Explain Java code

What this Java sample shows

classes

record Order(...) declares a class from its three fields. Java writes the constructor, the accessors customer(), status() and cents(), and equals, hashCode and toString. The block inside is a compact constructor: it runs on every new Order(...) and throws IllegalArgumentException for a negative total.

streams

paidByCustomer reads the list as a stream. filter keeps the paid orders, then Collectors.groupingBy puts them in a TreeMap keyed by customer and adds up their cents with summingInt. Paid orders of 1250 and 2750 for Ana and 3000 for Ben give {Ana=4000, Ben=3000}.

generics

topKey works for any map whose values can be compared. The bound V extends Comparable<V> is what lets Map.Entry.comparingByValue() compile: with a plain V, javac rejects the method with an incompatible types error.

Good to know

• A customer with no paid orders is left out of the result, not listed with 0. A customer whose only order is PENDING never reaches groupingBy.
• TreeMap::new keeps the customers in name order. Without it, groupingBy promises no particular map or order; the JDK hands back a HashMap.
• topKey returns an Optional because an empty map has no top key: it gives Optional.empty, never null. The spec doesn't say which entry wins a tie; the JDK keeps the one max met first, which in a TreeMap is the name that sorts first.
• o.status().equals("PAID") throws NullPointerException on an order whose status is null. Written as "PAID".equals(o.status()), it skips that order instead.

Watch out: summingInt adds in an int, so a customer's total silently wraps around once it passes 2,147,483,647 cents, about 21.5 million dollars. Two paid orders of 2,000,000,000 cents add up to -294967296. Use summingLong for totals that can grow that large.