Java-Result

A small Java library providing a Result type, inspired by Rust's Result<T, E>.

Usage Example

import com.github.bsolar17.Result;

Result<Integer, String> divide(int a, int b) {
    if (b == 0) {
        return Result.err("division by zero");
    } else {
        return Result.ok(a / b);
    }
}

Functional style:

Result<Integer, String> result = divide(10, 2);
String message = result.match(
    value -> "Success: " + value,
    error -> "Failure: " + error
);
System.out.println(message);

Classic if/else:

if (result.isOk()) {
    System.out.println("Result: " + result.value());
} else {
    System.out.println("Error: " + result.error());
}

Map the value if present, leaving error unchanged:

Result<String, String> mapped = result.map(Object::toString);

Map the error if present, leaveing value unchanged:

Result<Integer, Integer> mappedErr = result.mapErr(String::length);

Chaining map and mapErr:

Result<String, Integer> result = Result.ok(10)
.map(Object::toString)
.mapErr(i -> i + 1);

Exceptions

Calling error() on an Ok throws NoSuchElementException:

Result<Integer, String> ok = Result.ok(42);
ok.error(); // Throws NoSuchElementException

Calling value() on an Err throws NoSuchElementException:

Result<Integer, String> err = Result.err("fail");
err.value(); // Throws NoSuchElementException

Using valueOrElseThrow to throw a custom exception:

Result<Integer, String> err = Result.err("fail");
Integer value = err.valueOrElseThrow(error -> new IllegalArgumentException(error));
// Throws IllegalArgumentException with message "fail"