Expose an iterator for GeneratedMessage.ExtendableMessage.extensions

This iterates over the wrapper class `GeneratedMessage.ExtendableMessage.FieldValue` to keep the mutable underlying structure of the extensions FieldSet hidden. The entries' values exposed here are a subset of the values exposed by `GeneratedMessage.getallFields`. This iterator has less overhead than calling allFields and does not create a new map object.

PiperOrigin-RevId: 786884890

Author: protobuf-github-bot

java/core/src/main/java/com/google/protobuf/GeneratedMessage.java

@@ -949,6 +949,14 @@ protected ExtendableMessage(ExtendableBuilder<MessageT, ?> builder) {
       this.extensions = builder.buildExtensions();
     }
 
+    /**
+     * Returns an iterator over the set extensions lazily wrapped in {@link FieldEntry} objects.
+     * Order is unspecified.
+     */
+    public final Iterator<FieldEntry> extensionsIterator() {
+      return new FieldEntryIterator(extensions);
+    }
+
     private void verifyExtensionContainingType(final Extension<? extends MessageT, ?> extension) {
       if (extension.getDescriptor().getContainingType() != getDescriptorForType()) {
         // This can only happen if someone uses unchecked operations.
@@ -1232,6 +1240,45 @@ private void verifyContainingType(final FieldDescriptor field) {
         throw new IllegalArgumentException("FieldDescriptor does not match message type.");
       }
     }
+
+    /** A wrapper for a field descriptor and its value. */
+    public static final class FieldEntry {
+      private final FieldDescriptor descriptor;
+      private final Object value;
+
+      public FieldDescriptor getDescriptor() {
+        return descriptor;
+      }
+
+      public Object getValue() {
+        return value;
+      }
+
+      FieldEntry(FieldDescriptor descriptor, Object value) {
+        this.descriptor = descriptor;
+        this.value = value;
+      }
+    }
+
+    private static final class FieldEntryIterator implements Iterator<FieldEntry> {
+      private final Iterator<Map.Entry<FieldDescriptor, Object>> iter;
+
+      FieldEntryIterator(FieldSet<FieldDescriptor> fieldSet) {
+        this.iter = fieldSet.iterator();
+      }
+
+      @Override
+      public final boolean hasNext() {
+        return iter.hasNext();
+      }
+
+      @Override
+      public final FieldEntry next() {
+        // Just let the inner iterator throw the NoSuchElementException.
+        Map.Entry<FieldDescriptor, Object> entry = iter.next();
+        return new FieldEntry(entry.getKey(), entry.getValue());
+      }
+    }
   }
 
   /**