diff --git a/src/main/java/info/debatty/java/stringsimilarity/Damerau.java b/src/main/java/info/debatty/java/stringsimilarity/Damerau.java
index b6fe914..419c86f 100644
--- a/src/main/java/info/debatty/java/stringsimilarity/Damerau.java
+++ b/src/main/java/info/debatty/java/stringsimilarity/Damerau.java
@@ -57,13 +57,7 @@ public class Damerau implements MetricStringDistance {
      */
     public final double distance(final String s1, final String s2) {
 
-        if (s1 == null) {
-            throw new NullPointerException("s1 must not be null");
-        }
-
-        if (s2 == null) {
-            throw new NullPointerException("s2 must not be null");
-        }
+        StringUtils.requireNonNull(s1, s2);
 
         if (s1.equals(s2)) {
             return 0;
diff --git a/src/main/java/info/debatty/java/stringsimilarity/Jaccard.java b/src/main/java/info/debatty/java/stringsimilarity/Jaccard.java
index 84417ec..7c6a678 100644
--- a/src/main/java/info/debatty/java/stringsimilarity/Jaccard.java
+++ b/src/main/java/info/debatty/java/stringsimilarity/Jaccard.java
@@ -76,13 +76,7 @@ public Jaccard() {
      * @throws NullPointerException if s1 or s2 is null.
      */
     public final double similarity(final String s1, final String s2) {
-        if (s1 == null) {
-            throw new NullPointerException("s1 must not be null");
-        }
-
-        if (s2 == null) {
-            throw new NullPointerException("s2 must not be null");
-        }
+        StringUtils.requireNonNull(s1, s2);
 
         if (s1.equals(s2)) {
             return 1;
diff --git a/src/main/java/info/debatty/java/stringsimilarity/JaroWinkler.java b/src/main/java/info/debatty/java/stringsimilarity/JaroWinkler.java
index 267cbb4..726b276 100644
--- a/src/main/java/info/debatty/java/stringsimilarity/JaroWinkler.java
+++ b/src/main/java/info/debatty/java/stringsimilarity/JaroWinkler.java
@@ -22,8 +22,13 @@ public class JaroWinkler
         implements NormalizedStringSimilarity, NormalizedStringDistance {
 
     private static final double DEFAULT_THRESHOLD = 0.7;
-    private static final int THREE = 3;
-    private static final double JW_COEF = 0.1;
+    private static final int IDX_MATCHES       = 0;
+    private static final int IDX_TRANSPOSITIONS = 1;
+    private static final int IDX_PREFIX        = 2;
+    private static final int IDX_MAX_LEN       = 3;
+    private static final double JW_COEF        = 0.1;
+    private static final int    JARO_DIVISOR   = 3;
+
     private final double threshold;
 
     /**
@@ -62,29 +67,27 @@ public final double getThreshold() {
      * @throws NullPointerException if s1 or s2 is null.
      */
     public final double similarity(final String s1, final String s2) {
-        if (s1 == null) {
-            throw new NullPointerException("s1 must not be null");
-        }
-
-        if (s2 == null) {
-            throw new NullPointerException("s2 must not be null");
-        }
+        StringUtils.requireNonNull(s1, s2);
 
         if (s1.equals(s2)) {
             return 1;
         }
 
         int[] mtp = matches(s1, s2);
-        float m = mtp[0];
+        float m = mtp[IDX_MATCHES];
         if (m == 0) {
             return 0f;
         }
-        double j = ((m / s1.length() + m / s2.length() + (m - mtp[1]) / m))
-                / THREE;
+        double j = (  m / s1.length()
+                + m / s2.length()
+                + (m - mtp[IDX_TRANSPOSITIONS]) / m
+        ) / JARO_DIVISOR;
+
         double jw = j;
 
         if (j > getThreshold()) {
-            jw = j + Math.min(JW_COEF, 1.0 / mtp[THREE]) * mtp[2] * (1 - j);
+            jw = j + Math.min(JW_COEF, 1.0 / mtp[IDX_MAX_LEN])
+                    * mtp[IDX_PREFIX] * (1 - j);
         }
         return jw;
     }
diff --git a/src/main/java/info/debatty/java/stringsimilarity/Levenshtein.java b/src/main/java/info/debatty/java/stringsimilarity/Levenshtein.java
index d2ba4fb..25eb791 100644
--- a/src/main/java/info/debatty/java/stringsimilarity/Levenshtein.java
+++ b/src/main/java/info/debatty/java/stringsimilarity/Levenshtein.java
@@ -52,13 +52,7 @@ public final double distance(final String s1, final String s2) {
      */
     public final double distance(final String s1, final String s2,
                                  final int limit) {
-        if (s1 == null) {
-            throw new NullPointerException("s1 must not be null");
-        }
-
-        if (s2 == null) {
-            throw new NullPointerException("s2 must not be null");
-        }
+        StringUtils.requireNonNull(s1, s2);
 
         if (s1.equals(s2)) {
             return 0;
@@ -111,8 +105,7 @@ public final double distance(final String s1, final String s2,
                 return limit;
             }
 
