Introduction Link to heading

Java has released an exciting new language feature in preview mode called switch expressions (JEP-354). These are a variation of switch statements which can be evaluated to a value. This idea is similar to the ternary operator which allows in line if-else statements to return a value.

The code you see here can be ran with JDK 13 and the --enable-preview flag but, be warned! A preview feature is “fully specified, fully implemented, and yet impermanent” (JEP-12). However, there is a proposal to make switch statements permanent in JDK 14.

Statements vs Expressions Link to heading

When we talk about statements in source code, we are talking about a single line of code that completes some action. Here we see three distinct statements.

int a = 4*(7+3);
boolean contact = eligible && available;
String name = user.getName();

An expression is typically a fragment and will evaluate to some value. From above, we have the algebraic expression 4*(7+3) which evaluates to 40. The second statement has the boolean expression eligible && available which evaluates to either true or false. Finally, the third statement has the expression user.getName() which will evaluate to a String reference.

The Switch Expression Link to heading

Switch expressions borrow from the syntax of traditional switch statements but allow the switch to be evaluated to a value. Because they are expressions, they can be assigned to variables. For contrast, take a look at how we might assign a value using a traditional switch statement.

int errorCode = getError();
String error = null;

switch(errorCode){
    case 404:
        error = "Not Found";
        break;
    case 400:
        error = "Bad Request";
        break;
    case 502:
    case 500:
        error = "Server Error";
        break;
    default:
        error = "Unknown Error";
}

There are drawbacks with this traditional switch statement.

  1. The code is error-prone. It is easy to miss a break statement by mistake allowing execution to fall through to the next case block. Syntax that lends itself to bugs is syntax we want to avoid.

  2. It doesn’t feel natural. Perhaps this is subjective by case and developer, but we often like to assign values at the variable declaration when possible. If a switch statement is needed, we are forced to separate assignment from declaration by limitations of syntax.

  3. Verbosity abounds. Verbosity that takes away from readability is typically a bad thing. There are cleaner syntax alternatives for expressing this same concept.

How do switch expressions solve these problems? Let’s rewrite the same code using Java’s new switch expression then talk about it.

int errorCode = getError();

String error = switch(errorCode){
    case 404 -> "Not Found";
    case 400 -> "Bad Request";
    case 502, 500 -> "Server Error";
    default -> "Unknown Error";
};

Here, the switch expression becomes a part of the the variable declaration. The expression will be evaluated and stored in the local variable error. Notice how we borrow the arrow from lambda syntax for mapping cases to values. There are a few improvements to consider.

  1. We have removed the need for break statements! There is no way to accidentally fall through because every case yields a value and then exits the switch.
  2. The declaration of the variable and its assignment are coupled into one statement. It is guaranteed by the compiler that the switch will yield a value (or throw an exception).
  3. The code is much more concise. There’s no need to explicitly break. Also, multiple case labels can be combined in an intuitive way.

Multi-line Case Bodies Link to heading

Sometimes the body of a case might require more than a single expression. How can we write switch expressions with more complex case bodies? We will again borrow from lambda syntax and add braces after the arrow. Since this puts us in a position to write multiple statements instead of a single expression, we will need a way to signal the return value. Since return is reserved for exiting the method, we introduce a new keyword, yield. Let’s look at the code.

int errorCode = getError();

String error = switch(errorCode){
    case 404 -> "Not Found";
    case 400 -> "Bad Request";
    case 502, 500 -> "Server Error";
    default -> {
        logger.warning("Unknown error code " + errorCode);
        yield "Unknown error";
    }
};

In the default case, we were able to open braces after the arrow and enter a block of statements. When we were ready to return the value for that case, we used the yield keyword which will terminate the switch expression and assign the value back to variable error. It might be worth noting that while we use a lambda-ish syntax, no lambda functions are being created.

Allowing Fall Through Link to heading

Switch expressions do allow for fall through if you need it. However, I recommend avoiding this syntax as much as possible. When using the arrow notation, every case will yield a value and break. However, if we use a colon, like traditional switch statements, we can introduce fall through.

String error = switch(errorCode){
    case 404:
        yield "Not Found";
    case 400:
        yield "Bad Request";
    case 502:
        logger.error("This will fall through to case 500's body");
    case 500:
        yield "Server Error";
    default:
        logger.warning("Unknown error code " + errorCode);
        yield "Unknown error";
};

If errorCode was 502, Java will first log the error message and then fall through to the body of the 500 case. You can use either the colon syntax or arrows but you cannot mix the two case kinds. If you do, the compiler will print this error.

error: different case kinds used in the switch

Conclusion Link to heading

Switch expressions are a great new language feature to express something more concisely than we could before. It was released in preview mode in JDK 13. There is a (proposal) to release switch expressions in JDK 14. Until then, happy coding.

- Hunter Anderson