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

…ide schemas

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

@@ -21,10 +21,8 @@
 import com.google.common.cache.LoadingCache;
 import com.google.common.util.concurrent.UncheckedExecutionException;
 
-import org.checkerframework.checker.nullness.qual.NonNull;
 import org.checkerframework.checker.nullness.qual.Nullable;
 
-import java.util.Optional;
 import java.util.Set;
 import java.util.concurrent.ExecutionException;
 import java.util.concurrent.TimeUnit;
@@ -44,18 +42,24 @@ public LoadingCacheLookup(Lookup<T> delegate) {
     this.delegate = delegate;
     this.cache = CacheBuilder.newBuilder()
         .expireAfterWrite(1, TimeUnit.MINUTES)
-        .build(new CacheLoader<String, @NonNull T>() {
-          @Override public @NonNull T load(String name) throws Exception {
-            return Optional.ofNullable(delegate.get(name))
-                .orElseThrow(() -> new EntryNotFoundException());
+        .build(new CacheLoader<String, T>() {
+          @Override public T load(String name) throws Exception {
+            T result = delegate.get(name);
+            if (result != null) {
+              return result;
+            }
+            throw new EntryNotFoundException();
           }
         });
     this.cacheIgnoreCase = CacheBuilder.newBuilder()
         .expireAfterWrite(1, TimeUnit.MINUTES)
         .build(new CacheLoader<String, Named<T>>() {
           @Override public Named<T> load(String name) throws Exception {
-            return Optional.ofNullable(delegate.getIgnoreCase(name))
-                .orElseThrow(() -> new EntryNotFoundException());
+            Named<T> result = delegate.getIgnoreCase(name);
+            if (result != null) {
+              return result;
+            }
+            throw new EntryNotFoundException();
           }
         });
   }