Add @pure annotations to SideInputReader

Author: kennknowles

runners/core-java/src/main/java/org/apache/beam/runners/core/SideInputReader.java

@@ -19,25 +19,22 @@
 
 import org.apache.beam.sdk.transforms.windowing.BoundedWindow;
 import org.apache.beam.sdk.values.PCollectionView;
-import org.checkerframework.checker.nullness.qual.Nullable;
+import org.checkerframework.dataflow.qual.Pure;
 
 /**
  * The interface to objects that provide side inputs. Particular implementations may read a side
  * input directly or use appropriate sorts of caching, etc.
  */
 public interface SideInputReader {
-  /**
-   * Returns the value of the given {@link PCollectionView} for the given {@link BoundedWindow}.
-   *
-   * <p>It is valid for a side input to be {@code null}. It is <i>not</i> valid for this to return
-   * {@code null} for any other reason.
-   */
-  @Nullable
+  /** Returns the value of the given {@link PCollectionView} for the given {@link BoundedWindow}. */
+  @Pure
   <T> T get(PCollectionView<T> view, BoundedWindow window);
 
   /** Returns true if the given {@link PCollectionView} is valid for this reader. */
+  @Pure
   <T> boolean contains(PCollectionView<T> view);
 
   /** Returns true if there are no side inputs in this reader. */
+  @Pure
   boolean isEmpty();
 }