8011200: (coll) Optimize empty HashMap and ArrayList

Reviewed-by: mduigou, alanb, bchristi, martin
Contributed-by: NSergey Linetskiy &lt;sergey.linetskiy@oracle.com&gt;, John Rose &lt;john.r.rose@oracle.com&gt;, Mike Duigou <mike.duigou@oracle.com>

src/share/classes/java/util/ArrayList.java

 /*
- * Copyright (c) 1997, 2012, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 1997, 2013, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -104,9 +104,21 @@ public class ArrayList<E> extends AbstractList<E>
 {
     private static final long serialVersionUID = 8683452581122892189L;
 
+    /**
+     * Default initial capacity.
+     */
+    private static final int DEFAULT_CAPACITY = 10;
+
+    /**
+     * Shared empty array instance used for empty instances.
+     */
+    private static final Object[] EMPTY_ELEMENTDATA = {};
+
     /**
      * The array buffer into which the elements of the ArrayList are stored.
-     * The capacity of the ArrayList is the length of this array buffer.
+     * The capacity of the ArrayList is the length of this array buffer. Any
+     * empty ArrayList with elementData == EMPTY_ELEMENTDATA will be expanded to
+     * DEFAULT_CAPACITY when the first element is added.
      */
     private transient Object[] elementData;
 
@@ -136,7 +148,8 @@ public class ArrayList<E> extends AbstractList<E>
      * Constructs an empty list with an initial capacity of ten.
      */
     public ArrayList() {
-        this(10);
+        super();
+        this.elementData = EMPTY_ELEMENTDATA;
     }
 
     /**
@@ -162,8 +175,7 @@ public class ArrayList<E> extends AbstractList<E>
      */
     public void trimToSize() {
         modCount++;
-        int oldCapacity = elementData.length;
-        if (size < oldCapacity) {
+        if (size < elementData.length) {
             elementData = Arrays.copyOf(elementData, size);
         }
     }
@@ -176,12 +188,29 @@ public class ArrayList<E> extends AbstractList<E>
      * @param   minCapacity   the desired minimum capacity
      */
     public void ensureCapacity(int minCapacity) {
-        if (minCapacity > 0)
-            ensureCapacityInternal(minCapacity);
+        int minExpand = (elementData != EMPTY_ELEMENTDATA)
+            // any size if real element table
+            ? 0
+            // larger than default for empty table. It's already supposed to be
+            // at default size.
+            : DEFAULT_CAPACITY;
+
+        if (minCapacity > minExpand) {
+            ensureExplicitCapacity(minCapacity);
+        }
     }
 
     private void ensureCapacityInternal(int minCapacity) {
+        if (elementData == EMPTY_ELEMENTDATA) {
+            minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
+        }
+
+        ensureExplicitCapacity(minCapacity);
+    }
+
+    private void ensureExplicitCapacity(int minCapacity) {
         modCount++;
+
         // overflow-conscious code
         if (minCapacity - elementData.length > 0)
             grow(minCapacity);
@@ -450,7 +479,7 @@ public class ArrayList<E> extends AbstractList<E>
         if (numMoved > 0)
             System.arraycopy(elementData, index+1, elementData, index,
                              numMoved);
-        elementData[--size] = null; // Let gc do its work
+        elementData[--size] = null; // clear to let GC do its work
 
         return oldValue;
     }
@@ -495,7 +524,7 @@ public class ArrayList<E> extends AbstractList<E>
         if (numMoved > 0)
             System.arraycopy(elementData, index+1, elementData, index,
                              numMoved);
-        elementData[--size] = null; // Let gc do its work
+        elementData[--size] = null; // clear to let GC do its work
     }
 
     /**
@@ -505,7 +534,7 @@ public class ArrayList<E> extends AbstractList<E>
     public void clear() {
         modCount++;
 
-        // Let gc do its work
+        // clear to let GC do its work
         for (int i = 0; i < size; i++)
             elementData[i] = null;
 
@@ -586,10 +615,12 @@ public class ArrayList<E> extends AbstractList<E>
         System.arraycopy(elementData, toIndex, elementData, fromIndex,
                          numMoved);
 
-        // Let gc do its work
+        // clear to let GC do its work
         int newSize = size - (toIndex-fromIndex);
-        while (size != newSize)
-            elementData[--size] = null;
+        for (int i = newSize; i < size; i++) {
+            elementData[i] = null;
+        }
+        size = newSize;
     }
 
