How to Count the Number of Set Bits in a java.util.BitSet in Java

How do you count the number of bits set in a java.util.BitSet in Java?

BitSet bitSet = new BitSet();
bitSet.set(0);
bitSet.set(2);
int setBitsCount = bitSet.cardinality(); // returns 2

In Java, the `java.util.BitSet` class provides a way to manage a sequence of bits. To count the number of bits set to true (1s), you can use the `cardinality()` method, which returns the count of set bits in the BitSet object.

BitSet bitSet = new BitSet();
bitSet.set(0);
bitSet.set(2);
int setBitsCount = bitSet.cardinality(); // This will return 2 because bits 0 and 2 are set.

Causes

• Misunderstanding how `BitSet` represents bits.
• Confusion between the size of the BitSet and the count of set bits.

Solutions

• Use the `cardinality()` method to get the count of set bits.
• Ensure that you set bits in the BitSet correctly before counting.

Mistake: Not initializing a BitSet before setting bits.

Solution: Ensure you instantiate a BitSet object.

Mistake: Confusing `size()` with `cardinality()`, which returns set bits only.

Solution: Use `cardinality()` for the count of set bits and `size()` for the total capacity.