The method FlowPane.getRuns() can be optimized in several ways.

1) Use Parent.getManagedChildren() instead of looping over all children every time. Assuming that the children of a FlowLayout do not change that often, using getManagedChildren() is more efficient, especially if only few children are managed.

2) Avoid unnecessary array construction. Right now every element is copied into two ArrayLists (once in getRuns() and once in normalizeRun()). Plus with horizontal orientation two double arrays are created in normalizeRun(). All of that can be avoided.

3) The functionality in normalizeRun() can be calculated on-the-fly while an element is added to a run. This does not improve runtime performance, but reduces the memory footprint, because we do not need to keep the values required by a separate normalize-step. This data is stored in the lists of Runs, it gets very likely promoted to older generations.