     /**
@@ -677,6 +708,7 @@ public class ArrayList<E> extends AbstractList<E>
                 w += size - r;
             }
             if (w != size) {
+                // clear to let GC do its work
                 for (int i = w; i < size; i++)
                     elementData[i] = null;
                 modCount += size - w;
@@ -701,17 +733,17 @@ public class ArrayList<E> extends AbstractList<E>
         int expectedModCount = modCount;
         s.defaultWriteObject();
 
-        // Write out array length
-        s.writeInt(elementData.length);
+        // Write out size as capacity for behavioural compatibility with clone()
+        s.writeInt(size);
 
         // Write out all elements in the proper order.
-        for (int i=0; i<size; i++)
+        for (int i=0; i<size; i++) {
             s.writeObject(elementData[i]);
+        }
 
         if (modCount != expectedModCount) {
             throw new ConcurrentModificationException();
         }
-
     }
 
     /**
@@ -720,16 +752,24 @@ public class ArrayList<E> extends AbstractList<E>
      */
     private void readObject(java.io.ObjectInputStream s)
         throws java.io.IOException, ClassNotFoundException {
+        elementData = EMPTY_ELEMENTDATA;
+
         // Read in size, and any hidden stuff
         s.defaultReadObject();
 
-        // Read in array length and allocate array
-        int arrayLength = s.readInt();
-        Object[] a = elementData = new Object[arrayLength];
+        // Read in capacity
+        s.readInt(); // ignored
+
+        if (size > 0) {
+            // be like clone(), allocate array based upon size not capacity
+            ensureCapacityInternal(size);
 
-        // Read in all elements in the proper order.
-        for (int i=0; i<size; i++)
-            a[i] = s.readObject();
+            Object[] a = elementData;
+            // Read in all elements in the proper order.
+            for (int i=0; i<size; i++) {
+                a[i] = s.readObject();
+            }
+        }
     }
 
     /**

src/share/classes/java/util/HashMap.java

@@ -129,7 +129,7 @@ public class HashMap<K,V>
     /**
      * The default initial capacity - MUST be a power of two.
      */
-    static final int DEFAULT_INITIAL_CAPACITY = 16;
+    static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
 
     /**
      * The maximum capacity, used if a higher value is implicitly specified
@@ -143,10 +143,15 @@ public class HashMap<K,V>
      */
     static final float DEFAULT_LOAD_FACTOR = 0.75f;
 
+    /**
+     * An empty table instance to share when the table is not inflated.
+     */
+    static final Entry<?,?>[] EMPTY_TABLE = {};
+
     /**
      * The table, resized as necessary. Length MUST Always be a power of two.
      */
-    transient Entry<?,?>[] table;
+    transient Entry<?,?>[] table = EMPTY_TABLE;
 
     /**
      * The number of key-value mappings contained in this map.
@@ -157,6 +162,8 @@ public class HashMap<K,V>
      * The next size value at which to resize (capacity * load factor).
      * @serial
      */
+    // If table == EMPTY_TABLE then this is the initial capacity at which the
+    // table will be created when inflated.
     int threshold;
 
     /**
@@ -223,14 +230,8 @@ public class HashMap<K,V>
             throw new IllegalArgumentException("Illegal load factor: " +
                                                loadFactor);
 
-        // Find a power of 2 >= initialCapacity
-        int capacity = 1;
-        while (capacity < initialCapacity)
-            capacity <<= 1;
-
         this.loadFactor = loadFactor;
-        threshold = (int)Math.min(capacity * loadFactor, MAXIMUM_CAPACITY + 1);
-        table = new Entry<?,?>[capacity];
+        threshold = initialCapacity;
         init();
     }
 
@@ -265,9 +266,33 @@ public class HashMap<K,V>
     public HashMap(Map<? extends K, ? extends V> m) {
         this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1,
                       DEFAULT_INITIAL_CAPACITY), DEFAULT_LOAD_FACTOR);
+        inflateTable(threshold);
+
         putAllForCreate(m);
     }
 
