↓Skip to main content

Notes: 34 Streams, Readers, and Streams

··2 mins

Final Exam #

Final Exam in Syllabus confirmed: Mon Dec 11th, 11am-1:30pm in D&M 442

Lab 12 #

Errata: Some valid approaches run out of memory on 1B primes on Inkfish. You may, optionally, change the PrimesCcShould#find_1b_primes test to use 2 threads instead of 8.

Problem: You want to find primes up to 10B, but BitSet only takes “int” args.

Solution: LongBitSet

Show adding LongBitSet to lab12.

Sequences #

Last time we talked about collections, which can be viewed as a sequence of items (explictly, with a for-each loop).

There are a couple other kinds of sequence that anyone who is writing Java programs should know about.

Show the readWords method in the demo app.

    Stream<String> readWords() {
        var raw = WordCount.class.getResourceAsStream("/words.txt.gz");
        var unz = new GZIPInputStream(raw);
        var rdr = new InputStreamReader(unz);
        var buf = new BufferedReader(rdr);
        return buf.lines();
    }

There are a couple different things going on in that method:

  • When you open a file or network socket, you get an InputStream. This is a single-use sequence of bytes.
  • Java makes a strong distinction between bytes and text characters, so bytes need to be decoded to get text.
    • Talk about ASCII, latin-1, Uncode, ucs2, utf16, utf8
    • InputStreamReader adapts our InputStream to a Reader.
  • BufferedReader adds some methods that require looking forward in the stream - specifically, readLine()
  • The buf.lines() method returns a Stream<String>, where the word “stream” is unrelated to InputStream.

InputStream has:

  • read, which reads one byte
  • read(array), which reads a full fixed size array worth of bytes

Reader has:

  • read, which reads one character
  • read(array), which reads a full array

BufferedReader has:

  • an internal buffer (think ArrayList<Char>) so it can internally store however many characters it needs to find the end of a line
  • readLine, which reads a full line of text

Stream is a completely new way to deal with arbitrary sequences of objects. It’s like a Collection, except:

  • You can only go through a Stream once
    • This makes sense in the example - lines are produced one at a time by reading the InputStream.
  • Streams are lazy - operations on streams don’t happen until they have to: either something happens to the values, or they are collected into a List or array.
  • Streams provide a bunch of traditionally functional stuff like filter, map, etc.
  • Streams can sometimes execute in parallel.

Let’s use our stream and collections to:

  • Count the words
  • Find the longest words
  • Count words longer than 5 characters
  • Count words of each length
  • Find longest word starting with each letter