Chapter 14 — Functional Programming on Collections

Level: intermediate Estimated time: 6 h Prerequisites: chapter 05 (loops), chapter 06 (collections), chapter 07 (functions), chapter 13 (exceptions) What you will be able to do by the end: transform, filter, sort, and summarize any list of players, enemies, or inventory items in a few declarative lines, without writing a single for loop.

14.0 — Chapter Objectives

By the end of this chapter, you will be able to:

• distinguish the imperative style (how to do it) from the declarative style (what to get);
• write an anonymous function and an arrow function to pass to a method;
• iterate over a collection with forEach();
• transform a collection with map(), including changing its type;
• explain what an Iterable is and why toList() is often required;
• filter a collection with where();
• search for an element with firstWhere(), lastWhere(), singleWhere(), and indexWhere();
• answer a yes-or-no question with any(), every(), and contains();
• reduce a collection to a single value with reduce() and fold();
• choose between reduce() and fold() with full knowledge of the trade-offs;
• sort a list with sort() and write a comparator;
• use compareTo() on numbers and on strings;
• sort objects by score, by name, and then by several criteria;
• sort a copy without modifying the original list;
• slice a collection with take() and skip();
• flatten nested lists with expand();
• remove duplicates with toSet();
• assemble a collection into a string with join();
• use the spread operator ... and its null-aware version ...?;
• write an if and a for directly inside a list literal;
• chain several operations into a single readable expression;
• decide, using clear criteria, when it is better to go back to a for loop.