diff --git a/pmd-java/src/main/java/net/sourceforge/pmd/lang/java/oom/AbstractClassMetric.java b/pmd-java/src/main/java/net/sourceforge/pmd/lang/java/oom/AbstractClassMetric.java
index 8eae08f6b6..bf88ebb3a4 100644
--- a/pmd-java/src/main/java/net/sourceforge/pmd/lang/java/oom/AbstractClassMetric.java
+++ b/pmd-java/src/main/java/net/sourceforge/pmd/lang/java/oom/AbstractClassMetric.java
@@ -12,6 +12,7 @@ import net.sourceforge.pmd.lang.java.ast.ASTClassOrInterfaceBodyDeclaration;
 import net.sourceforge.pmd.lang.java.ast.ASTClassOrInterfaceDeclaration;
 import net.sourceforge.pmd.lang.java.ast.ASTMethodOrConstructorDeclaration;
 import net.sourceforge.pmd.lang.java.oom.interfaces.ClassMetric;
+import net.sourceforge.pmd.lang.java.oom.interfaces.OperationMetricKey;
 
 /**
  * Shares common logic across class metrics. Many class metrics actually sum the metric over the operations of
diff --git a/pmd-java/src/main/java/net/sourceforge/pmd/lang/java/oom/ClassStats.java b/pmd-java/src/main/java/net/sourceforge/pmd/lang/java/oom/ClassStats.java
index 29fae6d31c..3f5cc66017 100644
--- a/pmd-java/src/main/java/net/sourceforge/pmd/lang/java/oom/ClassStats.java
+++ b/pmd-java/src/main/java/net/sourceforge/pmd/lang/java/oom/ClassStats.java
@@ -13,7 +13,9 @@ import net.sourceforge.pmd.lang.java.ast.ASTClassOrInterfaceDeclaration;
 import net.sourceforge.pmd.lang.java.ast.ASTMethodOrConstructorDeclaration;
 import net.sourceforge.pmd.lang.java.ast.QualifiedName;
 import net.sourceforge.pmd.lang.java.oom.interfaces.ClassMetric;
+import net.sourceforge.pmd.lang.java.oom.interfaces.ClassMetricKey;
 import net.sourceforge.pmd.lang.java.oom.interfaces.MetricVersion;
+import net.sourceforge.pmd.lang.java.oom.interfaces.OperationMetricKey;
 import net.sourceforge.pmd.lang.java.oom.signature.FieldSigMask;
 import net.sourceforge.pmd.lang.java.oom.signature.FieldSignature;
 import net.sourceforge.pmd.lang.java.oom.signature.OperationSigMask;
@@ -152,7 +154,7 @@ import net.sourceforge.pmd.lang.java.oom.signature.OperationSignature;
      * @return The result of the computation, or {@code Double.NaN} if it couldn't be performed.
      */
     /* default */ double compute(OperationMetricKey key, ASTMethodOrConstructorDeclaration node, String name,
-                                  boolean force, MetricVersion version) {
+                                 boolean force, MetricVersion version) {
         Map<String, OperationStats> sigMap = operations.get(OperationSignature.buildFor(node));
         // TODO:cf the operation signature will be built many times, we might as well store it in the node
 
@@ -174,7 +176,7 @@ import net.sourceforge.pmd.lang.java.oom.signature.OperationSignature;
      * @return The result of the computation, or {@code Double.NaN} if it couldn't be performed.
      */
     /* default */ double compute(ClassMetricKey key, ASTClassOrInterfaceDeclaration node, boolean force,
-                                  MetricVersion version) {
+                                 MetricVersion version) {
         ParameterizedMetricKey paramKey = ParameterizedMetricKey.build(key, version);
         // if memo.get(key) == null then the metric has never been computed. NaN is a valid value.
         Double prev = memo.get(paramKey);
diff --git a/pmd-java/src/main/java/net/sourceforge/pmd/lang/java/oom/Metrics.java b/pmd-java/src/main/java/net/sourceforge/pmd/lang/java/oom/Metrics.java
index 0c2f0aa93c..1f94b4d42e 100644
--- a/pmd-java/src/main/java/net/sourceforge/pmd/lang/java/oom/Metrics.java
+++ b/pmd-java/src/main/java/net/sourceforge/pmd/lang/java/oom/Metrics.java
@@ -7,8 +7,10 @@ package net.sourceforge.pmd.lang.java.oom;
 
