diff --git a/moshi/src/main/java/com/squareup/moshi/ArrayJsonAdapter.java b/moshi/src/main/java/com/squareup/moshi/ArrayJsonAdapter.java
index a05da0b..aaae560 100644
--- a/moshi/src/main/java/com/squareup/moshi/ArrayJsonAdapter.java
+++ b/moshi/src/main/java/com/squareup/moshi/ArrayJsonAdapter.java
@@ -26,7 +26,7 @@ import java.util.List;
  * Converts arrays to JSON arrays containing their converted contents. This
  * supports both primitive and object arrays.
  */
-class ArrayJsonAdapter extends JsonAdapter<Object> {
+final class ArrayJsonAdapter extends JsonAdapter<Object> {
   public static final Factory FACTORY = new Factory() {
     @Override public JsonAdapter<?> create(Type type, AnnotatedElement annotations, Moshi moshi) {
       Type elementType = Types.arrayComponentType(type);
diff --git a/moshi/src/main/java/com/squareup/moshi/JsonReader.java b/moshi/src/main/java/com/squareup/moshi/JsonReader.java
index 01b5d00..957bcaf 100644
--- a/moshi/src/main/java/com/squareup/moshi/JsonReader.java
+++ b/moshi/src/main/java/com/squareup/moshi/JsonReader.java
@@ -173,7 +173,7 @@ import okio.Source;
  * <p>Each {@code JsonReader} may be used to read a single JSON stream. Instances
  * of this class are not thread safe.
  */
-public class JsonReader implements Closeable {
+public final class JsonReader implements Closeable {
   private static final long MIN_INCOMPLETE_INTEGER = Long.MIN_VALUE / 10;
 
   private static final ByteString SINGLE_QUOTE_OR_SLASH = ByteString.encodeUtf8("'\\");
diff --git a/moshi/src/main/java/com/squareup/moshi/JsonWriter.java b/moshi/src/main/java/com/squareup/moshi/JsonWriter.java
index 4d6ed52..86da482 100644
--- a/moshi/src/main/java/com/squareup/moshi/JsonWriter.java
+++ b/moshi/src/main/java/com/squareup/moshi/JsonWriter.java
@@ -124,7 +124,7 @@ import static com.squareup.moshi.JsonScope.NONEMPTY_OBJECT;
  * Instances of this class are not thread safe. Calls that would result in a
  * malformed JSON string will fail with an {@link IllegalStateException}.
  */
-public class JsonWriter implements Closeable, Flushable {
+public final class JsonWriter implements Closeable, Flushable {
 
   /*
    * From RFC 4627, "All Unicode characters may be placed within the