+    private static int roundUpToPowerOf2(int number) {
+        // assert number >= 0 : "number must be non-negative";
+        int rounded = number >= MAXIMUM_CAPACITY
+                ? MAXIMUM_CAPACITY
+                : (rounded = Integer.highestOneBit(number)) != 0
+                    ? (Integer.bitCount(number) > 1) ? rounded << 1 : rounded
+                    : 1;
+
+        return rounded;
+    }
+
+    /**
+     * Inflates the table.
+     */
+    private void inflateTable(int toSize) {
+        // Find a power of 2 >= toSize
+        int capacity = roundUpToPowerOf2(toSize);
+
+        threshold = (int) Math.min(capacity * loadFactor, MAXIMUM_CAPACITY + 1);
+        table = new Entry[capacity];
+    }
+
     // internal utilities
 
     /**
@@ -305,6 +330,7 @@ public class HashMap<K,V>
      * Returns index for hash code h.
      */
     static int indexFor(int h, int length) {
+        // assert Integer.bitCount(length) == 1 : "length must be a non-zero power of 2";
         return h & (length-1);
     }
 
@@ -369,6 +395,10 @@ public class HashMap<K,V>
      */
     @SuppressWarnings("unchecked")
     final Entry<K,V> getEntry(Object key) {
+        if (isEmpty()) {
+            return null;
+        }
+
         int hash = (key == null) ? 0 : hash(key);
         for (Entry<?,?> e = table[indexFor(hash, table.length)];
              e != null;
@@ -381,7 +411,6 @@ public class HashMap<K,V>
         return null;
     }
 
-
     /**
      * Associates the specified value with the specified key in this map.
      * If the map previously contained a mapping for the key, the old
@@ -395,6 +424,9 @@ public class HashMap<K,V>
      *         previously associated <tt>null</tt> with <tt>key</tt>.)
      */
     public V put(K key, V value) {
+        if (table == EMPTY_TABLE) {
+            inflateTable(threshold);
+        }
         if (key == null)
             return putForNullKey(value);
         int hash = hash(key);
@@ -529,6 +561,10 @@ public class HashMap<K,V>
         if (numKeysToBeAdded == 0)
             return;
 
+        if (table == EMPTY_TABLE) {
+            inflateTable((int) Math.max(numKeysToBeAdded * loadFactor, threshold));
+        }
+
         /*
          * Expand the map if the map if the number of mappings to be added
          * is greater than or equal to threshold.  This is conservative; the
@@ -573,6 +609,9 @@ public class HashMap<K,V>
      * for this key.
      */
     final Entry<K,V> removeEntryForKey(Object key) {
+        if (isEmpty()) {
+            return null;
+        }
         int hash = (key == null) ? 0 : hash(key);
         int i = indexFor(hash, table.length);
         @SuppressWarnings("unchecked")
@@ -605,7 +644,7 @@ public class HashMap<K,V>
      * for matching.
      */
     final Entry<K,V> removeMapping(Object o) {
-        if (!(o instanceof Map.Entry))
+        if (isEmpty() || !(o instanceof Map.Entry))
             return null;
 
         Map.Entry<?,?> entry = (Map.Entry<?,?>) o;
@@ -641,9 +680,7 @@ public class HashMap<K,V>
      */
     public void clear() {
         modCount++;
-        Entry<?,?>[] tab = table;
-        for (int i = 0; i < tab.length; i++)
-            tab[i] = null;
+        Arrays.fill(table, null);
         size = 0;
     }
 
@@ -693,7 +730,14 @@ public class HashMap<K,V>
         } catch (CloneNotSupportedException e) {
             // assert false;
         }
-        result.table = new Entry<?,?>[table.length];
+        if (result.table != EMPTY_TABLE) {
+            result.inflateTable(Math.min(
+                (int) Math.min(
+                    size * Math.min(1 / loadFactor, 4.0f),
+                    // we have limits...
+                    HashMap.MAXIMUM_CAPACITY),
+                table.length));
+        }
         result.entrySet = null;
         result.modCount = 0;
         result.size = 0;
@@ -749,8 +793,7 @@ public class HashMap<K,V>
         }
 
         public final int hashCode() {
-            return (key==null   ? 0 : key.hashCode()) ^
-                   (value==null ? 0 : value.hashCode());
+            return Objects.hashCode(getKey()) ^ Objects.hashCode(getValue());
         }
 
         public final String toString() {
@@ -1017,14 +1060,15 @@ public class HashMap<K,V>
     private void writeObject(java.io.ObjectOutputStream s)
         throws IOException
     {
-        Iterator<Map.Entry<K,V>> i =
-            (size > 0) ? entrySet0().iterator() : null;
-
         // Write out the threshold, loadfactor, and any hidden stuff
         s.defaultWriteObject();
 