 import net.sourceforge.pmd.lang.java.ast.ASTClassOrInterfaceDeclaration;
 import net.sourceforge.pmd.lang.java.ast.ASTMethodOrConstructorDeclaration;
+import net.sourceforge.pmd.lang.java.oom.interfaces.ClassMetricKey;
 import net.sourceforge.pmd.lang.java.oom.interfaces.Metric.Version;
 import net.sourceforge.pmd.lang.java.oom.interfaces.MetricVersion;
+import net.sourceforge.pmd.lang.java.oom.interfaces.OperationMetricKey;
 
 
 /**
@@ -33,6 +35,7 @@ public final class Metrics {
         return TOP_LEVEL_PACKAGE;
     }
 
+
     /**
      * Computes a metric identified by its code on a class AST node.
      *
@@ -46,6 +49,7 @@ public final class Metrics {
         return TOP_LEVEL_PACKAGE.compute(key, node, false, Version.STANDARD);
     }
 
+
     /**
      * Computes a metric identified by its code on a operation AST node.
      *
@@ -59,14 +63,34 @@ public final class Metrics {
         return TOP_LEVEL_PACKAGE.compute(key, node, false, Version.STANDARD);
     }
 
-    public static double get(OperationMetricKey key, ASTMethodOrConstructorDeclaration node, MetricVersion option) {
-        MetricVersion safeOption = (option == null) ? Version.STANDARD : option;
+
+    /**
+     * Computes a metric identified by its code on a class AST node.
+     *
+     * @param key     The key identifying the metric to be computed
+     * @param node    The node on which to compute the metric
+     * @param version The version of the metric.
+     *
+     * @return The value of the metric, or {@code Double.NaN} if the value couln't be computed.
+     */
+    public static double get(ClassMetricKey key, ASTClassOrInterfaceDeclaration node, MetricVersion version) {
+        MetricVersion safeOption = (version == null) ? Version.STANDARD : version;
 
         return TOP_LEVEL_PACKAGE.compute(key, node, false, safeOption);
     }
 
-    public static double get(ClassMetricKey key, ASTClassOrInterfaceDeclaration node, MetricVersion option) {
-        MetricVersion safeOption = (option == null) ? Version.STANDARD : option;
+
+    /**
+     * Computes a metric identified by its code on a operation AST node.
+     *
+     * @param key     The key identifying the metric to be computed
+     * @param node    The node on which to compute the metric
+     * @param version The version of the metric.
+     *
+     * @return The value of the metric, or {@code Double.NaN} if the value couln't be computed.
+     */
+    public static double get(OperationMetricKey key, ASTMethodOrConstructorDeclaration node, MetricVersion version) {
+        MetricVersion safeOption = (version == null) ? Version.STANDARD : version;
 
         return TOP_LEVEL_PACKAGE.compute(key, node, false, safeOption);
     }
diff --git a/pmd-java/src/main/java/net/sourceforge/pmd/lang/java/oom/OperationStats.java b/pmd-java/src/main/java/net/sourceforge/pmd/lang/java/oom/OperationStats.java
index 4092739c30..a1a624502f 100644
--- a/pmd-java/src/main/java/net/sourceforge/pmd/lang/java/oom/OperationStats.java
+++ b/pmd-java/src/main/java/net/sourceforge/pmd/lang/java/oom/OperationStats.java
@@ -10,6 +10,7 @@ import java.util.Map;
 import net.sourceforge.pmd.lang.java.ast.ASTMethodOrConstructorDeclaration;
 import net.sourceforge.pmd.lang.java.oom.interfaces.MetricVersion;
 import net.sourceforge.pmd.lang.java.oom.interfaces.OperationMetric;
+import net.sourceforge.pmd.lang.java.oom.interfaces.OperationMetricKey;
 