-            // copy v1 (current row) to v0 (previous row) for next iteration
-            //System.arraycopy(v1, 0, v0, 0, v0.length);
+
 
             // Flip references to current and previous row
             vtemp = v0;
diff --git a/src/main/java/info/debatty/java/stringsimilarity/LongestCommonSubsequence.java b/src/main/java/info/debatty/java/stringsimilarity/LongestCommonSubsequence.java
index e8d091a..36452a4 100644
--- a/src/main/java/info/debatty/java/stringsimilarity/LongestCommonSubsequence.java
+++ b/src/main/java/info/debatty/java/stringsimilarity/LongestCommonSubsequence.java
@@ -37,13 +37,7 @@ public class LongestCommonSubsequence implements StringDistance {
      * @throws NullPointerException if s1 or s2 is null.
      */
     public final double distance(final String s1, final String s2) {
-        if (s1 == null) {
-            throw new NullPointerException("s1 must not be null");
-        }
-
-        if (s2 == null) {
-            throw new NullPointerException("s2 must not be null");
-        }
+        StringUtils.requireNonNull(s1, s2);
 
         if (s1.equals(s2)) {
             return 0;
diff --git a/src/main/java/info/debatty/java/stringsimilarity/MetricLCS.java b/src/main/java/info/debatty/java/stringsimilarity/MetricLCS.java
index ab4cc61..f503bb4 100644
--- a/src/main/java/info/debatty/java/stringsimilarity/MetricLCS.java
+++ b/src/main/java/info/debatty/java/stringsimilarity/MetricLCS.java
@@ -50,13 +50,7 @@ public class MetricLCS
      * @throws NullPointerException if s1 or s2 is null.
      */
     public final double distance(final String s1, final String s2) {
-        if (s1 == null) {
-            throw new NullPointerException("s1 must not be null");
-        }
-
-        if (s2 == null) {
-            throw new NullPointerException("s2 must not be null");
-        }
+        StringUtils.requireNonNull(s1, s2);
 
         if (s1.equals(s2)) {
             return 0;
diff --git a/src/main/java/info/debatty/java/stringsimilarity/NGram.java b/src/main/java/info/debatty/java/stringsimilarity/NGram.java
index c2a5f2d..8562bf6 100644
--- a/src/main/java/info/debatty/java/stringsimilarity/NGram.java
+++ b/src/main/java/info/debatty/java/stringsimilarity/NGram.java
@@ -43,13 +43,7 @@ public NGram() {
      * @throws NullPointerException if s0 or s1 is null.
      */
     public final double distance(final String s0, final String s1) {
-        if (s0 == null) {
-            throw new NullPointerException("s0 must not be null");
-        }
-
-        if (s1 == null) {
-            throw new NullPointerException("s1 must not be null");
-        }
+        StringUtils.requireNonNull(s0, s1);
 
         if (s0.equals(s1)) {
             return 0;
diff --git a/src/main/java/info/debatty/java/stringsimilarity/NormalizedLevenshtein.java b/src/main/java/info/debatty/java/stringsimilarity/NormalizedLevenshtein.java
index 49eaa21..a020fd2 100644
--- a/src/main/java/info/debatty/java/stringsimilarity/NormalizedLevenshtein.java
+++ b/src/main/java/info/debatty/java/stringsimilarity/NormalizedLevenshtein.java
@@ -50,13 +50,7 @@ public class NormalizedLevenshtein implements
      */
     public final double distance(final String s1, final String s2) {
 
