How to set an empty or not empty entry for a switch case statement?

As commented, what you need in Java is a series of if tests.

In the String class, you can test either:

• The string has no characters at all (isEmpty)
• The string has no characters OR has only whitespace characters (isBlank)

if ( str.isEmpty() ) { System.out.println( "ERROR - String has no characters." ); }
else if ( str.isBlank() ) { System.out.println( "ERROR - String has only whitespace." ); }
else if ( str.equals( "CoDe" ) ) { System.out.println( "Code received." ); }
else { System.out.println( "ERROR - Incorrect entry." ); }

See that code run live at Ideone.com.

Before that code block, I would add a null-check.

Objects.requireNonNull( str ); // Throw exception if null.

I find the if - else if - else construct in Java to be awkward in terms of readability. The switch syntax is much better at making obvious that a series of possibilities is being tested, mutually-exclusive, with a single result.

I do wish Java offered a cascading test for a series of boolean expressions. I have used such a feature in another language. There I found the cascading-boolean-tests to be quite practical for handling a series of business rules. But, alas, no such feature in Java. If only Brian Goetz were taking my calls.

I do not know Groovy. Perhaps Groovy provides another facility.

I tested it, and I think I have the answer for you.

    switch(str) {
        case "": // empty
            println("Entered an empty value")
            break
        case "CoDe": // str == CoDe
            println("Entered special code "+str)
            break
        default: // String not empty and not "CoDe"
            println("Not empty")
            break
    }

It works because you have case "" which is the empty string. Meaning that everithing else is not empty. everything else is the default:

If you are not convinced, I'll give you a different example with the same meaning.

    if (str.isEmpty()) {
       // do something
    } else if (!str.isEmpty()) { // <- this if is redundant
       // do something else
    }

You don't need the second if, because, you enter the else branch only if str is not empty! I appleid the same logic to the case. And it works because case "": in Java is valid.

Your code has a smell, as str != '' would block all further cases.

I would re-organize it and use straight-forward basic Groovy switch-statement:

String str = 'value'
switch(str){
    case 'CoDe':
        println "Entered special code $str"
        break
    case '':
        println 'Entered an empty value'
        break
    case { str != '' && str != null }:
        println "Entered value $str"
        break
    default:
        println 'Incorrect entry'
}

prints

Entered value value