Power of declarative syntax of Lambda

Since Java has introduced lambda, writing code has become a breeze especially when it comes to expressing what you want to computer to do rather than how to do it. The following is a question from Project Euler and is solved using lambdas in Java8.

Let d(n) be defined as the sum of proper divisors of n (numbers less than n which
divide evenly into n). If d(a) = b and d(b) = a, where ab, then a and b are an
amicable pair and each of a and b are called amicable numbers.

For example, the proper divisors of 220 are 1, 2, 4, 5, 10, 11, 20, 22, 44, 55 and
110; therefore d(220) = 284. The proper divisors of 284 are 1, 2, 4, 71 and 142;
so d(284) = 220.

Evaluate the sum of all the amicable numbers under 10000.
 A simple brute force solution is to check every number from 2 to 10000 and if its  an amicable number then add it to sum. Listing it in steps you are basically doing 3 things:
  1. Take numbers from 2 to 10000
  2. If number is amicable
  3. Add it

In Java 7 one will basically start a for loop, make another method for checking amicable property which in turn will have another method for getting divisors and its total. Adding to all this boilerplate there will be a lot of mutable variables like sum, temp just to track of the operations that you have listed. So much verbosity will easily divert somebody reading the code from actually knowing what the program is trying to achieve. One will have to dwell in the individual methods and track when the mutable variable is being accessed and what would be the outcome. Not saying that Java 7 leads to cluttered code, one can always write eye-pleasing code but at a cost of boilerplate classes and interfaces. In Java 8 it is natural for the code to be expressive, look at the Lambda solution for this question below:

capture

It exactly matches the list of 3 things that we made to solve the problem. It iterates from 2 to 10000 and checks if the number is amicable and adds it, without the boilerplate. This style when applied to enterprise level applications makes them low maintenance and one can easily map the code to requirements.

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