-        if (s1 == null) {
-            throw new NullPointerException("s1 must not be null");
-        }
-
-        if (s2 == null) {
-            throw new NullPointerException("s2 must not be null");
-        }
+        StringUtils.requireNonNull(s1, s2);
 
         if (s1.equals(s2)) {
             return 0;
diff --git a/src/main/java/info/debatty/java/stringsimilarity/OptimalStringAlignment.java b/src/main/java/info/debatty/java/stringsimilarity/OptimalStringAlignment.java
index f7fbba5..32d2c0a 100644
--- a/src/main/java/info/debatty/java/stringsimilarity/OptimalStringAlignment.java
+++ b/src/main/java/info/debatty/java/stringsimilarity/OptimalStringAlignment.java
@@ -52,13 +52,7 @@ public final class OptimalStringAlignment implements StringDistance {
      * @throws NullPointerException if s1 or s2 is null.
      */
     public double distance(final String s1, final String s2) {
-        if (s1 == null) {
-            throw new NullPointerException("s1 must not be null");
-        }
-
-        if (s2 == null) {
-            throw new NullPointerException("s2 must not be null");
-        }
+        StringUtils.requireNonNull(s1, s2);
 
         if (s1.equals(s2)) {
             return 0;
diff --git a/src/main/java/info/debatty/java/stringsimilarity/QGram.java b/src/main/java/info/debatty/java/stringsimilarity/QGram.java
index ae6f9fc..41c9d13 100644
--- a/src/main/java/info/debatty/java/stringsimilarity/QGram.java
+++ b/src/main/java/info/debatty/java/stringsimilarity/QGram.java
@@ -60,13 +60,7 @@ public QGram() {
      * @throws NullPointerException if s1 or s2 is null.
      */
     public final double distance(final String s1, final String s2) {
-        if (s1 == null) {
-            throw new NullPointerException("s1 must not be null");
-        }
-
-        if (s2 == null) {
-            throw new NullPointerException("s2 must not be null");
-        }
+        StringUtils.requireNonNull(s1, s2);
 
         if (s1.equals(s2)) {
             return 0;
diff --git a/src/main/java/info/debatty/java/stringsimilarity/RatcliffObershelp.java b/src/main/java/info/debatty/java/stringsimilarity/RatcliffObershelp.java
index 22e4ea3..c015676 100644
--- a/src/main/java/info/debatty/java/stringsimilarity/RatcliffObershelp.java
+++ b/src/main/java/info/debatty/java/stringsimilarity/RatcliffObershelp.java
@@ -57,13 +57,7 @@ public class RatcliffObershelp implements
      */
     @Override
     public final double similarity(final String s1, final String s2) {
-        if (s1 == null) {
-            throw new NullPointerException("s1 must not be null");
-        }
-
-        if (s2 == null) {
-            throw new NullPointerException("s2 must not be null");
-        }
+        StringUtils.requireNonNull(s1, s2);
 
         if (s1.equals(s2)) {
             return 1.0d;
diff --git a/src/main/java/info/debatty/java/stringsimilarity/ShingleBased.java b/src/main/java/info/debatty/java/stringsimilarity/ShingleBased.java
index 24017e2..2d103bc 100644
--- a/src/main/java/info/debatty/java/stringsimilarity/ShingleBased.java
+++ b/src/main/java/info/debatty/java/stringsimilarity/ShingleBased.java
@@ -77,7 +77,7 @@ public ShingleBased(final int k) {
     /**
      *
      */
-    ShingleBased() {
+    protected ShingleBased() {
         this(DEFAULT_K);
     }
 
diff --git a/src/main/java/info/debatty/java/stringsimilarity/SorensenDice.java b/src/main/java/info/debatty/java/stringsimilarity/SorensenDice.java
index dec376b..c7710f9 100644
--- a/src/main/java/info/debatty/java/stringsimilarity/SorensenDice.java
+++ b/src/main/java/info/debatty/java/stringsimilarity/SorensenDice.java
@@ -78,13 +78,7 @@ public SorensenDice() {
      * @throws NullPointerException if s1 or s2 is null.
      */
     public final double similarity(final String s1, final String s2) {
-        if (s1 == null) {
-            throw new NullPointerException("s1 must not be null");
-        }
-
-        if (s2 == null) {
-            throw new NullPointerException("s2 must not be null");
-        }
+        StringUtils.requireNonNull(s1, s2);
 
