I am assuming that if the two strings have the same length, any of them will be OK as the longer.

I am finding the length by counting the code points in the string. In most cases this will count the characters, unlike using String.length(). More discussion of defining the length of a string below.

Use Collections.max() and a Comparator

As Anonymous said in a comment:

private static final Comparator<String> compareByLengthInCodePoints
        = Comparator.comparingInt(s -> s.codePointCount(0, s.length()));

public static void printLonger(String s1, String s2) {
    String longer = Collections.max(List.of(s1, s2), compareByLengthInCodePoints);
    System.out.println(longer + " length " + longer.codePointCount(0, longer.length()));
}

Try it out:

    String threeLetters = "abc";
    String twoSmileys = "\uD83D\uDE42\uD83D\uDE20";
    printLonger(threeLetters, twoSmileys);

Output:

abc length 3

Or use a loop

As another solution we can use a while or for loop to simulate an if statement. This auxiliary method does that:

private static String getLongerString(String s1, String s2) {
    while (s1.codePointCount(0, s1.length()) > s2.codePointCount(0, s2.length())) {
        return s1;
    }
    return s2;
}

Just call it from the printLonger() method from before. My IDE warns me: 'while' statement does not loop. Which is exactly point.

What is the length of a string?

The Java char type is virtually deprecated since is does can no longer represent a Unicode character. By extension also the String.length() method no longer gives a meaningful value since it gives the number of chars in the string.

A Java char is 16 bits, so there are 65 000 possible char values. Last time I checked, there were 292 000 different Unicode characters, and the number is still growing. So nearly 3 times as many. Therefore some characters use two Java chars, for example smileys.

A character is therefore represented by a 32 bits code point, or by two consecutive chars in a string.

Counting code points still isn’t perfect. Some code points represent non-printing characters, for example change of reading direction or accents or other diacritical marks.

Yet other thinkable definitions of the length of a string would be the width in pixels when rendering the string using some specific font.