Question

In Java, I would like to store (>10’000) arrays of boolean values (boolean[]) with length 32 to the disk and read them again later on for further computation and comparison.

Since a single array will have a length of 32, I wonder whether it makes sense to store it as an integer value to speed up the reading and writing (on a 32 bit machine). Would you suggest using BitSet and then convert to int? Or even forget about int and use bytes?

Answer 1

For binary storage, use int and a DataOutputStream (DataInputStream for reading).

I think boolean arrays are stored as byte or int arrays internally in Java, so you may want to consider avoiding the overhead and keeping the int encoding all the time, i.e. not use boolean[] at all.

Instead, have something like

public class BooleanArray32 {
  private int values;

  public boolean get(int pos) {
    return (values & (1 << pos)) != 0;
  }

  public void set(int pos, boolean value) {
     int mask = 1 << pos;
     values = (values & ~mask) | (value ? mask : 0);
  }

  public void write(DataOutputStream dos) throws IOException {
    dos.writeInt(values);
  }

  public void read(DataInputStream dis) throws IOException {
    values = dis.readInt();
  }

  public int compare(BooleanArray32 b2) {
     return countBits(b2.values & values);
  }

  // From http://graphics.stanford.edu/~seander/bithacks.html
  // Disclaimer: I did not fully double check whether this works for Java's signed ints
  public static int countBits(int v) {
    v = v - ((v >>> 1) & 0x55555555);                    // reuse input as temporary
    v = (v & 0x33333333) + ((v >>> 2) & 0x33333333);     // temp
    return ((v + (v >>> 4) & 0xF0F0F0F) * 0x1010101) >>> 24; 
  }
}