private ComponentPlaceholder getMatchingPlaceholders(
			Collection<ComponentPlaceholder> placeholders, ComponentPlaceholder matchee) {

		String name = matchee.getName();
		String group = matchee.getGroup();
		Iterator<ComponentPlaceholder> it = placeholders.iterator();
		while (it.hasNext()) {
			ComponentPlaceholder placeholder = it.next();
			if (placeholder.getName().equals(name) && placeholder.getGroup().equals(group)) {
				return placeholder;
			}
		}
		return null;
	}
A class that tracks:
<ul>
 	<li>placeholders that are being used for a given provider</li>
 	<li>placeholders that are no longer being used, which are available for reuse </li>
</ul>
Ghidra/Framework/Docking/src/main/java/docking/PlaceholderSet.java