A simple pattern matching demo in java

2 min readSep 22, 2024

Pattern matching allows to take a decision based upon a type and to destructure its value. Let’s see it in use:

private void performSend(Sendable sendable) {
    switch (sendable) {
        case Email email -> emailSender.send(email);
        case SMS sms -> smsSender.send(sms);
    }
}

Here is how the above method can be invoked:

.performSend(new SMS("+33612345678", "+33687654321", "Hello Tom from Sam", Instant.now()))

There are several ways (and operators) to use pattern matching in java. I have chosen the switch/case way which is available since java 17.

Here, depending on whether the sendable parameter to the switch selector is of type Email or SMS, the first or second case expression will be executed, resulting in a email or a SMS being sent.

Pattern matching has been available in the scala programming language for a long time but is relatively new in java. Before then, it could only be simulated in java by using the instanceof operator together with a cast, therefore breaking the type-safety of the program.

More information and some theory about pattern matching

In the above example, Email and SMS are algebraic data types (ADT) called product types: they can have any number of instances depending on the values passed to their constructor:

public record Email(String to, String from, String subject, String message, Instant date) implements Sendable {
}

public record SMS(String to, String from, String message, Instant date) implements Sendable {
}

They are typically implemented with records in java.

Sendable is another ADT called the sum type in that it can take a finite number of values. It can be implemented with enums or sealed interfaces:

public sealed interface Sendable permits Email, SMS {
}

You see from the above code that Sendable can have only two types: Email and SMS. This is what allows us to omit the default case expression here:

switch (sendable) {
    case Email email -> emailSender.send(email);
    case SMS sms -> smsSender.send(sms);
    //No default case here!!
}

Conclusion: We have seen the power of pattern matching in java. I have shown how to use it with the switch/case syntax. Please check out the official documentation for other syntaxes.

A simple pattern matching demo in java

More information and some theory about pattern matching

Written by Julien Martin

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