Java 21 Certification Guide - Part 2

Switch for pattern matching and Enum

Welcome to the second article in my series on preparing for the Java 21 certification.
Over the next several posts, we’ll walk through the most important language changes, APIs, and tricky details you’ll need to know for the exam.

In this article, we’ll focus on switch expressions, utilizing pattern matching for switches and enums.

To make things practical, we’ll analyze a question in the style of the certification exam.

❓ Question

What is the answer to this code?

enum Card {
    CLUB, DIAMOND, HEART, SPADE;
}

public class Question3 {
    public static void main(String[] args) {
        Card card = Card.CLUB;
        printCardColor(card);
    }

    private static void printCardColor(Card card) {
        String color = null;
        if (card == Card.HEART || card == Card.DIAMOND) {
            color = "red";
        }
        if (card == Card.SPADE || card == Card.CLUB) {
            color = "black";
        }
        System.out.println(color);
    }
}

Which option can replace the printCardColor method? Choose the right answer.

A)

    private static void printCardColor(Card card) {
        String color = null;
        if (card != null) {
            color = switch (card.ordinal()) {
                case 0, 1 -> "red";
                case 2, 3 -> "black";
            };
        }
        System.out.println(color);
    }

B)

private static void printCardColor(Card card) {
    String color = switch (Card.valueOf(card.toString())) {
        case DIAMOND, HEART -> "red";
        case SPADE, CLUB -> "black";
    };
    System.out.println(color);
}

C)

private static void printCardColor(Card card) {
    String color = switch (card) {
        case HEART, DIAMOND -> "red";
        case SPADE, CLUB -> "black";
        default -> null;
    };
    System.out.println(color);
}

D)

private static void printCardColor(Card card) {
    String color = switch (card) {
        case null -> null;
        case HEART, DIAMOND -> "red";
        case SPADE, CLUB -> "black";
    };
    System.out.println(color);
}

🔎 Explanation

A) Using ordinal

• Problem: This code does not compile because the switch on int (from card.ordinal()) is not exhaustive — there’s no default branch.

• Fix: Add a default branch to make it compile, but it’s still a bad practice for enums.

B) Using valueOf()

• Problem: If card is null, card.toString() throws a NullPointerException.

C) Using switch with default

• Problem: The switch is exhaustive for all Card constants, so default is technically unnecessary. However, because card might be null, a NullPointerException is thrown before entering the switch, making this option incorrect for robust code.

D) Using switch with null case

• Correct Solution. In this case, we have a switch null‑safe, and the same result as the previous code.

✅ Correct Answer

D

Did you get it right? Don’t worry if you missed some; this is a hard question. The goal here is to expose you to several tricky details that often show up in the Java 21 certification exam.

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⚡ Next in this series: we’ll dive into Stream API.

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