         // Write out number of buckets
-        s.writeInt(table.length);
+        if (table==EMPTY_TABLE) {
+            s.writeInt(roundUpToPowerOf2(threshold));
+        } else {
+           s.writeInt(table.length);
+        }
 
         // Write out size (number of Mappings)
         s.writeInt(size);
@@ -1049,16 +1093,18 @@ public class HashMap<K,V>
     {
         // Read in the threshold (ignored), loadfactor, and any hidden stuff
         s.defaultReadObject();
-        if (loadFactor <= 0 || Float.isNaN(loadFactor))
+        if (loadFactor <= 0 || Float.isNaN(loadFactor)) {
             throw new InvalidObjectException("Illegal load factor: " +
                                                loadFactor);
+        }
 
-        // set hashMask
+        // set other fields that need values
         Holder.UNSAFE.putIntVolatile(this, Holder.HASHSEED_OFFSET,
                 sun.misc.Hashing.randomHashSeed(this));
+        table = EMPTY_TABLE;
 
-        // Read in number of buckets and allocate the bucket array;
-        s.readInt(); // ignored
+        // Read in number of buckets
+        s.readInt(); // ignored.
 
         // Read number of mappings
         int mappings = s.readInt();
@@ -1066,23 +1112,21 @@ public class HashMap<K,V>
             throw new InvalidObjectException("Illegal mappings count: " +
                                                mappings);
 
-        int initialCapacity = (int) Math.min(
-                // capacity chosen by number of mappings
-                // and desired load (if >= 0.25)
-                mappings * Math.min(1 / loadFactor, 4.0f),
-                // we have limits...
-                HashMap.MAXIMUM_CAPACITY);
-        int capacity = 1;
-        // find smallest power of two which holds all mappings
-        while (capacity < initialCapacity) {
-            capacity <<= 1;
+        // capacity chosen by number of mappings and desired load (if >= 0.25)
+        int capacity = (int) Math.min(
+                    mappings * Math.min(1 / loadFactor, 4.0f),
+                    // we have limits...
+                    HashMap.MAXIMUM_CAPACITY);
+
+        // allocate the bucket array;
+        if (mappings > 0) {
+            inflateTable(capacity);
+        } else {
+            threshold = capacity;
         }
 
-        table = new Entry<?,?>[capacity];
-        threshold = (int) Math.min(capacity * loadFactor, MAXIMUM_CAPACITY + 1);
         init();  // Give subclass a chance to do its thing.
 
-
         // Read the keys and values, and put the mappings in the HashMap
         for (int i=0; i<mappings; i++) {
             @SuppressWarnings("unchecked")

