Refactor: separate SplittableTruncateSizedRestrictions

sdks/java/core/src/main/java/org/apache/beam/sdk/transforms/splittabledofn/RestrictionTracker.java

@@ -211,6 +211,6 @@ public static <RestrictionT> TruncateResult of(RestrictionT restriction) {
       return new AutoValue_RestrictionTracker_TruncateResult(restriction);
     }
 
-    public abstract @Nullable RestrictionT getTruncatedRestriction();
+    public abstract RestrictionT getTruncatedRestriction();
   }
 }

sdks/java/core/src/main/java/org/apache/beam/sdk/util/WindowedValue.java

@@ -49,6 +49,7 @@
 import org.apache.beam.vendor.guava.v32_1_2_jre.com.google.common.base.MoreObjects;
 import org.apache.beam.vendor.guava.v32_1_2_jre.com.google.common.collect.ImmutableList;
 import org.checkerframework.checker.nullness.qual.Nullable;
+import org.checkerframework.dataflow.qual.Pure;
 import org.joda.time.Instant;
 
 /**
@@ -143,21 +144,27 @@ public static <T> WindowedValue<T> timestampedValueInGlobalWindow(
    * Returns a new {@code WindowedValue} that is a copy of this one, but with a different value,
    * which may have a new type {@code NewT}.
    */
+  @Pure
   public abstract <NewT> WindowedValue<NewT> withValue(NewT value);
 
   /** Returns the value of this {@code WindowedValue}. */
+  @Pure
   public abstract T getValue();
 
   /** Returns the timestamp of this {@code WindowedValue}. */
+  @Pure
   public abstract Instant getTimestamp();
 
   /** Returns the windows of this {@code WindowedValue}. */
+  @Pure
   public abstract Collection<? extends BoundedWindow> getWindows();
 
   /** Returns the pane of this {@code WindowedValue} in its window. */
+  @Pure
   public abstract PaneInfo getPane();
 
   /** Returns {@code true} if this WindowedValue has exactly one window. */
+  @Pure
   public boolean isSingleWindowedValue() {
     return false;
   }

sdks/java/core/src/main/java/org/apache/beam/sdk/values/KV.java

@@ -40,6 +40,7 @@
  */
 public class KV<K, V> implements Serializable {
   /** Returns a {@link KV} with the given key and value. */
+  @Pure
   public static <K, V> KV<K, V> of(K key, V value) {
     return new KV<>(key, value);
   }