Hack University - Streams: filter, map, collect

The stream pipeline

Most collection work is the same three verbs in different orders: keep some elements, transform them, gather a result. Written as loops, the verbs smear across mutable helper variables; written as a stream, each verb is one named stage, and a reviewer reads the intent line by line. Streams are the house style for this in modern Java codebases, so you will read them daily even before you write them. A stream processes a collection in stages, each stage taking a lambda. It is Java's answer to Python's list comprehensions:

[n * n for n in nums if n % 2 == 0]

nums.stream()
    .filter(n -> n % 2 == 0)
    .map(n -> n * n)
    .collect(Collectors.toList());

stream() opens the pipeline.
filter(predicate) keeps elements that pass the test.
map(function) transforms each element.
A terminal operation ends the pipeline and produces the result: collect(Collectors.toList()) builds a list, count() counts, mapToInt(...).sum() totals.

Nothing runs until the terminal operation, and the source collection is never modified.

Elements flow through the pipeline one stage at a time: filtered, transformed, then gathered by the terminal collect.

Code exercise · java

Run the pipeline. Note the two imports, and that the original nums list is untouched afterward.

More stages and endings

A few more pieces cover most day-to-day pipelines. Mid-pipeline stages: distinct() drops duplicates, sorted() orders elements, limit(n) keeps the first n. Terminal operations besides collect:

Terminal	Produces
`count()`	how many elements survived, as a `long`
`anyMatch(predicate)`	`true` as soon as one element passes
`mapToInt(n -> n).sum()`	a total (int math needs the mapToInt bridge)
`toList()`	a `List` — modern shorthand for `collect(Collectors.toList())`

One rule to keep pipelines honest: a stream never modifies its source, and each stream can be consumed by exactly one terminal operation — build a fresh stream() for each question you ask.

Code exercise · java

Run this. Four questions about one list, each as its own small pipeline: a count, a cleaned-and-sorted copy, a yes/no, and a sum. The source list is never modified.

Quiz

Which line of a stream pipeline actually triggers the work?

Code exercise · java

Your turn. From the words list, build a list of the words with length at most 4, uppercased, and print it. Then count the words longer than 4 characters and print the count.