In the topic called duplicate calculation in recursion, the author mention the exercise proposal below. I don't quite understand what would a more general and non-intrusive way of applying memoization looks like. Also how to apply design pattern called "decorator".

As an exercise, you could try to make memoization more general and non-intrusive, i.e. applying memoization without changing the original function. (Hint: one can refer to a design pattern called decorator).

The orginal solution which lack of generalization and non-intrusive was given as below :

import java.util.HashMap;

public class Main {

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

  private int fib(int N) {
    if (cache.containsKey(N)) {
      return cache.get(N);
    }
    int result;
    if (N < 2) {
      result = N;
    } else {
      result = fib(N-1) + fib(N-2);
    }
    // keep the result in cache.
    cache.put(N, result);
    return result;
  }
}