Question

In Java, I have an array of a million or so flags true/false to store in mind. Should BitSet help? Although it implements a Set, will it be possible to iterate its elements as fast as if it would be an array boolean[]?

Sorry if the question has been asked. First I tried to split an array into chunks of binary represented ints and form int[] as a result of those binaries, so I could reduce the size by 32, but this is quite low-level.

I found some critics of the BitSet elsewhere and that boolean[] stores a lot of extra memory => bad for large arrays.

Any better idea to store a million of flags?

Answer 1

I have an array of a million or so flags true/false to store in mind. Should BitSet help?

You can have billions of bits in a BitSet.

Although it implements a Set, will it be possible to iterate its elements as fast as if it would be an array boolean[]?

A boolean[] uses one byte per bit (on most JVMs) whereas BitSet uses one bit per bit. For small arrays, a boolean[] is faster, but when you are testing the size of your CPU cache a BitSet can be more efficient.

BTW: Using a BitSet is slightly slower for small sizes because it need to extract out a bit out of each a word of memory. A byte[] has the same issue, so if you want to set bit yourself, I suggest you use a int[] like BitSet does.

---

An example using BitSet

BitSet bitSet = new BitSet();
// set bit 100
bitSet.set(100);
// get bit 99
System.out.println("bit 99 is " + bitSet.get(99));
System.out.println("bit 100 is " + bitSet.get(100) + " after set");
bitSet.clear(100);
System.out.println("bit 100 is " + bitSet.get(100) + " after clear");

prints

bit 99 is false
bit 100 is true after set
bit 100 is false after clear