Overview (Lucene 3.6.1 API)

Packages
Package	Description
org.apache.lucene	Top-level package.
org.apache.lucene.analysis	API and code to convert text into indexable/searchable tokens.
org.apache.lucene.analysis.standard	Standards-based analyzers implemented with JFlex.
org.apache.lucene.analysis.standard.std31	Backwards-compatible implementation to match `Version.LUCENE_31`
org.apache.lucene.analysis.standard.std34	Backwards-compatible implementation to match `Version.LUCENE_34`
org.apache.lucene.analysis.tokenattributes	Useful `Attribute`s for text analysis.
org.apache.lucene.collation	`CollationKeyFilter` converts each token into its binary `CollationKey` using the provided `Collator`, and then encode the `CollationKey` as a String using `IndexableBinaryStringTools`, to allow it to be stored as an index term.
org.apache.lucene.document	The logical representation of a `Document` for indexing and searching.
org.apache.lucene.index	Code to maintain and access indices.
org.apache.lucene.queryParser	A simple query parser implemented with JavaCC.
org.apache.lucene.search	Code to search indices.
org.apache.lucene.search.function	Programmatic control over documents scores.
org.apache.lucene.search.payloads	The payloads package provides Query mechanisms for finding and using payloads.
org.apache.lucene.search.spans	The calculus of spans.
org.apache.lucene.store	Binary i/o API, used for all index data.
org.apache.lucene.util	Some utility classes.
org.apache.lucene.util.fst	Finite state transducers
org.apache.lucene.util.packed	The packed package provides random access capable arrays of positive longs.

Apache Lucene is a high-performance, full-featured text search engine library. Here's a simple example how to use Lucene for indexing and searching (using JUnit to check if the results are what we expect):

    Analyzer analyzer = new StandardAnalyzer(Version.LUCENE_CURRENT);

    // Store the index in memory:
    Directory directory = new RAMDirectory();
    // To store an index on disk, use this instead:
    //Directory directory = FSDirectory.open("/tmp/testindex");
    IndexWriter iwriter = new IndexWriter(directory, analyzer, true,
                                          new IndexWriter.MaxFieldLength(25000));
    Document doc = new Document();
    String text = "This is the text to be indexed.";
    doc.add(new Field("fieldname", text, Field.Store.YES,
        Field.Index.ANALYZED));
    iwriter.addDocument(doc);
    iwriter.close();
    
    // Now search the index:
    IndexReader ireader = IndexReader.open(directory); // read-only=true
    IndexSearcher isearcher = new IndexSearcher(ireader);
    // Parse a simple query that searches for "text":
    QueryParser parser = new QueryParser("fieldname", analyzer);
    Query query = parser.parse("text");
    ScoreDoc[] hits = isearcher.search(query, null, 1000).scoreDocs;
    assertEquals(1, hits.length);
    // Iterate through the results:
    for (int i = 0; i < hits.length; i++) {
      Document hitDoc = isearcher.doc(hits[i].doc);
      assertEquals("This is the text to be indexed.", hitDoc.get("fieldname"));
    }
    isearcher.close();
    ireader.close();
    directory.close();

The Lucene API is divided into several packages:

org.apache.lucene.analysis defines an abstract Analyzer API for converting text from a java.io.Reader into a TokenStream, an enumeration of token Attributes. A TokenStream can be composed by applying TokenFilters to the output of a Tokenizer. Tokenizers and TokenFilters are strung together and applied with an Analyzer. A handful of Analyzer implementations are provided, including StopAnalyzer and the grammar-based StandardAnalyzer.
org.apache.lucene.document provides a simple Document class. A Document is simply a set of named Fields, whose values may be strings or instances of java.io.Reader.
org.apache.lucene.index provides two primary classes: IndexWriter, which creates and adds documents to indices; and IndexReader, which accesses the data in the index.
org.apache.lucene.search provides data structures to represent queries (ie TermQuery for individual words, PhraseQuery for phrases, and BooleanQuery for boolean combinations of queries) and the abstract Searcher which turns queries into TopDocs. IndexSearcher implements search over a single IndexReader.
org.apache.lucene.queryParser uses JavaCC to implement a QueryParser.
org.apache.lucene.store defines an abstract class for storing persistent data, the Directory, which is a collection of named files written by an IndexOutput and read by an IndexInput. Multiple implementations are provided, including FSDirectory, which uses a file system directory to store files, and RAMDirectory which implements files as memory-resident data structures.
org.apache.lucene.util contains a few handy data structures and util classes, ie BitVector and PriorityQueue.

To use Lucene, an application should:

Create Documents by adding Fields;
Create an IndexWriter and add documents to it with addDocument();
Call QueryParser.parse() to build a query from a string; and
Create an IndexSearcher and pass the query to its search() method.

Some simple examples of code which does this are:

IndexFiles.java creates an index for all the files contained in a directory.
SearchFiles.java prompts for queries and searches an index.

To demonstrate these, try something like:

> java -cp lucene.jar:lucene-demo.jar:lucene-analyzers-common.jar org.apache.lucene.demo.IndexFiles rec.food.recipes/soups
adding rec.food.recipes/soups/abalone-chowder
[ ... ]
> java -cp lucene.jar:lucene-demo.jar:lucene-analyzers-common.jar org.apache.lucene.demo.SearchFiles
Query: chowder
Searching for: chowder
34 total matching documents
1. rec.food.recipes/soups/spam-chowder
[ ... thirty-four documents contain the word "chowder" ... ]
Query: "clam chowder" AND Manhattan
Searching for: +"clam chowder" +manhattan
2 total matching documents
1. rec.food.recipes/soups/clam-chowder
[ ... two documents contain the phrase "clam chowder" and the word "manhattan" ... ]
[ Note: "+" and "-" are canonical, but "AND", "OR" and "NOT" may be used. ]

Lucene 3.6.1 core API