Bitwise Operators and Bit Masks

1. masks

// Constants to hold bit masks for desired flagsstatic final int flagAllOff = 0;  //         000...00000000 (empty mask)static final int flagbit1 = 1;    // 2^^0    000...00000001static final int flagbit2 = 2;    // 2^^1    000...00000010static final int flagbit3 = 4;    // 2^^2    000...00000100static final int flagbit4 = 8;    // 2^^3    000...00001000static final int flagbit5 = 16;   // 2^^4    000...00010000static final int flagbit6 = 32;   // 2^^5    000...00100000static final int flagbit7 = 64;   // 2^^6    000...01000000static final int flagbit8 = 128;  // 2^^7    000...10000000//...static final int flagbit31 = (int) Math.pow(2, 30);   // 2^^30//...// Variable to hold the status of all flagsint flags = 0;

 

2. individual masks to obtain combo masks:

int maskReadAllowed = 4;    // 00001000int maskWriteAllowed = 16;  // 00010000// 00001000 | 00010000 = 00011000 represents both read and write permissionsint maskReadAndWriteAllowed = maskReadAllowed | maskWriteAllowed;// Start out with no permissionsint user1Permissions = 0;   // 00000000int user2Permissions = 0;   // 00000000// Allow user 1 to read// 00000000 | 00001000 = 00001000user1Permissions |= maskReadAllowed;// Allow user 2 to read as well as write in one shot// 00000000 | 00011000 = 00011000user2Permissions |= maskReadAndWriteAllowed;// Later, allow user 1 to write also// 00001000 | 00010000 = 00011000user1Permissions |= maskWriteAllowed;// Or, to make sure that user 1 has read permission as well, just in case// 00001000 | 00011000 = 00011000 (same as before)user1Permissions |= maskReadAndWriteAllowed;

 

3. BitMask

 

      0 0 0 0 0 0 1 0     (MASK constant)AND   1 0 0 1 0 1 1 0     (your flags variable)      ---------------      0 0 0 0 0 0 1 0     (result of operation)

 

 

4. sample

 

    // Define masks for lowest level permissions as constants which    // are powers of 2. This ensures that only one bit in an integer    // is set to 1.  Make these private because combinations will be    // used to publically access valid permission sets.

   public static final int VIEW = 4; public static final int ADD = 16; public static final int EDIT = 32; public static final int DELETE = 256;    /* Valid permission sets    - No permission to do anything    - View Only    - View and Add    - View and Edit    - View, Add and Edit    - All permissions (View, Add, Edit and Delete)    */    // Obtain valid permission sets by Bitwise ORing lower level permissions    public static final int NOTHING_ALLOWED = 0;    public static final int VIEW_ALLOWED = VIEW;    public static final int VIEW_ADD_ALLOWED = VIEW | ADD;    public static final int VIEW_EDIT_ALLOWED = VIEW | EDIT;    public static final int VIEW_ADD_EDIT_ALLOWED = VIEW | ADD | EDIT;    public static final int ALL_ALLOWED = VIEW | ADD | EDIT | DELETE;    // Or, alternately    //public static final int ALL_ALLOWED = VIEW_ADD_EDIT_ALLOWED | DELETE;

 

 

    // Check permission(s)    public static boolean isPermitted(        int myPermissions,        int permissionToCheck    ) {        return ((myPermissions & permissionToCheck) == permissionToCheck);    }

 

 

Reference:

1. Bitwise Operators and Bit Masks

2. <Apress Beginning C From Novice to Professional 4th Edition>

P119 Using Bitwise Operators