test/java/util/Map/BasicSerialization.java

+/*
+ * Copyright (c) 2012, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+/*
+ * @test
+ * @bug 8011200
+ * @run testng BasicSerialization
+ * @summary Ensure Maps can be serialized and deserialized.
+ * @author Mike Duigou
+ */
+import java.io.ByteArrayOutputStream;
+import java.io.InputStream;
+import java.io.IOException;
+import java.io.ObjectInputStream;
+import java.io.ObjectOutputStream;
+import java.io.ByteArrayInputStream;
+import java.lang.reflect.Constructor;
+import java.lang.reflect.Method;
+import java.util.*;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.ConcurrentSkipListMap;
+
+import org.testng.annotations.Test;
+import org.testng.annotations.DataProvider;
+import static org.testng.Assert.fail;
+import static org.testng.Assert.assertEquals;
+import static org.testng.Assert.assertTrue;
+import static org.testng.Assert.assertFalse;
+import static org.testng.Assert.assertSame;
+
+public class BasicSerialization {
+
+    enum IntegerEnum {
+
+        e0, e1, e2, e3, e4, e5, e6, e7, e8, e9,
+        e10, e11, e12, e13, e14, e15, e16, e17, e18, e19,
+        e20, e21, e22, e23, e24, e25, e26, e27, e28, e29,
+        e30, e31, e32, e33, e34, e35, e36, e37, e38, e39,
+        e40, e41, e42, e43, e44, e45, e46, e47, e48, e49,
+        e50, e51, e52, e53, e54, e55, e56, e57, e58, e59,
+        e60, e61, e62, e63, e64, e65, e66, e67, e68, e69,
+        e70, e71, e72, e73, e74, e75, e76, e77, e78, e79,
+        e80, e81, e82, e83, e84, e85, e86, e87, e88, e89,
+        e90, e91, e92, e93, e94, e95, e96, e97, e98, e99,
+        EXTRA_KEY;
+        public static final int SIZE = values().length;
+    };
+    private static final int TEST_SIZE = IntegerEnum.SIZE - 1;
+    /**
+     * Realized keys ensure that there is always a hard ref to all test objects.
+     */
+    private static final IntegerEnum[] KEYS = new IntegerEnum[TEST_SIZE];
+    /**
+     * Realized values ensure that there is always a hard ref to all test
+     * objects.
+     */
+    private static final String[] VALUES = new String[TEST_SIZE];
+
+    static {
+        IntegerEnum[] keys = IntegerEnum.values();
+        for (int each = 0; each < TEST_SIZE; each++) {
+            KEYS[each] = keys[each];
+            VALUES[each] = keys[each].name();
+        }
+    }
+    private static final IntegerEnum EXTRA_KEY = IntegerEnum.EXTRA_KEY;
+    private static final String EXTRA_VALUE = IntegerEnum.EXTRA_KEY.name();
+
+    public static <K, V> Map<K, V> mapClone(Map<K, V> map) {
+        Method cloneMethod;
+
+        try {
+            cloneMethod = map.getClass().getMethod("clone", new Class[]{});
+        } catch (NoSuchMethodException | SecurityException all) {
+            cloneMethod = null;
+        }
+
+        if (null != cloneMethod) {
+            try {
+                Map<K, V> result = (Map<K, V>)cloneMethod.invoke(map, new Object[]{});
+                return result;
+            } catch (Exception all) {
+                fail("clone() failed " + map.getClass().getSimpleName(), all);
+                return null;
+            }
+        } else {
+            Constructor<? extends Map> copyConstructor;
+            try {
+                copyConstructor = (Constructor<? extends Map>)map.getClass().getConstructor(new Class[]{Map.class});
+
+                Map<K, V> result = (Map<K, V>)copyConstructor.newInstance(new Object[]{map});
+
+                return result;
+            } catch (Exception all) {
+                return serialClone(map);
+            }
+        }
+    }
+
+    @Test(dataProvider = "Map<IntegerEnum,String>")
+    public void testSerialization(String description, Map<IntegerEnum, String> map) {
+        Object foo = new Object();
+
+        Map<IntegerEnum, String> clone = mapClone(map);
+        Map<IntegerEnum, String> serialClone = serialClone(map);
+
+        assertEquals(map, map, description + ":should equal self");
+        assertEquals(clone, map, description + ":should equal clone");
+        assertEquals(map, clone, description + ": should equal orginal map");
+        assertEquals(serialClone, map, description + ": should equal deserialized clone");
+        assertEquals(map, serialClone, description + ": should equal original map");
+        assertEquals(serialClone, clone, description + ": deserialized clone should equal clone");
+        assertEquals(clone, serialClone, description + ": clone should equal deserialized clone");
+
+        assertFalse(map.containsKey(EXTRA_KEY), description + ":unexpected key");
+        assertFalse(clone.containsKey(EXTRA_KEY), description + ":unexpected key");
+        assertFalse(serialClone.containsKey(EXTRA_KEY), description + ":unexpected key");
