Java adaptation of some great concepts on which Rust was built.

rs4j

Rust encourages developers to handle errors and non-values properly by design.
This leads to fewer non-logic bugs in production and the code might look cleaner as well.

The rs4j library is written in pure Java and has just adapted some concepts, which are also used in Rust. The method names are kept similar (snake_case to camelCase) for developers using both languages.
There are some additional wrapper methods, such as andThenContinue() in both Result and Option to get rid of return statements. A new instance will then be returned automatically.

Installation

Gradle

repositories {
    maven { url 'https://registry.provided.space' }
}

dependencies {
    implementation 'space.provided:rs4j:VERSION'
}

Maven

<repositories>
    <repository>
        <id>provided</id>
        <name>provided.space</name>
        <url>https://registry.provided.space</url>
    </repository>
</repositories>

<dependencies>
    <dependency>
        <groupId>space.provided</groupId>
        <artifactId>rs4j</artifactId>
        <version>VERSION</version>
    </dependency>
</dependencies>

Exceptionless

Java has two types of Exceptions; checked and unchecked. Unchecked Exceptions don't have to be declared to be thrown in a method, which increases the likelihood of them not being handled.
This is where Results come in handy with their representation of Ok and Error. Calling unwrap on Error or unwrapError on Ok will lead to a ValueAccessError.

final Result<User, ?> userResult = userRpc.login(email, password);
if (userResult.isOk()) {
    final User user = userResult.unwrap();
    messages.queue("Hello %1$s.".formatted(user.getUsername()));
}

Null safety

It's not always obvious if a method always returns an object or if it could be null as well.
Instead of null, an Option can be used, which represents one of two states (Some and None). Calling unwrap on None will lead to a ValueAccessError.

final Option<Date> terminationDateOption = user.getTerminationDate();
if (terminationDateOption.isSome()) {
    final Date terminationDate = terminationDateOption.unwrap();
    messages.queue("Your login will be deactivated after %1$s.".formatted(dateFormatter.format(terminationDate)));
}

Monads for Result and Option

Monads can improve the codes aesthetics by getting rid of some if statements, variable declarations and unwrap calls.

userRpc.login(email, password).andThenContinue(user -> {
    final String username = user.getUsername();

    user.getTerminationDate()
        .andThenContinue(terminationDate -> {
            final String formattedDate = dateFormatter.format(terminationDate);
            messages.queue("Hello %1$s. Your login will be deactivated after %2$s.".formatted(username, formattedDate));
        })
        .orElseContinue(() -> {
            messages.queue("Hello %1$s.".formatted(username));
        });
});