BitSet

Bits are efficient. Imagine we need to store many true or false values. Using bits, not bytes, saves 7 bits per value—and these savings can add up.

Some programs written in Java may use excess memory—with a collection like BitSet, we can improve performance in these programs. Sometimes the results are dramatic.

Set, get example

We begin with a simple example of BitSet: we set two bits, at positions 10 and 100. Then we get some bits. Position 5 is false, but 10 and 100 are true.

import java.util.BitSet;

public class Program {
    public static void main(String[] args) {

        // Set two bits in a BitSet.
        BitSet b = new BitSet();
        b.set(10);
        b.set(100);

        // Get values of these bit positions.
        boolean bit1 = b.get(5);
        boolean bit2 = b.get(10);
        boolean bit3 = b.get(100);

        System.out.println(bit1);
        System.out.println(bit2);
        System.out.println(bit3);
    }
}
false
true
true

Flip

This method makes some programs simpler. It changes the value of a 0 bit to 1, and the value of a 1 bit to zero. By default in BitSet, a bit is zero.

import java.util.BitSet;

public class Program {
    public static void main(String[] args) {
        BitSet b = new BitSet();

        // Set this bit.
        b.set(3);
        System.out.println(b.get(3));

        // Flip the bit.
        b.flip(3);
        System.out.println(b.get(3));
    }
}
true
false

Set range

Bits can be set in a range. In this way we can avoid writing excess loops to set bits. Here we set the bits at indexes 2, 3 and 4 to true. We display them in a for-loop.

import java.util.BitSet;

public class Program {
    public static void main(String[] args) {
        BitSet b = new BitSet();

        // Set bits in this range to true.
        b.set(2, 5);

        // Display first five bits.
        for (int i = 0; i < 5; i++) {
            System.out.println(b.get(i));
        }
    }
}
false
false
true
true
true

ToByteArray

A BitSet is composed of bytes. With toByteArray we can access the backing store, a byte array. Each byte has a value determined by the bits that are set.

import java.util.BitSet;

public class Program {
    public static void main(String[] args) {
        BitSet b = new BitSet();

        // Set bit 0 and bit 8 to true.
        b.set(0);
        b.set(8);

        // Convert to byte array.
        for (byte value : b.toByteArray()) {
            System.out.println(value);
        }
    }
}
1
1

Get range

With get, we usually access a single bit. But we can get a range of bits, like a substring, as a completely new BitSet instance. Note how the positions of the set bits change.

import java.util.BitSet;

public class Program {
    public static void main(String[] args) {

        BitSet b = new BitSet();
        b.set(3);
        b.set(5);

        // Get range of bits from original set.
        BitSet b2 = b.get(3, 6);

        // Display first five bits.
        for (int i = 0; i < 5; i++) {
            System.out.println(b2.get(i));
        }
    }
}
true
false
true
false
false

Clear

This method sets a bit to zero. It is the same thing as passing a false value as the second argument to the set method. We also can clear a range of bits.

import java.util.BitSet;

public class Program {
    public static void main(String[] args) {

        BitSet b = new BitSet();

        // Set first four bits to true.
        b.set(0, 4);

        // Clear first two bits.
        b.clear(0, 2);

        // Display first four bits.
        System.out.println(b.get(0));
        System.out.println(b.get(1));
        System.out.println(b.get(2));
        System.out.println(b.get(3));
    }
}
false
false
true
true

With less memory usage, programs often are faster too. BitSet introduces some complexity, but is simpler than directly manipulating bits.