[CALCITE-6728] Introduce new methods to lookup tables and schemas ins…

…ide schemas

core/src/main/java/org/apache/calcite/schema/lookup/LoadingCacheLookup.java

@@ -26,6 +26,7 @@
 import java.util.Set;
 import java.util.concurrent.ExecutionException;
 import java.util.concurrent.TimeUnit;
+import java.util.function.Function;
 
 /**
  * This class can be used to cache lookups.
@@ -42,24 +43,10 @@ public LoadingCacheLookup(Lookup<T> delegate) {
     this.delegate = delegate;
     this.cache = CacheBuilder.newBuilder()
         .expireAfterWrite(1, TimeUnit.MINUTES)
-        .build(
-            CacheLoader.from(name -> {
-            T result = delegate.get(name);
-            if (result != null) {
-              return result;
-            }
-            throw new EntryNotFoundException();
-        }));
+        .build(new NullSafeCacheLoader<String, T>(delegate::get));
     this.cacheIgnoreCase = CacheBuilder.newBuilder()
         .expireAfterWrite(1, TimeUnit.MINUTES)
-        .build(
-            CacheLoader.from(name -> {
-            Named<T> result = delegate.getIgnoreCase(name);
-            if (result != null) {
-              return result;
-            }
-            throw new EntryNotFoundException();
-        }));
+        .build(new NullSafeCacheLoader<>(delegate::getIgnoreCase));
   }
 
   @Override public @Nullable T get(String name) {
@@ -92,4 +79,25 @@ public LoadingCacheLookup(Lookup<T> delegate) {
     return delegate.getNames(pattern);
   }
 
+  /**
+   * CacheLoader which doesn't return null values.
+   *
+   * @param <K> Key
+   * @param <V> Value
+   */
+  private static class NullSafeCacheLoader<K, V> extends CacheLoader<K, V> {
+    private final Function<K, @Nullable V> function;
+
+    NullSafeCacheLoader(Function<K, @Nullable V> function) {
+      this.function = function;
+    }
+
+    @Override public V load(final K key) {
+      V result = function.apply(key);
+      if (result != null) {
+        return result;
+      }
+      throw new EntryNotFoundException();
+    }
+  }
 }