Functional Programming with Java

Comparison of imperative to functional / declarative programming in Java

Why do i need functional programming in Java

• Immutable State: The state of an object doesn't change and hence need not be protected or synchronized. In functional programming, change in state occurs via series of transformations which keeps the object immutable and yet achieves change in state.
• No side-effects: The execution of a function has no side-effects, a function code will always return same result for same argument when called multiple times. This makes it easy to understand and predict behaviour of program.

A nice side-effect of functional programming in Java is, that it shortens your source code e.g.:

// Imperative Way
List<Person> females_imperative = new ArrayList<>();
for (Person person: people) {
    if(Gender.FEMALE.equals(person.getGender())){
        females_imperative.add(person);
    }
}
// Functional Way
Predicate<Person> personPredicate = person -> Gender.FEMALE.equals(person.getGender());
List<Person> females_declarative = people.stream().filter(personPredicate).collect(Collectors.toList());

Installation

First clone the repository and cd into it:

git clone https://github.com/christopherfriedrich/javafunctional.git
cd javafunctional/

Then install the Maven dependencies using

mvn install

Running the tests

The tests mainly compare the imperative to the functional approach. They can be run via

mvn test