Is the accumulator of reduce in Java 8 allowed to modify its arguments?

Question

In Java 8, Stream has a method reduce:

T reduce(T identity, BinaryOperator accumulator);

Is the accumulator operator allowed to mo

Answer 1

This is allowed syntactically, but I think it runs against the design pattern and is a bad idea.

  static void accumulatorTest() {
     ArrayList points = new ArrayList<>();
     points.add(new Point(5, 6));
     points.add(new Point(0, 6));
     points.add(new Point(1, 9));
     points.add(new Point(4, 16));
     BinaryOperator sumPoints = new BinaryOperator() {
        public Point apply(Point p1, Point p2) {
           p2.x += p1.x;
           p2.y += p1.y;
           return new Point(p2); //return p2 and the list is transformed into running total
        }
     };
     Point sum = points.stream().reduce(new Point(0, 0), sumPoints); 
     System.out.println(sum);
     System.out.println(points);
  }

The answer is correct; we get the sum of all of the x and y coordinates. The original list is modified, confirmed by the output:

java.awt.Point[x=10,y=37] [java.awt.Point[x=5,y=6], java.awt.Point[x=5,y=12], java.awt.Point[x=6,y=21], java.awt.Point[x=10,y=37]]