+        map.put(EXTRA_KEY, EXTRA_VALUE);
+        clone.put(EXTRA_KEY, EXTRA_VALUE);
+        serialClone.put(EXTRA_KEY, EXTRA_VALUE);
+        assertTrue(map.containsKey(EXTRA_KEY), description + ":missing key");
+        assertTrue(clone.containsKey(EXTRA_KEY), description + ":missing key");
+        assertTrue(serialClone.containsKey(EXTRA_KEY), description + ":missing key");
+        assertSame(map.get(EXTRA_KEY), EXTRA_VALUE, description + ":wrong value");
+        assertSame(clone.get(EXTRA_KEY), EXTRA_VALUE, description + ":wrong value");
+        assertSame(serialClone.get(EXTRA_KEY), EXTRA_VALUE, description + ":wrong value");
+
+        assertEquals(map, map, description + ":should equal self");
+        assertEquals(clone, map, description + ":should equal clone");
+        assertEquals(map, clone, description + ": should equal orginal map");
+        assertEquals(serialClone, map, description + ": should equal deserialized clone");
+        assertEquals(map, serialClone, description + ": should equal original map");
+        assertEquals(serialClone, clone, description + ": deserialized clone should equal clone");
+        assertEquals(clone, serialClone, description + ": clone should equal deserialized clone");
+    }
+
+    static byte[] serializedForm(Object obj) {
+        try {
+            ByteArrayOutputStream baos = new ByteArrayOutputStream();
+            new ObjectOutputStream(baos).writeObject(obj);
+            return baos.toByteArray();
+        } catch (IOException e) {
+            fail("Unexpected Exception", e);
+            return null;
+        }
+    }
+
+    static Object readObject(byte[] bytes) throws IOException, ClassNotFoundException {
+        InputStream is = new ByteArrayInputStream(bytes);
+        return new ObjectInputStream(is).readObject();
+    }
+
+    @SuppressWarnings("unchecked")
+    static <T> T serialClone(T obj) {
+        try {
+            return (T)readObject(serializedForm(obj));
+        } catch (IOException | ClassNotFoundException e) {
+            fail("Unexpected Exception", e);
+            return null;
+        }
+    }
+
+    @DataProvider(name = "Map<IntegerEnum,String>", parallel = true)
+    private static Iterator<Object[]> makeMaps() {
+        return Arrays.asList(
+            // empty
+            new Object[]{"HashMap", new HashMap()},
+            new Object[]{"LinkedHashMap", new LinkedHashMap()},
+            new Object[]{"Collections.checkedMap(HashMap)", Collections.checkedMap(new HashMap(), IntegerEnum.class, String.class)},
+            new Object[]{"Collections.synchronizedMap(HashMap)", Collections.synchronizedMap(new HashMap())},
+            // null hostile
+            new Object[]{"EnumMap", new EnumMap(IntegerEnum.class)},
+            new Object[]{"Hashtable", new Hashtable()},
+            new Object[]{"TreeMap", new TreeMap()},
+            new Object[]{"ConcurrentHashMap", new ConcurrentHashMap()},
+            new Object[]{"ConcurrentSkipListMap", new ConcurrentSkipListMap()},
+            new Object[]{"Collections.checkedMap(ConcurrentHashMap)", Collections.checkedMap(new ConcurrentHashMap(), IntegerEnum.class, String.class)},
+            new Object[]{"Collections.synchronizedMap(EnumMap)", Collections.synchronizedMap(new EnumMap(IntegerEnum.class))},
+            // filled
+            new Object[]{"HashMap", fillMap(new HashMap())},
+            new Object[]{"LinkedHashMap", fillMap(new LinkedHashMap())},
+            new Object[]{"Collections.checkedMap(HashMap)", Collections.checkedMap(fillMap(new HashMap()), IntegerEnum.class, String.class)},
+            new Object[]{"Collections.synchronizedMap(HashMap)", Collections.synchronizedMap(fillMap(new HashMap()))},
+            // null hostile
+            new Object[]{"EnumMap", fillMap(new EnumMap(IntegerEnum.class))},
+            new Object[]{"Hashtable", fillMap(new Hashtable())},
+            new Object[]{"TreeMap", fillMap(new TreeMap())},
+            new Object[]{"ConcurrentHashMap", fillMap(new ConcurrentHashMap())},
+            new Object[]{"ConcurrentSkipListMap", fillMap(new ConcurrentSkipListMap())},
+            new Object[]{"Collections.checkedMap(ConcurrentHashMap)", Collections.checkedMap(fillMap(new ConcurrentHashMap()), IntegerEnum.class, String.class)},
+            new Object[]{"Collections.synchronizedMap(EnumMap)", Collections.synchronizedMap(fillMap(new EnumMap(IntegerEnum.class)))}).iterator();
+    }
+
+    private static Map<IntegerEnum, String> fillMap(Map<IntegerEnum, String> result) {
+        for (int each = 0; each < TEST_SIZE; each++) {
+            result.put(KEYS[each], VALUES[each]);
+        }
+
+        return result;
+    }
+}