Transition to Option

Option.java

import java.util.function.Function;

public abstract class Option<T> {
    private static class Some<T> extends Option<T> {
        private T val;

        Some(final T val) {
            this.val = val;
        }

        @Override
        public String toString() {
            return "Some<" +
                    this.val
                            .getClass()
                            .toString()
                            .split(" ")[1] +
                    ">(" + this.val + ")";
        }

        @Override
        public <R> Option<R> ifNotNull(Function<T, Option<R>> f) {
            return f.apply(val);
        }
    }

    private static class None<T> extends Option<T> {
        None() {}

        @Override
        public <R> Option<R> ifNotNull(Function<T, Option<R>> f) {
            return new None<>();
        }

        @Override
        public String toString() {
            return "Nothing()";
        }
    }

    private Option() { }

    public static <T> Option<T> from(final T t) {
        if (t == null) return None();
        else return Some(t);
    }

    public static <T> Option<T> Some(final T t) {
        return new Some<T>(t);
    }

    public static <T> Option<T> None() {
        return new None<T>();
    }

    public abstract  <R> Option<R> ifNotNull(final Function<T, Option<R>> f);
}

simpleOption1.java

public class Main {
    static Integer intVar(String input) {
        if (input == null) return null;
        else {
            try {
                return Integer.parseInt(input);
            } catch (NumberFormatException ex) {
                return null;
            }
        }
    }

    static Integer checkValid(Integer i) {
        if (i == null) return null;
        else {
            if ((i > 0) && (i < 10)) return i;
            else return null;
        }
    }

    public static void main(String[] args) {
        String myIntVar = System.getenv("MYINTVAR");
        Integer iVar = intVar(myIntVar);
        Integer validVar = checkValid(iVar);

        System.out.println(validVar);
    }
}

simpleOption2.java

public class Main {
    static Integer intVar(String input) {
        try {
            return Integer.parseInt(input);
        } catch (NumberFormatException ex) {
            return null;
        }
    }

    static Integer checkValid(Integer i) {
        if ((i > 0) && (i < 10)) return i;
        else return null;
    }

    public static void main(String[] args) {
        String myIntVar = System.getenv("MYINTVAR");

        if (myIntVar != null) {
            Integer iVar = intVar(myIntVar);

            if (iVar != null) {
                Integer validVar = checkValid(iVar);

                System.out.println(validVar);
            }
        }
    }
}

simpleOption3.java

public class Main {
    static Option<Integer> intVar(String input) {
        try {
            return Option.Some(Integer.parseInt(input));
        } catch (NumberFormatException ex) {
            return Option.None();
        }
    }

    static Option<Integer> checkValid(Integer i) {
        if ((i > 0) && (i < 10)) return Option.Some(i);
        else return Option.None();
    }

    public static void main(String[] args) {
        Option<String> myIntVar = Option.from(System.getenv("MYINTVAR"));
        Option<Integer> iVar = myIntVar.ifNotNull(Main::intVar);
        Option<Integer> validVar = iVar.ifNotNull(Main::checkValid);

        System.out.println(validVar);
    }
}

simpleOption4.java

public class Main {
    static Option<Integer> intVar(String input) {
        try {
            return Option.Some(Integer.parseInt(input));
        } catch (NumberFormatException ex) {
            return Option.None();
        }
    }

    static Option<Integer> checkValid(Integer i) {
        if ((i > 0) && (i < 10)) return Option.Some(i);
        else return Option.None();
    }

    public static void main(String[] args) {
        Option<Integer> validVar =
            Option.from(System.getenv("MYINTVAR"))
                .ifNotNull(Main::intVar)
                .ifNotNull(Main::checkValid);

        System.out.println(validVar);
    }
}