StreamCollectFailure.java

import java.util.stream.IntStream;

public class StreamCollectFailure {
  public static void main(String[] args) {
    int[] points = { 80, 95, 70, 60 };
    
    double ave = IntStream.of(points).average().orElse(0.0);
    double var = IntStream.of(points).collect(
      () -> new Double(0.0),
      (r,n) -> { r += Math.pow(n - ave, 2); },
      (r,s) -> { r += s; });
    // This implementation doesn't work (as we'd expect),
    // because Double is immutable and of autoboxing/unboxing spec.
    System.out.println("ave="+ave);
    System.out.println("var="+var);
  }
}