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    <p>Thanks for all your suggestions. I implemented my solution by creating new nested classes for the values of <code>P</code> and <code>F</code> already calculated, then used a <code>HashMap</code> to store the results. The <code>HashMap</code> is then queried for the result before computation takes place; if it is present it just returns the result, if it is not it computes the result and adds it to the <code>HashMap</code>.</p> <p>The final product looks a bit like this:</p> <pre><code>public class ProbabilityCalculator { private NavigableSet&lt;DataPoint&gt; points; private ProbabilityCalculator(NavigableSet&lt;DataPoint&gt; points) { this.points = points; } private static class P { public final DataPoint left; public final Event leftEvent; public final DataPoint right; public final Event rightEvent; public P(DataPoint left, Event leftEvent, DataPoint right, Event rightEvent) { this.left = left; this.leftEvent = leftEvent; this.right = right; this.rightEvent = rightEvent; } public boolean equals(Object o) { if(!(o instanceof P)) return false; P p = (P) o; if(!(this.leftEvent == null ? p.leftEvent == null : this.leftEvent.equals(p.leftEvent))) return false; if(!(this.rightEvent == null ? p.rightEvent == null : this.rightEvent.equals(p.rightEvent))) return false; return this.left.equals(p.left) &amp;&amp; this.right.equals(p.right); } public int hashCode() { int result = 93; result = 31 * result + this.left.hashCode(); result = 31 * result + this.right.hashCode(); result = this.leftEvent != null ? 31 * result + this.leftEvent.hashCode() : 31 * result; result = this.rightEvent != null ? 31 * result + this.rightEvent.hashCode() : 31 * result; return result; } } private Map&lt;P, Double&gt; usedPs = new HashMap&lt;P, Double&gt;(); private static class F { public final DataPoint left; public final Event leftEvent; public final NavigableSet&lt;DataPoint&gt; dataPointsToLeft; public F(DataPoint dataPoint, Event dataPointEvent, NavigableSet&lt;DataPoint&gt; dataPointsToLeft) { this.dataPoint = dataPoint; this.dataPointEvent = dataPointEvent; this.dataPointsToLeft = dataPointsToLeft; } public boolean equals(Object o) { if(!(o instanceof F)) return false; F f = (F) o; return this.dataPoint.equals(f.dataPoint) &amp;&amp; this.dataPointEvent.equals(f.dataPointEvent) &amp;&amp; this.dataPointsToLeft.equals(f.dataPointsToLeft); } public int hashCode() { int result = 7; result = 31 * result + this.dataPoint.hashCode(); result = 31 * result + this.dataPointEvent.hashCode(); result = 31 * result + this.dataPointsToLeft.hashCode(); return result; } } private Map&lt;F, Double&gt; usedFs = new HashMap&lt;F, Double&gt;(); private Double p(DataPoint right, Event rightEvent, DataPoint left, Event leftEvent) { P newP = new P(right, rightEvent, left, leftEvent); if(this.usedPs.containsKey(newP)) return usedPs.get(newP); // do some stuff usedPs.put(newP, result); return result; } private Double f(DataPoint right, Event rightEvent) { NavigableSet&lt;DataPoint&gt; dataPointsToLeft = dataPoints.headSet(right, false); F newF = new F(right, rightEvent, dataPointsToLeft); if(usedFs.containsKey(newF)) return usedFs.get(newF); DataPoint left = points.lower(right); Double result = 0.0; if(left.isLefthandNode()) { result = 0.25 * p(right, rightEvent, left, null); } else if(left.isQ()) { result = p(right, rightEvent, left, left.getQEvent()) * f(left, left.getQEvent(), points); } else { // if M_k for(Event leftEvent : left.getEvents()) result += p(right, rightEvent, left, leftEvent) * f(left, leftEvent, points); } usedFs.put(newF, result) return result; } public Double S() { return f(points.last(), points.last().getRightNodeEvent(), points) } public static probabilityOfQ(DataPoint q, NavigableSet&lt;DataPoint&gt; points) { ProbabilityCalculator pc = new ProbabilityCalculator(points); Double S1 = S(); points.add(q); Double S2 = S(); return S2/S1; } } </code></pre>
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