Learning objectives

• stream processing (finite vs. infinite/unbounded)
• pipes and filters architecture
• representing streams using the Iterator design pattern
• separation of processing and I/O concerns
• testability using the Observer design pattern
• time/space complexity and scalability

System requirements

• Java 11 SDK or later (Java 17 LTS release recommended)
• SBT

You may also be able to install these requirements through your package manager or SDKMAN!.

Alternatively, you can use an IDE such as IntelliJ IDEA or Android Studio.

Description

This application maintains a running FIFO queue of the last n words read from the standard input (stdin). For each word read, the application prints the updated queue to the standard output (stdout) in the format

[w1, w2, ..., wn]

where wn is the most recent word read, and w1 is the oldest word still in the queue.

The default queue capacity is 10, and a different capacity can be passed as a command-line argument.

If multiple words are entered on the same line, the application processes them separately, using non-word characters, such as whitespace, as separators.

Examples

$ sbt "run 3"
[info] ...
Multiple main classes detected, select one to run:

 [1] hw.Main
 [2] hw.MainLeaky
 [3] hw.MainTestableva-sbt
1
[info] running hw.Main
w1 w2
[w1]
[w1, w2]
w3
[w1, w2, w3]
w4 w5
[w2, w3, w4]
[w3, w4, w5]
w6
[w4, w5, w6]
EOF

Running the application

Without command-line arguments:

$ sbt run

With a specific command-line argument (sliding queue capacity):

$ sbt "run 3"

SBT will then prompt you to choose the specific main class you want to run.

Running a specific main class directly

$ sbt "runMain edu.luc.cs.consoleapp.Main"

or

$ sbt "runMain edu.luc.cs.consoleapp.Main 3"

Running the tests

$ sbt test

Generating the test coverage reports

$ sbt jacoco

You will then see a summary report in the terminal, and you can view the full report in a web browser. (Note that these will get generated only after all tests pass.)

On macOS:

$ open target/scala-2.12/jacoco/report/html/index.html

On Linux:

$ xdg-open target/scala-2.12/jacoco/report/html/index.html

On Windows: please let me know if you know how to do this from the WSL command line. Otherwise you can open the index file in your web browser.

Note that the report will show 0% coverage as long as there are failing tests.

Running the application outside SBT

This avoids the performance overhead of running the application through SBT and allows passing command-line arguments directly:

On Linux or Mac OS X:

$ sbt stage
$ ./target/universal/stage/bin/main 3

On Windows:

> sbt stage
> .\target\universal\stage\bin\main 3