Speeding Up Calculations with Memoization (Caching)

This lesson covers speeding up calculations in Java with memoization (caching), so you understand how to avoid repeating the same work. It's for anyone searching "Java memoization usage."

Memoization saves a calculation's result in a "storage box" once, and retrieves it from the box instead of recalculating when the same input comes up again. It's like "giving the same answer you already remembered, if you're asked the same question again."

The example stores completed calculations in a HashMap called cache, and the slowSquare() method checks cache.containsKey(n) every time it's called to see if it's already been computed. From the second call with the same input onward, a "retrieved from cache" message appears instead.

A common early mistake is not judging when caching is actually worth it. Caching recursive calculations or API call results can give a noticeably real speedup, while it can be unnecessary overhead for lightweight logic.

This is a classic example of the "time versus memory" trade-off — using more memory to save on calculation time — a genuinely useful performance-optimization idea in real work.

📖 Reference code

Map<Integer, Integer> cache = new HashMap<>();

int slowSquare(int n) {
    if (cache.containsKey(n)) {
        System.out.println("Retrieved from cache: " + n);
        return cache.get(n);
    }
    System.out.println("Calculating: " + n);
    int result = n * n;
    cache.put(n, result);
    return result;
}

System.out.println(slowSquare(5));
System.out.println(slowSquare(5));

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