Mastering Java Switch Expressions: From Classic Switch to Yield and Pattern Matching

Introduction

During a code review of a colleague's implementation, the author noticed an outdated switch statement and decided to explore how to modernize it.

Classic Switch Example

private static String createPlayer(PlayerTypes playerType) {
    switch (playerType) {
        case TENNIS:
            return "网球运动员费德勒";
        case FOOTBALL:
            return "足球运动员C罗";
        case BASKETBALL:
            return "篮球运动员詹姆斯";
        case UNKNOWN:
            throw new IllegalArgumentException("未知");
        default:
            throw new IllegalArgumentException("运动员类型: " + playerType);
    }
}

The original code works but requires a default clause to satisfy the compiler.

Adding a Variable and Breaks

private static String createPlayer(PlayerTypes playerType) {
    String player = null;
    switch (playerType) {
        case TENNIS:
            player = "网球运动员费德勒";
            break;
        case FOOTBALL:
            player = "足球运动员C罗";
            break;
        case BASKETBALL:
            player = "篮球运动员詹姆斯";
            break;
        case UNKNOWN:
            throw new IllegalArgumentException("未知");
        default:
            throw new IllegalArgumentException("运动员类型: " + playerType);
    }
    return player;
}

By introducing a player variable and break statements, the default clause becomes optional.

Switch Expression (JDK 12+)

private static String createPlayer(PlayerTypes playerType) {
    return switch (playerType) {
        case TENNIS -> "网球运动员费德勒";
        case FOOTBALL -> "足球运动员C罗";
        case BASKETBALL -> "篮球运动员詹姆斯";
        case UNKNOWN -> throw new IllegalArgumentException("未知");
    };
}

The new arrow syntax eliminates the need for break and makes the switch a true expression that returns a value.

Handling New Enum Values

public enum PlayerTypes {
    TENNIS,
    FOOTBALL,
    BASKETBALL,
    PINGPANG,
    UNKNOWN
}

When a new constant PINGPANG is added, the compiler issues a warning that the switch does not cover all enum values. Adding the missing case resolves the warning.

private static String createPlayer(PlayerTypes playerType) {
    return switch (playerType) {
        case TENNIS -> "网球运动员费德勒";
        case FOOTBALL -> "足球运动员C罗";
        case BASKETBALL -> "篮球运动员詹姆斯";
        case PINGPANG -> "乒乓球运动员马龙";
        case UNKNOWN -> throw new IllegalArgumentException("未知");
    };
}

Multiple Labels in One Case

private static String createPlayer(PlayerTypes playerType) {
    return switch (playerType) {
        case BASKETBALL, PINGPANG -> "牛逼运动员沉默王二";
        // other cases omitted for brevity
    };
}

Multiple enum constants can be combined using commas.

Using yield in Switch Expressions

private static String createPlayer(PlayerTypes playerType) {
    return switch (playerType) {
        case TENNIS -> {
            System.out.println("网球");
            yield "网球运动员费德勒";
        }
        case FOOTBALL -> {
            System.out.println("足球");
            yield "足球运动员C罗";
        }
        // other cases omitted
        case UNKNOWN -> throw new IllegalArgumentException("未知");
    };
}

The yield statement produces the value for the enclosing switch expression, similar to return but usable inside a block.

Bytecode Insight

private static String createPlayer(NewSwitchDemo3.PlayerTypes playerType) {
    String var10000;
    switch (playerType) {
        case TENNIS:
            System.out.println("网球");
            var10000 = "网球运动员费德勒";
            break;
        // other cases omitted
        case UNKNOWN:
            throw new IllegalArgumentException("未知");
        default:
            throw new IncompatibleClassChangeError();
    }
    return var10000;
}

In the compiled bytecode, yield is translated into a traditional break with a temporary variable.

Conclusion

While the new switch expression syntax looks fashionable, developers should weigh readability, compatibility with older JDK versions, and team familiarity before adopting it; sometimes the classic switch remains the most pragmatic choice.