simple algorithm for storing and re-storing the expansion state of a treeview in javafx

ExpansionState.java

import java.util.HashMap;
import java.util.Map;

import javafx.scene.control.TreeItem;
import javafx.scene.control.TreeView;

public class ExpansionState {
	
	private TreeView treeView;
	private Map<Integer, Object> expansionGraph;
	
	public ExpansionState(TreeView treeView) {
		this.treeView = treeView;
	}
	
	public void storeExpansionState() {
		TreeItem rootItem = this.treeView.getRoot();
		if(rootItem != null) {
			this.expansionGraph = this.calculateExpansionGraph(rootItem);
		}	
	}
	
	public void restoreExpansionState() {
		if(this.expansionGraph != null) {
			TreeItem rootItem = this.treeView.getRoot();
			if(rootItem != null) {
				this.applyExpansionGraph(rootItem, this.expansionGraph);
			}
		}
	}
	
	private Map<Integer, Object> calculateExpansionGraph(TreeItem treeItem) {
		Map<Integer, Object> expGraph = new HashMap<Integer, Object>();
		
		for(int t = 0; t < treeItem.getChildren().size(); t++) {
			TreeItem childItem = (TreeItem) treeItem.getChildren().get(t);
			if(childItem.isExpanded()) {
				if(childItem.getChildren().size() < 1) {
					expGraph.put(t, this.calculateExpansionGraph(childItem));
				} else {
					expGraph.put(t, true);
				}
			}
		}
		
		return expGraph;
	}
	
	private void applyExpansionGraph(TreeItem treeItem, Map<Integer, Object> expansionGraph) {
		for(int t = 0; t < treeItem.getChildren().size(); t++) {
			if(expansionGraph.containsKey(t)) {
				TreeItem childItem = (TreeItem) treeItem.getChildren().get(t);
				if(expansionGraph.get(t) instanceof Map) {
					Map<Integer, Object> subExpansionGraph = (Map<Integer, Object>) expansionGraph.get(t);
					this.applyExpansionGraph(childItem, subExpansionGraph);
				} else {
					Boolean expanded = (Boolean) expansionGraph.get(t);
					childItem.setExpanded(expanded);
				}
			}
		}
	}
}