Bloom Filter (Java)

A Bloom filter is a space-efficient probabilistic data structure used to test whether an element is a member of a set.
It can return false positives but never false negatives, meaning a query will return either:

• Possibly in the set, or
• Definitely not in the set

This implementation uses an array of AtomicBoolean for thread-safe operations.

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Features

• Add elements to the filter.
• Check membership (absence = definitely not in set).
• Remove elements (non-standard; may cause false negatives).
• Count the number of bits currently set.
• Thread-safe using AtomicBoolean[].

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How It Works

1. When an element is added, the bit corresponding to its filterValue is set to true.
2. Membership check reads the bit at the same index:
   • 0 → definitely not present
   • 1 → possibly present (false positives possible)
3. remove() clears the bit, but this can introduce false negatives and is non-standard.

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Usage Example

import org.bloomfilter.BloomFilter;
import org.bloomfilter.BloomFilterValue;

class MyValue implements BloomFilterValue {
    private final int value;

    MyValue(int value) {
        this.value = value;
    }

    @Override
    public int getFilterValue() {
        return value % 100; // simple example
    }
}

public class Main {
    public static void main(String[] args) {
        BloomFilter<MyValue> filter = new BloomFilter<>(100);
        MyValue item = new MyValue(42);

        filter.add(item);
        System.out.println(filter.isAbsent(item)); // false
        System.out.println("Values set: " + filter.valuesCount());

        filter.remove(42);
        System.out.println(filter.isAbsent(item)); // true
    }
}