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");
    IndexWriterConfig config = new IndexWriterConfig(Version.LUCENE_CURRENT, analyzer);
    IndexWriter iwriter = new IndexWriter(directory, config);
    Document doc = new Document();
    String text = "This is the text to be indexed.";
    doc.add(new Field("fieldname", text, TextField.TYPE_STORED));
    iwriter.addDocument(doc);
    iwriter.close();
    
    // Now search the index:
    DirectoryReader ireader = DirectoryReader.open(directory);
    IndexSearcher isearcher = new IndexSearcher(ireader);
    // Parse a simple query that searches for "text":
    QueryParser parser = new QueryParser(Version.LUCENE_CURRENT, "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"));
    }
    ireader.close();
    directory.close();

The Lucene API is divided into several packages:

To use Lucene, an application should:
  1. Create {@link org.apache.lucene.document.Document Document}s by adding {@link org.apache.lucene.document.Field Field}s;
  2. Create an {@link org.apache.lucene.index.IndexWriter IndexWriter} and add documents to it with {@link org.apache.lucene.index.IndexWriter#addDocument(Iterable) addDocument()};
  3. Call QueryParser.parse() to build a query from a string; and
  4. Create an {@link org.apache.lucene.search.IndexSearcher IndexSearcher} and pass the query to its {@link org.apache.lucene.search.IndexSearcher#search(org.apache.lucene.search.Query, int) search()} method.
Some simple examples of code which does this are: To demonstrate these, try something like:
> java -cp lucene-core.jar:lucene-demo.jar:lucene-analyzers-common.jar org.apache.lucene.demo.IndexFiles -index index -docs rec.food.recipes/soups
adding rec.food.recipes/soups/abalone-chowder
  [ ... ]

> java -cp lucene-core.jar:lucene-demo.jar:lucene-queryparser.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. ]