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.
Note The set() function sets a bit to true (or 1). To set a bit to zero, please use the clear method or specify false in set.
Note 2 The get() function returns a boolean indicating the value of the bit. True is 1 and false is 0.
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.
Here We set the first bit of the first byte to 1, so the first byte equals 1. The second byte, at position 8, also equals 1.
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.
Info With no arguments, clear() will erase all bits in the BitSet. This is similar to creating an entirely new instance.
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.
Dot Net Perls is a collection of tested code examples. Pages are continually updated to stay current, with code correctness a top priority.
Sam Allen is passionate about computer languages. In the past, his work has been recommended by Apple and Microsoft and he has studied computers at a selective university in the United States.
This page was last updated on Jun 27, 2023 (edit).