 
 /**
diff --git a/pmd-java/src/main/java/net/sourceforge/pmd/lang/java/oom/PackageStats.java b/pmd-java/src/main/java/net/sourceforge/pmd/lang/java/oom/PackageStats.java
index 8932ab4c1e..66157c2f46 100644
--- a/pmd-java/src/main/java/net/sourceforge/pmd/lang/java/oom/PackageStats.java
+++ b/pmd-java/src/main/java/net/sourceforge/pmd/lang/java/oom/PackageStats.java
@@ -10,7 +10,9 @@ import java.util.Map;
 import net.sourceforge.pmd.lang.java.ast.ASTClassOrInterfaceDeclaration;
 import net.sourceforge.pmd.lang.java.ast.ASTMethodOrConstructorDeclaration;
 import net.sourceforge.pmd.lang.java.ast.QualifiedName;
+import net.sourceforge.pmd.lang.java.oom.interfaces.ClassMetricKey;
 import net.sourceforge.pmd.lang.java.oom.interfaces.MetricVersion;
+import net.sourceforge.pmd.lang.java.oom.interfaces.OperationMetricKey;
 import net.sourceforge.pmd.lang.java.oom.signature.FieldSigMask;
 import net.sourceforge.pmd.lang.java.oom.signature.OperationSigMask;
 
diff --git a/pmd-java/src/main/java/net/sourceforge/pmd/lang/java/oom/ParameterizedMetricKey.java b/pmd-java/src/main/java/net/sourceforge/pmd/lang/java/oom/ParameterizedMetricKey.java
index aa137e98f0..f66971252f 100644
--- a/pmd-java/src/main/java/net/sourceforge/pmd/lang/java/oom/ParameterizedMetricKey.java
+++ b/pmd-java/src/main/java/net/sourceforge/pmd/lang/java/oom/ParameterizedMetricKey.java
@@ -7,11 +7,12 @@ package net.sourceforge.pmd.lang.java.oom;
 import java.util.HashMap;
 import java.util.Map;
 
+import net.sourceforge.pmd.lang.java.oom.interfaces.Metric;
 import net.sourceforge.pmd.lang.java.oom.interfaces.MetricKey;
 import net.sourceforge.pmd.lang.java.oom.interfaces.MetricVersion;
 
 /**
- * Represents a key parameterized with its options. Used to index memoization maps.
+ * Represents a key parameterized with its version. Used to index memoization maps.
  *
  * @author Clément Fournier
  */
@@ -20,18 +21,18 @@ public final class ParameterizedMetricKey {
     private static final Map<Integer, ParameterizedMetricKey> POOL = new HashMap<>();
 
     /** The metric key. */
-    public final MetricKey key;
+    public final MetricKey<? extends Metric> key;
     /** The version of the metric. */
     public final MetricVersion version;
 
     /** Used internally by the pooler. */
-    private ParameterizedMetricKey(MetricKey key, MetricVersion version) {
+    private ParameterizedMetricKey(MetricKey<? extends Metric> key, MetricVersion version) {
         this.key = key;
         this.version = version;
     }
 
     /** Builds a parameterized metric key. */
-    public static ParameterizedMetricKey build(MetricKey key, MetricVersion version) {
+    public static ParameterizedMetricKey build(MetricKey<? extends Metric> key, MetricVersion version) {
         int code = code(key, version);
         ParameterizedMetricKey paramKey = POOL.get(code);
         if (paramKey == null) {
diff --git a/pmd-java/src/main/java/net/sourceforge/pmd/lang/java/oom/ClassMetricKey.java b/pmd-java/src/main/java/net/sourceforge/pmd/lang/java/oom/interfaces/ClassMetricKey.java
similarity index 77%
rename from pmd-java/src/main/java/net/sourceforge/pmd/lang/java/oom/ClassMetricKey.java
rename to pmd-java/src/main/java/net/sourceforge/pmd/lang/java/oom/interfaces/ClassMetricKey.java
index f6b3cd25cd..23f11dac94 100644
--- a/pmd-java/src/main/java/net/sourceforge/pmd/lang/java/oom/ClassMetricKey.java
+++ b/pmd-java/src/main/java/net/sourceforge/pmd/lang/java/oom/interfaces/ClassMetricKey.java
@@ -2,12 +2,11 @@
  * BSD-style license; for more info see http://pmd.sourceforge.net/license.html
  */
 
