.out.println(input); // null
System.out.println(input == null); // true

String[] inputs = request.getParameterValues("inputs");
System.out.println(inputs); // null
System.out.println(inputs == null); // true

Submit of an empty field will give you an empty string.

<form>
    <input name="input" />
    <input type="submit" />
</form>

String input = request.getParameter("input");
System.out.println(input); // empty string
System.out.println(input == null); // false
System.out.println(input.isEmpty()); // true

String[] inputs = request.getParameterValues("input");
System.out.println(inputs == null); // false
System.out.println(inputs.length); // 1
System.out.println(Arrays.toString(inputs)); // []
System.out.println(inputs[0].isEmpty()); // true

Submit of empty fields will give you an array of empty strings.

<form>
    <input name="input" />
    <input name="input" />
    <input name="input" />
    <input name="input" />
    <input name="input" />
    <input type="submit" />
</form>

String[] inputs = request.getParameterValues("input");
System.out.println(inputs == null); // false
System.out.println(inputs.length); // 5
System.out.println(Arrays.toString(inputs)); // [, , , , ]
System.out.println(inputs[0].isEmpty()); // true
System.out.println(inputs[1].isEmpty()); // true
System.out.println(inputs[2].isEmpty()); // true
System.out.println(inputs[3].isEmpty()); // true
System.out.println(inputs[4].isEmpty()); // true

You basically need to adjust your precondition as follows:

if (inputs != null) {
    for (String input : inputs) {
        if (!input.isEmpty()) {
            // This input is not empty. Process it.
        }
    }
}

If you'd like to check beforehand if all inputs are null or empty, then do so:

if (inputs == null) {
    // Inputs were not included in form.
}
else if (Arrays.stream(inputs).allMatch(String::isEmpty)) {
    // Inputs were included in form, but they are all empty.
}
else {
    // At least one input is included in form and not empty.
}

Answered By - BalusC

Answer Checked By - Marie Seifert (PHPFixing Admin)