I think should check value before set it to bits.

src/main/java/com/clearspring/analytics/stream/cardinality/RegisterSet.java

@@ -57,6 +57,7 @@ public static int getSizeForCount(int count) {
     }
 
     public void set(int position, int value) {
+        value = value & 0x1F; //make sure value would not longger than REGISTER_SIZE
         int bucketPos = position / LOG2_BITS_PER_WORD;
         int shift = REGISTER_SIZE * (position - (bucketPos * LOG2_BITS_PER_WORD));
         this.M[bucketPos] = (this.M[bucketPos] & ~(0x1f << shift)) | (value << shift);
@@ -69,6 +70,7 @@ public int get(int position) {
     }
 
     public boolean updateIfGreater(int position, int value) {
+        value = value & 0x1F; //make sure value would not longger than REGISTER_SIZE
         int bucket = position / LOG2_BITS_PER_WORD;
         int shift = REGISTER_SIZE * (position - (bucket * LOG2_BITS_PER_WORD));
         int mask = 0x1f << shift;

src/main/java/com/clearspring/analytics/stream/membership/BloomFilter.java

@@ -178,6 +178,17 @@ public static BloomFilter deserialize(byte[] bytes) {
 
         return filter;
     }
+    /**
+     * this method return the cardinality estimation of current bloom filter
+     * base on article "cardinality estimation for dynamic bloomfilter"
+     * */
+    public long getCardinality(){
+        int nhash = this.getHashCount();
+        int mbits = this.buckets();
+
+        int bitsUsed = this.buckets() - this.emptyBuckets();
+        return Math.round(1.0/nhash * Math.log(1.0-1.0*bitsUsed/mbits)/Math.log(1-1.0/mbits));
+    }
 }
 
 class BloomFilterSerializer implements ICompactSerializer<BloomFilter> {

src/test/java/com/clearspring/analytics/stream/cardinality/RegisterSetTest.java

@@ -28,7 +28,14 @@ public class RegisterSetTest {
     public void testGetAndSet() throws Exception {
         RegisterSet rs = new RegisterSet((int) Math.pow(2, 4));
         rs.set(0, 11);
+        rs.set(1, 11);
+        rs.set(2, 11);
         assertEquals(11, rs.get(0));
+        assertEquals(11, rs.get(1));
+        assertEquals(11, rs.get(2));
+        rs.set(0, 0xFF);
+        assertEquals(11, rs.get(1));
+        assertEquals(11, rs.get(2));
     }
 
     @Test

src/test/java/com/clearspring/analytics/stream/membership/BloomFilterTest.java

@@ -27,6 +27,7 @@
 import java.util.Set;
 import java.util.UUID;
 
+import com.clearspring.analytics.TestUtils;
 import com.clearspring.analytics.stream.membership.KeyGenerator.RandomStringGenerator;
 
 import org.junit.Before;
@@ -56,7 +57,15 @@ public BloomFilterTest() {
     public void clear() {
         bf.clear();
     }
-
+    @Test
+    public void testCardinality(){
+        Random r = new Random();
+        for(int i =0;i<7799;i++){
+            String str = Integer.toHexString(r.nextInt());
+            bf.add(str);
+        }
+        System.out.println(bf.getCardinality());
+    }
     @Test
     public void testOne() {
         bf.add("a");