-package net.sourceforge.pmd.lang.java.oom;
+package net.sourceforge.pmd.lang.java.oom.interfaces;
 
-import net.sourceforge.pmd.lang.java.oom.interfaces.ClassMetric;
-import net.sourceforge.pmd.lang.java.oom.interfaces.MetricKey;
 import net.sourceforge.pmd.lang.java.oom.metrics.AtfdMetric;
 import net.sourceforge.pmd.lang.java.oom.metrics.CycloMetric;
+import net.sourceforge.pmd.lang.java.oom.metrics.NcssMetric;
 import net.sourceforge.pmd.lang.java.oom.metrics.WmcMetric;
 
 /**
@@ -19,7 +18,9 @@ public enum ClassMetricKey implements MetricKey<ClassMetric> {
     /** Weighed Method Count. */
     WMC(new WmcMetric()),
     /** Cyclomatic complexity. */
-    CYCLO(new CycloMetric());
+    CYCLO(new CycloMetric()),
+    /** Non Commenting Source Statements. */
+    NCSS(new NcssMetric());
 
     private final ClassMetric calculator;
 
diff --git a/pmd-java/src/main/java/net/sourceforge/pmd/lang/java/oom/OperationMetricKey.java b/pmd-java/src/main/java/net/sourceforge/pmd/lang/java/oom/interfaces/OperationMetricKey.java
similarity index 82%
rename from pmd-java/src/main/java/net/sourceforge/pmd/lang/java/oom/OperationMetricKey.java
rename to pmd-java/src/main/java/net/sourceforge/pmd/lang/java/oom/interfaces/OperationMetricKey.java
index a912bdd526..91cb42e1fc 100644
--- a/pmd-java/src/main/java/net/sourceforge/pmd/lang/java/oom/OperationMetricKey.java
+++ b/pmd-java/src/main/java/net/sourceforge/pmd/lang/java/oom/interfaces/OperationMetricKey.java
@@ -2,10 +2,8 @@
  * BSD-style license; for more info see http://pmd.sourceforge.net/license.html
  */
 
-package net.sourceforge.pmd.lang.java.oom;
+package net.sourceforge.pmd.lang.java.oom.interfaces;
 
-import net.sourceforge.pmd.lang.java.oom.interfaces.MetricKey;
-import net.sourceforge.pmd.lang.java.oom.interfaces.OperationMetric;
 import net.sourceforge.pmd.lang.java.oom.metrics.AtfdMetric;
 import net.sourceforge.pmd.lang.java.oom.metrics.CycloMetric;
 import net.sourceforge.pmd.lang.java.oom.metrics.NcssMetric;
@@ -19,6 +17,7 @@ public enum OperationMetricKey implements MetricKey<OperationMetric> {
     ATFD(new AtfdMetric()),
     /** Cyclometric complexity. */
     CYCLO(new CycloMetric()),
+    /** Non Commenting Source Statements. */
     NCSS(new NcssMetric());
 
     private final OperationMetric calculator;
diff --git a/pmd-java/src/main/java/net/sourceforge/pmd/lang/java/oom/metrics/CycloMetric.java b/pmd-java/src/main/java/net/sourceforge/pmd/lang/java/oom/metrics/CycloMetric.java
index 86504f76e0..47e0e456a5 100644
--- a/pmd-java/src/main/java/net/sourceforge/pmd/lang/java/oom/metrics/CycloMetric.java
+++ b/pmd-java/src/main/java/net/sourceforge/pmd/lang/java/oom/metrics/CycloMetric.java
@@ -9,19 +9,18 @@ import org.apache.commons.lang3.mutable.MutableInt;
 import net.sourceforge.pmd.lang.java.ast.ASTClassOrInterfaceDeclaration;
 import net.sourceforge.pmd.lang.java.ast.ASTMethodOrConstructorDeclaration;
 import net.sourceforge.pmd.lang.java.oom.AbstractClassMetric;
-import net.sourceforge.pmd.lang.java.oom.OperationMetricKey;
-import net.sourceforge.pmd.lang.java.oom.interfaces.ClassMetric;
 import net.sourceforge.pmd.lang.java.oom.interfaces.MetricVersion;
 import net.sourceforge.pmd.lang.java.oom.interfaces.OperationMetric;
+import net.sourceforge.pmd.lang.java.oom.interfaces.OperationMetricKey;
 import net.sourceforge.pmd.lang.java.oom.metrics.cyclo.CycloPathUnawareOperationVisitor;
 import net.sourceforge.pmd.lang.java.oom.metrics.cyclo.CycloVisitor;
 import net.sourceforge.pmd.lang.java.oom.metrics.cyclo.StandardCycloVisitor;
 
