Requesting 'count' on a SIZED Stream (like the result of Collection.stream or Arrays.stream) apparently triggers a linear traversal of the stream, even though the Stream already knows its size. This could fairly easily be optimized.

This isn't very useful for self-contained use cases (why not just get the size directly?), but comes up when you pass Stream or Supplier<Stream> objects between methods.

My performance test (the timing numbers aren't very useful, but definitely grow with size):

---

import java.util.*;
import java.util.stream.*;

public class StreamOptimization {

    public static void main(String... args) {
        for (int i = 100; i < 200_000_000; i = (int) (i * 1.3)) {
            testArrayCount(i);
            testListCount(i);
        }
    }

    public static void testListCount(int i) {
        List<Object> objs = new ArrayList<>(i);
        for (int j = 0; j < i; j++) objs.add(null);
        long start = System.currentTimeMillis();
        long count = objs.stream().count();
        long delta = System.currentTimeMillis() - start;
        System.out.println("list count=" + count + "; time=" + delta);
    }

    public static void testArrayCount(int i) {
        Object[] objs = new Object[i];
        long start = System.currentTimeMillis();
        long count = Arrays.stream(objs).count();
        long delta = System.currentTimeMillis() - start;
        System.out.println("array count=" + count + "; time=" + delta);
    }

}