Collect.java

import net.hamnaberg.funclite.*;

public class Collect {

  public static <A, B> Option<B> collect(A value, Predicate<A> p, Function<A, B> f) {
    if (p.apply(value)) {
      return Option.fromNullable(f.apply(value));
    }
    return Option.none();
  }


  public static void main(String[] args) {

    String hello = "hello";

    collect(hello, String::isEmpty, Functions::identity)
  }
}

Collect2.java

import net.hamnaberg.funclite.*;

public class Collect2 {
  public static class PartialFunction<A, B> implements Function<A, B> {
    private final Predicate<A> p;
    private final Function<A, B> f;

    public PartialFunction(Predicate<A> p, Function<A, B> f) {
      this.p = p;
      this.f = f;
    }

    public boolean isDefinedAt(A value) {
      return p.apply(value);
    }

    public B apply(A value) {
      return f.apply(value);
    }
  }

  public static <A, B> Option<B> collect(A value, PartialFunction<A, B> pf) {
    if (pf.isDefinedAt(value)) {
      return Option.fromNullable(pf.apply(value));
    }
    return Option.none();
  }


  public static void main(String[] args) {

    String hello = "hello";

    collect(hello, new PartialFunction(String::isEmpty, Functions::identity))
  }
}

Collect3.java

import net.hamnaberg.funclite.*;

public class Collect3 {
  public static interface PartialFunction<A, B> extends Function<A, B> {
    boolean isDefinedAt(A value);
    B apply(A value);
  }

  public static class MapPartialFunction<A,B> implements PartialFunction<A,B> {
    private Map<A, B> map;
    public MapPartialFunction(Map<A, B> m) {
      this.map = m;
    }

    public boolean isDefinedAt(A value) {
      return map.containsKey(value);
    }

    public B apply(A value) {
      return map.get(value);
    }
  }

  public static class PartialFunctionImpl<A, B> implements PartialFunction<A, B> {
    private final Predicate<A> p;
    private final Function<A, B> f;

    public PartialFunctionImpl(Predicate<A> p, Function<A, B> f) {
      this.p = p;
      this.f = f;
    }

    public boolean isDefinedAt(A value) {
      return p.apply(value);
    }

    public B apply(A value) {
      return f.apply(value);
    }
  }

  public static <A, B> Option<B> collect(A value, PartialFunction<A, B> pf) {
    if (pf.isDefinedAt(value)) {
      return Option.fromNullable(pf.apply(value));
    }
    return Option.none();
  }


  public static void main(String[] args) {

    String hello = "hello";

    collect(hello, new PartialFunction(String::isEmpty, Functions::identity))
  }
}