 /**
- * McCabe's Cyclomatic Complexity. Number of independent paths through a block of code. Formally, given that the control
- * flow graph of a method has n edges, m nodes and p connected components, the Cyclomatic complexity of the method is
- * given by CYCLO = n - m + 2p. In practice it can be calculated by counting control flow statements following the
- * standard rules given below.
+ * McCabe's Cyclomatic Complexity. Number of independent paths through a block of code [1, 2]. Formally, given that the
+ * control flow graph of the block has n vertices, e edges and p connected components, the Cyclomatic complexity of the
+ * block is given by {@code CYCLO = e - n + 2p} [2]. In practice it can be calculated by counting control flow
+ * statements following the standard rules given below.
  *
  * <p>The standard version of the metric complies with McCabe's original definition:
  *
@@ -35,15 +34,19 @@ import net.sourceforge.pmd.lang.java.oom.metrics.cyclo.StandardCycloVisitor;
  * statement in itself.
  * </ul>
  *
- * <p> Version {@link Version#DO_NOT_COUNT_EXPRESSION_PATHS}: Boolean operators are not counted, which means that empty
+ * <p>Version {@link Version#DO_NOT_COUNT_EXPRESSION_PATHS}: Boolean operators are not counted, which means that empty
  * fall-through cases in {@code switch} statements are not counted as well.
  *
- * <p>[1] Lanza. Object-Oriented Metrics in Practice.
+ * <p>References:
+ * <ul>
+ * <li> [1] Lanza, Object-Oriented Metrics in Practice, 2005.
+ * <li> [2] McCabe, A Complexity Measure, in Proceedings of the 2nd ICSE (1976).
+ * </ul>
  *
  * @author Clément Fournier
  * @since June 2017
  */
-public class CycloMetric extends AbstractClassMetric implements OperationMetric, ClassMetric {
+public class CycloMetric extends AbstractClassMetric implements OperationMetric {
 
     @Override
     public double computeFor(ASTClassOrInterfaceDeclaration node, MetricVersion version) {
diff --git a/pmd-java/src/main/java/net/sourceforge/pmd/lang/java/oom/metrics/WmcMetric.java b/pmd-java/src/main/java/net/sourceforge/pmd/lang/java/oom/metrics/WmcMetric.java
index 9e387e9103..8c08fe33e8 100644
--- a/pmd-java/src/main/java/net/sourceforge/pmd/lang/java/oom/metrics/WmcMetric.java
+++ b/pmd-java/src/main/java/net/sourceforge/pmd/lang/java/oom/metrics/WmcMetric.java
@@ -6,9 +6,9 @@ package net.sourceforge.pmd.lang.java.oom.metrics;
 
 import net.sourceforge.pmd.lang.java.ast.ASTClassOrInterfaceDeclaration;
 import net.sourceforge.pmd.lang.java.oom.AbstractClassMetric;
-import net.sourceforge.pmd.lang.java.oom.OperationMetricKey;
 import net.sourceforge.pmd.lang.java.oom.interfaces.ClassMetric;
 import net.sourceforge.pmd.lang.java.oom.interfaces.MetricVersion;
+import net.sourceforge.pmd.lang.java.oom.interfaces.OperationMetricKey;
 
 /**
  * Weighed Method Count. It is the sum of the statical complexity of all operations of a class. We use