StreamEx reducingWithZero demo

Test.java

import one.util.streamex.MoreCollectors;
import one.util.streamex.StreamEx;

import java.util.stream.Collector;

class Test {
  enum MyLattice {
    /*
          TOP
         /   \
       V1    V2
        \    /
        BOTTOM
     */
    TOP, V1, V2, BOTTOM;

    MyLattice join(MyLattice other) {
      if (this == BOTTOM) return other;
      if (other == BOTTOM) return this;
      if (this == other) return this;
      return TOP;
    }

    static Collector<MyLattice, ?, MyLattice> joining() {
      return MoreCollectors.reducingWithZero(
        TOP, BOTTOM, MyLattice::join);
    }
  }

  public static void main(String[] args) {
    MyLattice result = StreamEx.of("BOTTOM", "V1", "V2", "V1", "V1", "TOP")
      .peek(e -> System.out.println("Processing " + e))
      .map(MyLattice::valueOf)
      .collect(MyLattice.joining());
    System.out.println("Result: " + result);
  }
}