Stages.java

package com.example.stages;

import java.util.Optional;

interface Person<Name, Age> {
    Name name();
    Age age();
}

record Person_Any<Name, Age>(Name name, Age age) implements Person<Name, Age> {}

interface Person_Name<T, R> {
    default <Age> Person<R, Age> apply(Person<T, Age> person) {
        return new Person_Any<>(applyName(person.name()), person.age());
    }

    R applyName(T name);
}

record Natural(Integer value) {}

record PersonAgeStage1() {

    public <Name> Person<Name, Integer> apply(Person<Name, Optional<String>> person) {
        return new Result<>(person.name(), person.age().map(Integer::parseInt).orElse(0));
    }

    record Result<Name>(Name name, Integer age) implements Person<Name, Integer> {}
}

record PersonAgeStage2() {

    public <Name> Person<Name, Natural> apply(Person<Name, Integer> person) {
        return new Result<>(person.name(), new Natural(person.age()));
    }

    record Result<Name>(Name name, Natural age) implements Person<Name, Natural> {}
}

record PersonNameStage1() {

    public <Age> Person<String, Age> apply(Person<Optional<String>, Age> person) {
        return new Result<>(person.name().orElse(""), person.age());
    }

    record Result<Age>(String name, Age age) implements Person<String, Age> {}
}

record PersonNameStage1_Alt() implements Person_Name<Optional<String>, String> {
    @Override
    public String applyName(Optional<String> name) {
        return name.orElse("");
    }
}

record PersonRaw(Optional<String> name, Optional<String> age)
        implements Person<Optional<String>, Optional<String>> {}

record PersonFinal(String name, Natural age) implements Person<String, Natural> {
    static PersonFinal of(Person<String, Natural> person) {
        return new PersonFinal(person.name(), person.age());
    }
}

record PersonProcess(
    PersonAgeStage1 ageStage1,
    PersonAgeStage2 ageStage2,
    PersonNameStage1 nameStage1
) {
    public PersonFinal apply(PersonRaw person) {
        return PersonFinal.of(ageStage2.apply(ageStage1.apply(nameStage1.apply(person))));
    }
}

public class Stages {
}