Removed unneccesary top-level cache fields

Previously the cache implemented AbstractMap for the its convenient
putAll, equals, hashCode, and toString implementations. This came at
the cost of its private, unused keySet and values fields. Those Object
methods iterated over the entrySet, which returns a copy of the entry
to avoid instabiliy due to concurrent mutations. This changes to
internal iterators which avoids this unnecessary garbage creation.

Equality is a complex subject and the implementation's internal docs
now better clarify the semantics. Due to the cache potentially holding
entries that are pending removal (expiration, reference collection),
we expect that usages call Cache.cleanUp() first.

Removed an adapter for the async cache loader as not needed, dropping
two instance fields.

Simplified some serialization code by removing redundancies.

caffeine/src/javaPoet/java/com/github/benmanes/caffeine/cache/LocalCacheFactoryGenerator.java

@@ -18,8 +18,8 @@
 import static com.github.benmanes.caffeine.cache.Specifications.BOUNDED_LOCAL_CACHE;
 import static com.github.benmanes.caffeine.cache.Specifications.BUILDER;
 import static com.github.benmanes.caffeine.cache.Specifications.BUILDER_PARAM;
-import static com.github.benmanes.caffeine.cache.Specifications.CACHE_LOADER;
-import static com.github.benmanes.caffeine.cache.Specifications.CACHE_LOADER_PARAM;
+import static com.github.benmanes.caffeine.cache.Specifications.ASYNC_CACHE_LOADER;
+import static com.github.benmanes.caffeine.cache.Specifications.ASYNC_CACHE_LOADER_PARAM;
 import static com.github.benmanes.caffeine.cache.Specifications.kTypeVar;
 import static com.github.benmanes.caffeine.cache.Specifications.vTypeVar;
 import static java.nio.charset.StandardCharsets.UTF_8;
@@ -90,7 +90,7 @@ public final class LocalCacheFactoryGenerator {
       .build();
   static final FieldSpec FACTORY = FieldSpec.builder(MethodType.class, "FACTORY")
       .initializer("$T.methodType($T.class, $T.class, $T.class, $T.class)",
-          MethodType.class, void.class, BUILDER, CACHE_LOADER.rawType, TypeName.BOOLEAN)
+          MethodType.class, void.class, BUILDER, ASYNC_CACHE_LOADER.rawType, TypeName.BOOLEAN)
       .addModifiers(Modifier.PUBLIC, Modifier.STATIC, Modifier.FINAL)
       .build();
 
@@ -136,7 +136,7 @@ private void addFactoryMethods() {
         .addModifiers(Modifier.STATIC)
         .addCode(LocalCacheSelectorCode.get())
         .addParameter(BUILDER_PARAM)
-        .addParameter(CACHE_LOADER_PARAM.toBuilder().addAnnotation(Nullable.class).build())
+        .addParameter(ASYNC_CACHE_LOADER_PARAM.toBuilder().addAnnotation(Nullable.class).build())
         .addParameter(boolean.class, "async")
         .addJavadoc("Returns a cache optimized for this configuration.\n")
         .build());

caffeine/src/javaPoet/java/com/github/benmanes/caffeine/cache/Specifications.java

@@ -76,10 +76,10 @@ public final class Specifications {
       ClassName.get(PACKAGE_NAME, "BoundedLocalCache"), kTypeVar, vTypeVar);
   public static final TypeName NODE = ParameterizedTypeName.get(nodeType, kTypeVar, vTypeVar);
 
-  public static final ParameterizedTypeName CACHE_LOADER = ParameterizedTypeName.get(
-      ClassName.get(PACKAGE_NAME, "CacheLoader"), TypeVariableName.get("? super K"), vTypeVar);
-  public static final ParameterSpec CACHE_LOADER_PARAM =
-      ParameterSpec.builder(CACHE_LOADER, "cacheLoader").build();
+  public static final ParameterizedTypeName ASYNC_CACHE_LOADER = ParameterizedTypeName.get(
+      ClassName.get(PACKAGE_NAME, "AsyncCacheLoader"), TypeVariableName.get("? super K"), vTypeVar);
+  public static final ParameterSpec ASYNC_CACHE_LOADER_PARAM =
+      ParameterSpec.builder(ASYNC_CACHE_LOADER, "cacheLoader").build();
 
   public static final TypeName REMOVAL_LISTENER = ParameterizedTypeName.get(
       ClassName.get(PACKAGE_NAME, "RemovalListener"), kTypeVar, vTypeVar);

caffeine/src/javaPoet/java/com/github/benmanes/caffeine/cache/local/AddConstructor.java

@@ -15,9 +15,9 @@
  */
 package com.github.benmanes.caffeine.cache.local;
 
+import static com.github.benmanes.caffeine.cache.Specifications.ASYNC_CACHE_LOADER_PARAM;
 import static com.github.benmanes.caffeine.cache.Specifications.BOUNDED_LOCAL_CACHE;
 import static com.github.benmanes.caffeine.cache.Specifications.BUILDER_PARAM;
-import static com.github.benmanes.caffeine.cache.Specifications.CACHE_LOADER_PARAM;
 
 /**
  * Adds the constructor to the cache.
@@ -33,17 +33,16 @@ protected boolean applies() {
 
   @Override
   protected void execute() {
-    String cacheLoader;
+    context.constructor
+        .addParameter(BUILDER_PARAM)
+        .addParameter(ASYNC_CACHE_LOADER_PARAM)
+        .addParameter(boolean.class, "async");
     if (context.superClass.equals(BOUNDED_LOCAL_CACHE)) {
-      cacheLoader = "(CacheLoader<K, V>) cacheLoader";
       context.suppressedWarnings.add("unchecked");
+      context.constructor.addStatement(
+          "super(builder, (AsyncCacheLoader<K, V>) cacheLoader, async)");
     } else {
-      cacheLoader = "cacheLoader";
+      context.constructor.addStatement("super(builder, cacheLoader, async)");
     }
-    context.constructor
-        .addParameter(BUILDER_PARAM)
-        .addParameter(CACHE_LOADER_PARAM)
-        .addParameter(boolean.class, "async")
-        .addStatement("super(builder, $L, async)", cacheLoader);
   }
 }