         if (s1.equals(s2)) {
             return 1;
diff --git a/src/main/java/info/debatty/java/stringsimilarity/StringUtils.java b/src/main/java/info/debatty/java/stringsimilarity/StringUtils.java
new file mode 100644
index 0000000..bee31e8
--- /dev/null
+++ b/src/main/java/info/debatty/java/stringsimilarity/StringUtils.java
@@ -0,0 +1,17 @@
+package info.debatty.java.stringsimilarity;
+
+/**
+ * Shared validation helpers for all similarity/distance methods.
+ */
+
+
+public class StringUtils {
+    private StringUtils() {}  // utility class — no instantiation
+
+
+    static void requireNonNull(String s1, String s2) {
+        if (s1 == null) throw new NullPointerException("s1 must not be null");
+        if (s2 == null) throw new NullPointerException("s2 must not be null");
+    }
+
+}
diff --git a/src/main/java/info/debatty/java/stringsimilarity/WeightedLevenshtein.java b/src/main/java/info/debatty/java/stringsimilarity/WeightedLevenshtein.java
index 69413e2..6e018b1 100644
--- a/src/main/java/info/debatty/java/stringsimilarity/WeightedLevenshtein.java
+++ b/src/main/java/info/debatty/java/stringsimilarity/WeightedLevenshtein.java
@@ -80,13 +80,7 @@ public final double distance(final String s1, final String s2) {
      */
     public final double distance(final String s1, final String s2,
                                  final double limit) {
-        if (s1 == null) {
-            throw new NullPointerException("s1 must not be null");
-        }
-
-        if (s2 == null) {
-            throw new NullPointerException("s2 must not be null");
-        }
+        StringUtils.requireNonNull(s1, s2);
 
         if (s1.equals(s2)) {
             return 0;
diff --git a/src/main/java/info/debatty/java/stringsimilarity/experimental/Sift4.java b/src/main/java/info/debatty/java/stringsimilarity/experimental/Sift4.java
index 29a8946..d569c69 100644
--- a/src/main/java/info/debatty/java/stringsimilarity/experimental/Sift4.java
+++ b/src/main/java/info/debatty/java/stringsimilarity/experimental/Sift4.java
@@ -24,6 +24,8 @@
 package info.debatty.java.stringsimilarity.experimental;
 
 import info.debatty.java.stringsimilarity.interfaces.StringDistance;
+
+import java.util.Iterator;
 import java.util.LinkedList;
 
 /**
@@ -62,34 +64,52 @@ public final void setMaxOffset(final int max_offset) {
      * @param s2
      * @return
      */
+
+    // AFTER — Offset moved to private static nested class
+    private static final class Offset {
+        final int c1;
+        final int c2;
+        boolean trans;   // mutable — documented intentionally
+        Offset(int c1, int c2, boolean trans) {
+            this.c1 = c1; this.c2 = c2; this.trans = trans;
+        }
+    }
+
+    /** Validates that neither string is null, consistent with all other algorithms. */
+    private static void validateInputs(String s1, String s2) {
+        if (s1==null) throw new NullPointerException("s1 must not be null/NULL");
+        if (s2==null) throw new NullPointerException("s2 must not be null/NULL"); // added null handling
+    }
+
+    /** Resolves whether a new match at (c1, c2) is a transposition. */
+    private boolean resolveTransposition(
+            int c1, int c2, LinkedList<Offset> offsetArr, int[] trans) {
+        boolean isTrans = false;
+        Iterator<Offset> it = offsetArr.iterator();
+        while (it.hasNext()) {
+            Offset ofs = it.next();
+            if (c1 <= ofs.c1 || c2 <= ofs.c2) {
+                isTrans = Math.abs(c2 - c1) >= Math.abs(ofs.c2 - ofs.c1);
+                if (isTrans) { trans[0]++; }
+                else if (!ofs.trans) { ofs.trans = true; trans[0]++; }
+                break;
+            } else if (c1 > ofs.c2 && c2 > ofs.c1) { it.remove(); }
+        }
+        return isTrans;
+    }
+
     public final double distance(final String s1, final String s2) {
 
         /**
          * Used to store relation between same character in different positions
          * c1 and c2 in the input strings.
          */
-        class Offset {
-
-            private final int c1;
-            private final int c2;
-            private boolean trans;
-
-            Offset(final int c1, final int c2, final boolean trans) {
-                this.c1 = c1;
-                this.c2 = c2;
-                this.trans = trans;
-            }
-        }
-
-        if (s1 == null || s1.isEmpty()) {
-            if (s2 == null) {
-                return 0;
-            }
 
+        if (s1.isEmpty()) {
             return s2.length();
         }
 
-        if (s2 == null || s2.isEmpty()) {
+        if (s2.isEmpty()) {
             return s1.length();
         }