The Bidi class implements the " Unicode Version 3.0 Bidirectional Algorithm" for working with Arabic and Hebrew text in which letters run right-to-left and numbers run left-to-right. It is named after the first four letters of "bidirectional." A full description of the bidirectional text handling and the bidirectional algorithim is beyond the scope of this book, but the simplest use case for this class is outlined here. Create a Bidi object by passing an AttributedCharacterIterator or a String and one of the DIRECTION constants (to indicate the base direction of the text) to the Bidi( ) constructor. Or use createLineBidi( ) to return a substring of an existing Bidi object (this is usually done when formatting a paragraph of text to fit on individual lines).

Once you have a Bidi object, use isLeftToRight( ) and isRightToLeft( ) to determine whether all the text has the same direction. If both of these methods return false (which is the same as isMixed( ) returning TRue) then you cannot treat the text as a single run of uni-directional text. In this case, you must break it into two or more runs of unidirectional text. getrunCount( ) returns the number of distinct runs of text. For each such numbered run, getrunStart( ) returns the index of the first character of the run, and geTRunLimit( ) returns the index of the first character past the end of the run. getrunLevel( ) returns the level of the text, which is an integer that represents the direction and nesting level of the text. Even levels represent left-to-right text, and odd levels represent right-to-left text. The level divided by two is the nesting level of the text. For example, left-to-right text embedded within right-to-left text has a level of 2.

public final class Bidi {
// Public Constructors
     public Bidi(AttributedCharacterIterator paragraph);  
     public Bidi(String paragraph, int flags);  
     public Bidi(char[ ] text, int textStart, byte[ ] embeddings, 
        int embStart, int paragraphLength, int flags);  
// Public Constants
     public static final int DIRECTION_DEFAULT_LEFT_TO_RIGHT;            =-2
     public static final int DIRECTION_DEFAULT_RIGHT_TO_LEFT;            =-1
     public static final int DIRECTION_LEFT_TO_RIGHT;                    =0
     public static final int DIRECTION_RIGHT_TO_LEFT;                    =1
// Public Class Methods
     public static void reorderVisually(byte[ ] levels, int levelStart, 
        Object[ ] objects, int objectStart, int count);  
     public static boolean requiresBidi(char[ ] text, int start, int limit);  
// Public Instance Methods
     public boolean baseIsLeftToRight( );  
     public Bidi createLineBidi(int lineStart, int lineLimit);  
     public int getBaseLevel( );  
     public int getLength( );  
     public int getLevelAt(int offset);  
     public int getRunCount( );  
     public int getRunLevel(int run);  
     public int getRunLimit(int run);  
     public int getRunStart(int run);  
     public boolean isLeftToRight( );  
     public boolean isMixed( );  
     public boolean isRightToLeft( );  
// Public Methods Overriding Object
     public String toString( );  
}