A set of stateless operators that don't need to be instantiated

[akarnokd]

for lift() all the time.

[benjchristensen]

That's a nice change. This should improve GC behavior. Have you been able to measure it?

[davidmoten]

There's a javadoc typo that has been copy pasted throughout: 'an singleton instance'.

[akarnokd]

I haven't measured the GC gain; but I guess your use case that required queue pooling might benefit from another 16 bytes of allocation-reduction.

[akarnokd]

The following program prints 240 bytes/instance on head and 225 on this PR; that is ~15 MB of heap saved

public class MergeAlloc {
    public static void main(String[] args) {
        MemoryMXBean memoryMXBean = ManagementFactory.getMemoryMXBean();
        MemoryUsage memHeap = memoryMXBean.getHeapMemoryUsage();
        long initial = memHeap.getUsed();

        int m = 1_000_000;

        System.out.printf("Starting: %.3f MB%n", initial / 1024.0 / 1024.0);

        List<Observable<Integer>> list = new LinkedList<>();

        for (int i = 0; i < m; i++) {
            list.add(Observable.merge(Observable.just(1), Observable.just(2)));
        }

        memHeap = memoryMXBean.getHeapMemoryUsage();
        long after = memHeap.getUsed();
        System.out.printf("Starting: %.3f MB%n", after / 1024.0 / 1024.0);

        System.out.printf("Cost: %d bytes/item", (after - initial) / m);
    }
}

[akarnokd]

Triggering re-Travis.