performance: make isEmpty() allocate-free, tests added

platform/platform-tests/testSrc/com/intellij/util/containers/ConcurrentMapsTest.java

@@ -97,17 +97,20 @@ public class ConcurrentMapsTest {
   }
 
   private void checkKeyIsTossedAfterGCPressure(Map<Object, Object> map) {
+    assertTrue(map.isEmpty());
     map.put(new Object(), new Object());
 
     //noinspection SizeReplaceableByIsEmpty
     do {
       map.put(this, this);  // to run processQueues();
+      assertFalse(map.isEmpty());
       map.remove(this);
 
       GCUtil.tryGcSoftlyReachableObjects();
       System.gc();
     }
     while (map.size() != 0);
+    assertTrue(map.isEmpty());
     assertEquals(0, map.size());
     map.put(this, this);
     assertEquals(1, map.size());
@@ -255,6 +258,7 @@ public class ConcurrentMapsTest {
   }
 
   private static void checkTossedKeyAndValue(@NotNull RefKeyRefValueHashMap<Object, Object> map) {
+    assertTrue(map.isEmpty());
     map.put(new Object(), new Object());
 
     do {

platform/util/src/com/intellij/util/containers/ConcurrentRefHashMap.java

@@ -126,7 +126,8 @@ abstract class ConcurrentRefHashMap<K, V> extends AbstractMap<K, V> implements C
 
   @Override
   public boolean isEmpty() {
-    return myMap.isEmpty();
+    // make easier and alloc-free call to myMap first
+    return myMap.isEmpty() || entrySet().isEmpty();
   }
 
   @Override

platform/util/src/com/intellij/util/containers/RefHashMap.java

@@ -202,7 +202,8 @@ abstract class RefHashMap<K, V> extends AbstractMap<K, V> implements Map<K, V> {
 
   @Override
   public boolean isEmpty() {
-    return myMap.isEmpty();
+    // make easier and alloc-free call to myMap first
+    return myMap.isEmpty() || entrySet().isEmpty();
   }
 
   @Override
@@ -383,7 +384,8 @@ abstract class RefHashMap<K, V> extends AbstractMap<K, V> implements Map<K, V> {
   @NotNull
   @Override
   public Set<Entry<K, V>> entrySet() {
-    if (entrySet == null) entrySet = new EntrySet();
-    return entrySet;
+    Set<Entry<K, V>> es = entrySet;
+    if (es == null) entrySet = es = new EntrySet();
+    return es;
   }
 }