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Project 1 Inverted Index

Associated Assignments Project 1 Functionality
Project 1 Code Review

For this project, you will write a Java program that processes all text files in a directory and its subdirectories, cleans and parses the text into word stems, and builds an in-memory inverted index to store the mapping from word stems to the documents and position within those documents where those word stems were found.

For example, suppose we have the following mapping stored in our inverted index:

	"capybara": {
		"input/mammals.txt": [
	"platypus": {
		"input/dangerous/venomous.txt": [
		"input/mammals.txt": [

This indicates that after processing the word stems from files, the word capybara is found in the file input/mammals.html in position 11. The word platypus is found in two files, input/mammals.html and input/dangerous/venomous.html. In the file input/mammals.html, the word platypus appears twice in positions 3 and 8. In file input/dangerous/venomous.html, the word platypus is in position 2 in the file.

The process of stemming reduces a word to a base form (or “stem”), so that words like interesting, interested, and interests all map to the stem interest. Stemming is a common preprocessing step in many web search engines.


The core functionality of your project must satisfy the following requirements:

  • Process command-line arguments to determine the input to process and output to produce. See the Input and Output sections below for specifics.

  • Create a custom inverted index data structure that stores a mapping from a word stem to the file(s) the word was found, and the position(s) in that file the word is located. The positions should start at 1. This will require nesting multiple built-in data structures.

  • If provided a directory as input, find all files within that directory and all subdirectories and parse each text file found. If provided a single text file as input, only parse that individual file. Any files that end in the .text or .txt extension (case insensitive) should be considered a text file.

    • Use the UTF-8 character encoding for all file processing, including reading and writing.
  • Efficiently process text files into word stems by removing any non-letter symbols (including digits, punctuation, accents, special characters), convert the remaining alphabetic characters to lowercase, splitting the text into words by whitespace, and then stemming the word using the Apache OpenNLP toolkit.

    • Use the regular expression (?U)[^\\p{Alpha}\\p{Space}]+ to remove special characters from text.

    • Use the regular expression (?U)\\p{Space}+ to split text into words by whitespace.

    • Use the SnowballStemmer English stemming algorithm in OpenNLP to stem words.

  • If the appropriate command-line arguments are provided, output the inverted index in pretty JSON format. See the Output section below for specifics.

  • Output user-friendly error messages in the case of exceptions or invalid input. Under no circumstance should your main() method output a stack trace to the user!

The functionality of your project will be evaluated with the group of JUnit tests.


Your main method must be placed in a class named Driver. The Driver class should accept the following command-line arguments:

  • -path path where the flag -path indicates the next argument is a path to either a single text file or a directory of text files that must be processed and added to the inverted index

  • -index path where the flag -index is an optional flag that indicates the next argument is the path to use for the inverted index output file. If the path argument is not provided, use index.json as the default output path. If the -index flag is not provided, do not produce an output file.

  • -locations filepath where -locations is an optional flag that indicates the next argument is the path to use to output all of the locations and their word count. If the filepath argument is not provided, use locations.json as the default output filename. If the -locations flag is not provided, do not produce an output file of locations.

The command-line flag/value pairs may be provided in any order. Do not convert paths to absolute form when processing command-line input!


All output will be produced in “pretty” JSON format using tab characters for indentation. According to the JSON standard, numbers like integers should never be quoted. Any string or object key, however, should always be surrounded by " quotes. Objects (similar to maps) should use curly braces { and } and arrays should use square brackets [ and ]. Make sure there are no trailing commas after the last element.

The paths should be output in the form they were originally provided. The tests use normalized relative paths, so the output should also be normalized relative paths. As long as command-line parameters are not converted to absolute form, this should be the default output provided by the path object.

See below for more details on the index versus locations output files.

Index Output

The contents of your inverted index should be output in alphabetically sorted order as a nested JSON object using a “pretty” format. For example:

	"capybara": {
		"input/mammals.html": [
	"platypus": {
		"input/dangerous/venomous.html": [
		"input/mammals.html": [

See the JSON files in the project tests expected/index-test directory for more examples.

