Collections and streams

groupingBy with a summingDouble downstream collector is the stream analogue of GROUP BY currency with SUM(amount). Option A does not group. Option D sums everything into one number. Option C is a type error.

Source: https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/util/stream/Collectors.html

Intermediate operations are lazy and return a new stream. The pipeline runs only when you call a terminal operation. This is the same as Spark's transformation/action split and the laziness of a Python generator.

Source: https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/util/stream/package-summary.html

map is a 1:1 projection. flatMap maps each element to a stream and joins the results. One element becomes zero or more elements. This is the UNNEST/explode operation.

Source: https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/util/stream/Stream.html#flatMap(java.util.function.Function)

merge(key, 1, Integer::sum) inserts 1 if the key is absent. Otherwise it applies the remapping function to the old and new values. Option A throws NullPointerException on the first occurrence. Option D never writes.

Source: https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/util/Map.html#merge(K,V,java.util.function.BiFunction)

List.of / Map.of / Set.of create immutable collections. If you change them, Java throws UnsupportedOperationException. Wrap them in new ArrayList<>(...) when you need to modify them.

Source: https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/util/List.html#of()

A parallel stream splits the input into partitions and merges the partial results. For a correct result, the combiner must be associative. The identity must also be a true identity. This is the same associativity rule as parallel aggregation in Spark/MapReduce.

Source: https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/util/stream/Stream.html#reduce(T,java.util.function.BinaryOperator)

max returns Optional because the stream may be empty. The comparator orders by amount. Optional forces the caller to handle the empty case.

Source: https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/util/stream/Stream.html#max(java.util.Comparator)

Order::amount is a method reference. It is equivalent to the lambda o -> o.amount(). You pass it where the code expects a functional interface (e.g. Function or ToDoubleFunction).

Source: https://docs.oracle.com/javase/tutorial/java/javaOO/methodreferences.html