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();

    Path indexPath = Files.createTempDirectory("tempIndex");
    Directory directory = FSDirectory.open(indexPath);
    IndexWriterConfig config = new IndexWriterConfig(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("fieldname", analyzer);
    Query query = parser.parse("text");
    ScoreDoc[] hits = isearcher.search(query, 10).scoreDocs;
    assertEquals(1, hits.length);
    // Iterate through the results:
    StoredFields storedFields = isearcher.storedFields();
    for (int i = 0; i < hits.length; i++) {
      Document hitDoc = storedFields.document(hits[i].doc);
      assertEquals("This is the text to be indexed.", hitDoc.get("fieldname"));
    }
    ireader.close();
    directory.close();
    IOUtils.rm(indexPath);

The Lucene API is divided into several packages:

To use Lucene, an application should:
  1. Create Documents by adding Fields;
  2. Create an IndexWriter and add documents to it with addDocument();
  3. Call QueryParser.parse() to build a query from a string; and
  4. Create an IndexSearcher and pass the query to its 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-analysis-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-analysis-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. ]

Packages 
Package Description
org.apache.lucene.analysis
Text analysis.
org.apache.lucene.analysis.standard
Fast, general-purpose grammar-based tokenizer StandardTokenizer implements the Word Break rules from the Unicode Text Segmentation algorithm, as specified in Unicode Standard Annex #29.
org.apache.lucene.analysis.tokenattributes
General-purpose attributes for text analysis.
org.apache.lucene.codecs
Codecs API: API for customization of the encoding and structure of the index.
org.apache.lucene.codecs.compressing
Compressing helper classes.
org.apache.lucene.codecs.lucene90
Lucene 9.0 file format.
org.apache.lucene.codecs.lucene90.blocktree
BlockTree terms dictionary.
org.apache.lucene.codecs.lucene90.compressing
Lucene 9.0 compressing format.
org.apache.lucene.codecs.lucene94
Lucene 9.4 file format.
org.apache.lucene.codecs.lucene95
Lucene 9.5 file format.
org.apache.lucene.codecs.lucene99
Lucene 9.9 file format.
org.apache.lucene.codecs.perfield
Postings format that can delegate to different formats per-field.
org.apache.lucene.document
The logical representation of a Document for indexing and searching.
org.apache.lucene.geo
Geospatial Utility Implementations for Lucene Core
org.apache.lucene.index
Code to maintain and access indices.
org.apache.lucene.internal.tests
Internal bridges to package-private internals, for use by the lucene test framework only.
org.apache.lucene.internal.vectorization
Internal implementations to support SIMD vectorization.
org.apache.lucene.search
Code to search indices.
org.apache.lucene.search.comparators
Comparators, used to compare hits so as to determine their sort order when collecting the top results with TopFieldCollector.
org.apache.lucene.search.knn
Classes related to vector search: knn and vector fields.
org.apache.lucene.search.similarities
This package contains the various ranking models that can be used in Lucene.
org.apache.lucene.store
Binary i/o API, used for all index data.
org.apache.lucene.util
Some utility classes.
org.apache.lucene.util.automaton
Finite-state automaton for regular expressions.
org.apache.lucene.util.bkd
Block KD-tree, implementing the generic spatial data structure described in this paper.
org.apache.lucene.util.compress
Compression utilities.
org.apache.lucene.util.fst
Finite state transducers
org.apache.lucene.util.graph
Utility classes for working with token streams as graphs.
org.apache.lucene.util.hnsw
Navigable Small-World graph, nominally Hierarchical but currently only has a single layer.
org.apache.lucene.util.hppc
package holding hppc related classes.
org.apache.lucene.util.mutable
Comparable object wrappers
org.apache.lucene.util.packed
Packed integer arrays and streams.
org.apache.lucene.util.quantization
Provides quantization methods for scaling vector values to smaller data types and possibly fewer dimensions