public static double infiniteSum(DoubleStream ds) { ... }

double sum = infiniteSum(IntStream.iterate(1, (i -> i + 1))
                                  .mapToDouble(n -> 1 / ((double)n * n)));

public static void yeOldeWaye() {
    double sum = 0;
    for (double n = 1; ; n++) {
        double term = 1 / (n * n);
        if (Math.abs(term) <= 1e-12 * Math.abs(sum)) {
            break;
        }
        sum += term;
    }
    System.out.println(sum);
}

public static double infiniteSum1(DoubleStream ds) {
    double sum = 0;
    PrimitiveIterator.OfDouble it = ds.iterator();
    while (true) {
        double term = it.next();
        if (Math.abs(term) <= 1e-12 * Math.abs(sum)) {
            break;
        }
        sum += term;
    }
    return sum;
}

private static class DoubleAccumulator {
    public double sum;
    public DoubleAccumulator() {
        sum = 0;
    }
}

public static double infiniteSum(DoubleStream ds) {
    DoubleAccumulator summer = ds.limit(800000).collect
        (DoubleAccumulator::new,
         (s, d) -> s.sum += d,
         (s1, s2) -> s1.sum += s2.sum);
    return summer.sum;
}

public static double infiniteSum(DoubleStream ds) {
    DoubleAccumulator summer = ds.collect
        (DoubleAccumulator::new,
         (s, d) -> { if (Math.abs(d) <= 1e-12 * Math.abs(s.sum)) {
                        ds.close();  // AAACK
                     } else
                        s.sum += d;
                   },
         (s1, s2) -> s1.sum += s2.sum);
    return summer.sum;
}

class MutableDouble {
    double sum;
}
MutableDouble sumHolder=new MutableDouble();
DoubleStream ds=IntStream.iterate(1, (i -> i + 1))
                         .mapToDouble(n -> 1 / ((double)n * n));
ds.peek(term -> sumHolder.sum+=term)
  .filter(term -> Math.abs(term)<1e-12*Math.abs(sumHolder.sum))
  .findFirst();
System.out.println(sumHolder.sum);

double sum = 0;
for (double n = 1; ; n++) {
    double term = 1 / (n * n);
    sum += term;
    if (Math.abs(term) <= 1e-12 * Math.abs(sum)) {
        break;
    }
}

class MutableDouble {
    double sum, next;
    void add(double d) {
        sum=next;
        next+=d;
    }
}
MutableDouble sumHolder=new MutableDouble();
DoubleStream ds=IntStream.iterate(1, (i -> i + 1))
                         .mapToDouble(n -> 1 / ((double)n * n));
ds.peek(sumHolder::add)
  .filter(term -> Math.abs(term)<1e-12*Math.abs(sumHolder.sum))
  .findFirst();

public static void yeStreamsWaye() {
    System.out.println(stream().sum());
}

private static DoubleStream stream() {
    return StreamSupport.doubleStream(Spliterators.spliteratorUnknownSize(new Piterator(),
            Spliterator.ORDERED | Spliterator.IMMUTABLE | Spliterator.NONNULL), false);
}

static class Piterator implements PrimitiveIterator.OfDouble {
    private double t = 1;
    @Override public boolean hasNext() { return 1 / (t * t) > 1e-12; }
    @Override public double nextDouble() { return 1 / (t * t++); }
}