Java Stream "Monad" LOL

StreamTest.java

import java.util.List;
import java.util.ArrayList;
import java.util.stream.Stream;

public class StreamTest {
  public static void main(String[] args) {
    List<Integer> list = new ArrayList<Integer>();
    list.add(1);
    list.add(2);
    list.add(3);
    Stream<Integer> stream = list.stream();

    list.add(4);

    // Haha -- this doesn't work.
    // Java can't figure out that Integers are ints
    // System.out.println(stream.sum());

    System.out.println(stream.mapToInt(x -> x).sum());
    // 10!!


    System.out.println(stream.mapToInt(x -> x).sum());
    //Exception in thread "main" java.lang.IllegalStateException: stream has already been operated upon or closed
    //     at java.util.stream.AbstractPipeline.<init>(AbstractPipeline.java:203)
    //     at java.util.stream.IntPipeline.<init>(IntPipeline.java:91)
    //     at java.util.stream.IntPipeline$StatelessOp.<init>(IntPipeline.java:592)
    //     at java.util.stream.ReferencePipeline$4.<init>(ReferencePipeline.java:204)
    //     at java.util.stream.ReferencePipeline.mapToInt(ReferencePipeline.java:203)
    //     at StreamTest.main(StreamTest.java:23)

  }
}