Bit flags from boolean variables in Java and MySQL

Question

I just want to create a bit flag from a series of boolean variables so I can store one variable in a MySQL database instead of several booleans. For example, I have three boolean flags (A, B, C) and want to create a single bit flag based on the values of the boolean flags. I think this would work but I'm guessing there's a better (or more legible) way to do this:

int myBitFlag = A ? 1:0;
myBitFlag += B ? 2:0;
myBitFlag += C ? 4:0;

I'm not sure why I can't find anything about this and maybe it's because I'm doing it wrong or not using the correct terminology..

And to select all of the entries with B=true, would I use this?

SELECT * FROM table WHERE myBitFlag & 2 = 1

The book `Code Complete` has examples on how to do this (as well as when/when not to do this) and is a great read for many other reasons. If you are a programmer and haven't checked it out, you should give it a look. — BlackVegetable, Sep 13 '12 at 21:26

score 1 · Answer 1 · answered Sep 13 '12 at 21:51

I think the general approach for packing a bit into an int is to shift a value a certain number of bits to the left. For example if you want A at bit 0, B at bit 1 and C at bit 2 you can do something like:

  public static void main(String[] args)
  {
    boolean a = true;
    boolean b = false;
    boolean c = true;

    int flag = 0; 
    flag |= biteIt(a,0);
    flag |= biteIt(b,1);
    flag |= biteIt(c,2);
    System.out.println(flag);
  }

  private static int biteIt(boolean b, int place) {
    return b ? 1 << place : 0;
  }

score 0 · Answer 2 · answered Sep 13 '12 at 21:38

0

Store them as separate INT(1)s in SQL, let the database optimize storage for you. This way you can create indices on the flags which is essential for speedy selects.

answered Sep 13 '12 at 21:38

ron

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Darryl Miles · Answer 3 · 2012-09-13T21:51:42.500

No use the correct "BIT NOT NULL" SQL type to allow the database to optimize storage and perform WHERE clause expressions on independant columns. This is the purpose of using SQL to describe your data well enough for it to optimize searching.

If you really need bitmask values out you can always use the expression in the SELECT output part:

IF(bitColumnC, 4, 0)

To get an integer value 4 out if your application needs it for SELECT output. Also:

IF(bitColumnA, 1, 0) + IF(bitColumnB, 2, 0)

If you really must have it as a single integer.

Otherwise why keep them as bit field in SQL, just use an INT type in SQL and do the masking in the client software. Use of INT(1) is same as BIT.

Bit flags from boolean variables in Java and MySQL

3 Answers3