functional-java

A library to ease functional programming in Java.

functional-java has borrowed alot of the interfaces and ideas from how Rust does it. More importantly it adds mitigations to contain some of Java's shortcomings like breaking control flow with exceptions etc, and project it down to a more functional style.

Designed to also work with Android projects.

Motivation

This is an example of how to fend off parts of Javas ordinary control-flow (returning null/throwing IOException) by instead using Result. By using Result you are consistent with "error"-handling and have more explicit control.

// LoadJson.java
import static functional.ThrowsAsResult.throwsAsResult;

JSONObject standardLoadJson(String filename) throws IOException {
    // The standard way of loading a json file.
}

public class LoadJson {
    public static Result<JSONObject, IOException> loadJson(String filename) {
        return throwsAsResult(
            () -> standardLoadJson("config.json"),
            IOException.class
        ).get()
    }
}


// main.java
import static functional.Result.okOrNullException;
import static LoadJson.loadJson;

public Result<String, Exception> getConfigured(String key) {
    return ok("config.json", Exception.class)
            .andThen(::loadJson)
            .andThen(config -> okOrNullException(config.getString(key)));
    }
}

public int main() {
    int width = getConfigured("width").unwrapOr(80);

    System.out.println("width = " + width);
}

If you wrap the parts in Java that missbehaves you can start writing readable java with minimal clutter.

This example reads the configurated width or defaults to 80.

Requirements

functional-java relies on lambdas and therefore requires java 8 or backporting with retrolambda or similar.

Installation

Gradle

Add it in your root build.gradle at the end of repositories:

allprojects {
    repositories {
        ...
        maven { url 'https://jitpack.io' }
    }
}

Add the dependency

dependencies {
        compile 'com.github.distil:functional-java:1.0'
}