 The project tests account for different path separators (forward slash / for Linux/Mac systems, and backward slash \ for Windows systems). Your code does not have to convert between the two!

Location Output

The locations and their word count should be output as a JSON object with the path of the file as the key and the word count (after cleaning, parsing, and stemming) as the value. Here is the expected output for the word count of all the text files associated with this project:

	"text/guten/1400-0.txt": 187368,
	"text/guten/pg1228.txt": 157344,
	"text/guten/pg1322.txt": 124370,
	"text/guten/pg1661.txt": 107396,
	"text/guten/pg22577.txt": 63630,
	"text/guten/pg37134.txt": 16696,
	"text/rfcs/rfc475.txt": 3228,
	"text/rfcs/rfc5646.txt": 27075,
	"text/rfcs/rfc6797.txt": 12925,
	"text/rfcs/rfc6805.txt": 9785,
	"text/rfcs/rfc6838.txt": 9367,
	"text/rfcs/rfc7231.txt": 28811,
	"text/simple/.txt/hidden.txt": 1,
	"text/simple/a/b/c/d/subdir.txt": 1,
	"text/simple/animals.text": 11,
	"text/simple/animals_copy.text": 11,
	"text/simple/animals_double.text": 22,
	"text/simple/capital_extension.TXT": 1,
	"text/simple/capitals.txt": 4,
	"text/simple/digits.txt": 2,
	"text/simple/dir.txt/findme.Txt": 1,
	"text/simple/hello.txt": 6,
	"text/simple/position.teXt": 20,
	"text/simple/symbols.txt": 10,
	"text/simple/words.tExT": 24

You can also find this output in the project-tests/expected/locations.json file in the project-tests repository.

 This output is provided to help you debug your stemming. If your word count for each file does not match the expected output, then your index output will also mismatch.

 This output should look similar to that of one of your homeworks... you might be able to use it directly depending how you setup your project code!


The following are a few examples (non-comprehensive) to illustrate the usage of the command-line arguments. Consider the following example:

java Driver -path ../project-tests/text/simple/hello.txt
            -index index-simple-hello.json

The above arguments indicate that Driver should build an inverted index from the single hello.txt file in the text/simple subdirectory of the current working directory’s parent directory project-tests, and output the inverted index as JSON to the index-simple-hello.json file in the current working directory.

java Driver -path ../project-tests/text/simple -index

The above arguments indicate that Driver should build an inverted index from all of the text files found in the text/simple subdirectory of the current working directory’s parent directory, and output the inverted index as JSON to the default path index.json in the current working directory.

java Driver -path ../project-tests/text/simple

The above arguments indicate that Driver should build an inverted index from all of the HTML files found in the text/simple subdirectory of the current working directory’s parent directory, but it should NOT produce an output file. It must still build the inverted index however! (This will be useful in the future when we add the ability to search.)


It is important to develop the project iteratively. In fact, you may already have certain components complete thanks to the homework assignments. One possible breakdown of tasks are:

  • Create code that handles parsing command-line arguments into flag/value pairs, and supports default values if a flag is missing a value.

  • Create code that is able to traverse a directory and return a list of all the text files found within that directory. The File I/O (Featuring NIO.2) tutorials might be useful for this.

  • Create code that is able to parse text into words, including converting that text to lowercase, replacing special characters and digits, splitting that text into words by whitespaces, and finally stemming the words.

  • Create code that handles storing a word, file path, and location into an inverted index data structure.

  • Create code that is capable of writing a nested data structure (matching your inverted index data structure) to a file in JSON format.

Test each part of your code. Keep in mind the tests provided only test the final output of your Java program. You are responsible for testing the individual components of your code.

Once you have the above components working, then you can work on integrating your code together. This part must still be done iteratively! Do not try to combine all of the code at once. First, work on being able to traverse a directory based on command-line parameters. Then, clean the text files found and output the cleaned words to the console. Finally, add those words to the index and write that index as JSON.

 These hints may or may not be useful depending on your approach. Do not be overly concerned if you do not find these hints helpful for your approach for this project.