Package org.apache.juneau.commons.collections

Class KeywordSet

java.lang.Object
org.apache.juneau.commons.collections.KeywordSet
public class KeywordSet extends Object
A specialized set for storing and efficiently looking up language keywords.

This class provides a lightweight, immutable container optimized for fast keyword lookups using binary search. Keywords are stored in a sorted array, making lookups O(log n) efficient. This is particularly useful for parsers, compilers, and syntax highlighters that need to frequently check if identifiers are reserved keywords.

Features:
  • Immutable after construction - thread-safe for concurrent reads
  • Efficient O(log n) lookups using binary search
  • Compact memory footprint using sorted array
  • Automatic rejection of null and single-character strings
Implementation Details:

Keywords are sorted lexicographically during construction and stored in an internal array. The contains(String) method uses Arrays.binarySearch(Object[], Object) for efficient lookups. Strings shorter than 2 characters are automatically rejected without performing a search, as most programming languages don't have single-character keywords.

Examples:

// Create a keyword set for Java reserved words KeywordSet javaKeywords = new KeywordSet( "abstract", "assert", "boolean", "break", "byte", "case", "catch", "char", "class", "const" // ... more keywords ); // Check if identifiers are keywords if (javaKeywords.contains("class")) { // Handle keyword } // Safe handling of edge cases assertFalse(javaKeywords.contains(null)); // Returns false assertFalse(javaKeywords.contains("a")); // Single char - returns false assertFalse(javaKeywords.contains("myVar")); // Not a keyword // Use in a parser/lexer class Lexer { private static final KeywordSet KEYWORDS = new KeywordSet( "if", "else", "while", "for", "return" ); boolean isKeyword(String token) { return KEYWORDS.contains(token); } }

Use Cases:
  • Programming language parsers - checking if tokens are reserved words
  • Syntax highlighters - identifying keywords for special formatting
  • Code analyzers - distinguishing keywords from identifiers
  • Template engines - recognizing template keywords
  • Query languages - validating reserved words
Notes:
  • This class is immutable and thread-safe after construction. Multiple threads can safely call contains(String) concurrently.
  • Keywords are compared using exact string matching (case-sensitive). For case-insensitive matching, normalize your keywords and input strings to the same case.
  • The minimum keyword length is 2 characters. Single-character strings are automatically rejected.
  • Consider creating KeywordSet instances as static final constants to avoid repeated construction.
See Also:

  • Constructor Summary

    Constructors
    Constructor
    Description
    KeywordSet(String... keywords)
    Creates a new keyword set with the specified keywords.

  • Method Summary

    Modifier and Type
    Method
    Description
    boolean
    contains(String s)
    Checks if the specified string is a keyword in this set.
    boolean
    equals(Object o)
    Compares the specified object with this keyword set for equality.
    int
    hashCode()
    Returns the hash code value for this keyword set.
    String
    toString()
    Returns a string representation of this keyword set.

    Methods inherited from class java.lang.Object

    clone, finalize, getClass, notify, notifyAll, wait, wait, wait

  • Constructor Details

    • KeywordSet

      public KeywordSet(String... keywords)
      Creates a new keyword set with the specified keywords.

      Keywords are automatically sorted during construction. Duplicate keywords are allowed but provide no benefit. For best performance, pass unique keywords.

      Example:

      // Create a keyword set for SQL keywords KeywordSet sql = new KeywordSet( "SELECT", "FROM", "WHERE", "INSERT", "UPDATE", "DELETE", "CREATE", "DROP", "TABLE", "INDEX" ); // Keywords can be passed in any order KeywordSet keywords = new KeywordSet("zebra", "apple", "banana"); assertTrue(keywords.contains("apple")); // Sorted internally

      Parameters:
      keywords - The keywords to store. Can be empty but not null. Individual keywords can be any non-null string.

  • Method Details

    • contains

      public boolean contains(String s)
      Checks if the specified string is a keyword in this set.

      This method performs an O(log n) binary search on the sorted keyword array. Null strings and strings with fewer than 2 characters are automatically rejected without performing a search.

      Example:

      KeywordSet keywords = new KeywordSet("class", "interface", "enum"); // Standard checks assertTrue(keywords.contains("class")); // Keyword exists assertFalse(keywords.contains("MyClass")); // Not a keyword // Edge cases handled gracefully assertFalse(keywords.contains(null)); // null returns false assertFalse(keywords.contains("")); // Empty string returns false assertFalse(keywords.contains("a")); // Single char returns false // Case-sensitive matching assertTrue(keywords.contains("class")); assertFalse(keywords.contains("CLASS")); // Different case

      Performance:
      • Time complexity: O(log n) using binary search
      • Space complexity: O(1) - no additional memory allocated
      • Short-circuit: Strings with length < 2 return immediately without searching
      Parameters:
      s - The string to check. Can be null.
      Returns:
      true if the string exists in this keyword set, false if it doesn't exist, is null, or has fewer than 2 characters.

    • toString

      public String toString()
      Returns a string representation of this keyword set.

      The format follows the standard Java set convention: "[keyword1, keyword2, ...]"

      Overrides:
      toString in class Object
      Returns:
      A string representation of this keyword set.

    • equals

      public boolean equals(Object o)
      Compares the specified object with this keyword set for equality.

      Returns true if the given object is also a keyword set and contains the same keywords in the same order. Two keyword sets are equal if their internal arrays are equal.

      Overrides:
      equals in class Object
      Parameters:
      o - Object to be compared for equality with this keyword set.
      Returns:
      true if the specified object is equal to this keyword set.

    • hashCode

      public int hashCode()
      Returns the hash code value for this keyword set.

      The hash code is computed from the internal array of keywords using Arrays.hashCode(Object[]). This ensures that ks1.equals(ks2) implies that ks1.hashCode()==ks2.hashCode() for any two keyword sets ks1 and ks2, as required by the general contract of Object.hashCode().

      Overrides:
      hashCode in class Object
      Returns:
      The hash code value for this keyword set.