nullable annotations to ReadableMap, WritableMap, ReadableArray, Writable. (#23329)

Summary:
Kotlin is getting traction and more developers write RN native modules in it. This PR adds nullable annotations to help with Kotlin null inference and improve developer experience. Also it'll help checking code quality using lint.

I skimmed through JavaOnlyMap.java, JavaOnlyArray.java, ReadableNativeArray.java, ReadableNativeMap.java, WritableNativeArray.java and WritableNativeMap.java to infer nullability.

This is breaking change to Kotlin code.

[Android] [Changed] - Add nullable annotations to ReadableMap, WritableMap, ReadableArray, Writable.
Pull Request resolved: https://github.com/facebook/react-native/pull/23329

Differential Revision: D14002571

Pulled By: cpojer

fbshipit-source-id: 899d8b3b0a5dad43e8300e6c4ea4208cca0f01a9

ReactAndroid/src/main/java/com/facebook/react/bridge/JavaOnlyMap.java

@@ -11,6 +11,8 @@ import java.util.HashMap;
 import java.util.Iterator;
 import java.util.Map;
 
+import javax.annotation.Nonnull;
+
 /**
  * Java {@link HashMap} backed implementation of {@link ReadableMap} and {@link WritableMap}
  * Instances of this class SHOULD NOT be used for communication between java and JS, use instances
@@ -77,52 +79,52 @@ public class JavaOnlyMap implements ReadableMap, WritableMap {
   }
 
   @Override
-  public boolean hasKey(String name) {
+  public boolean hasKey(@Nonnull String name) {
     return mBackingMap.containsKey(name);
   }
 
   @Override
-  public boolean isNull(String name) {
+  public boolean isNull(@Nonnull String name) {
     return mBackingMap.get(name) == null;
   }
 
   @Override
-  public boolean getBoolean(String name) {
+  public boolean getBoolean(@Nonnull String name) {
     return (Boolean) mBackingMap.get(name);
   }
 
   @Override
-  public double getDouble(String name) {
+  public double getDouble(@Nonnull String name) {
     return ((Number) mBackingMap.get(name)).doubleValue();
   }
 
   @Override
-  public int getInt(String name) {
+  public int getInt(@Nonnull String name) {
     return ((Number) mBackingMap.get(name)).intValue();
   }
 
   @Override
-  public String getString(String name) {
+  public String getString(@Nonnull String name) {
     return (String) mBackingMap.get(name);
   }
 
   @Override
-  public ReadableMap getMap(String name) {
+  public ReadableMap getMap(@Nonnull String name) {
     return (ReadableMap) mBackingMap.get(name);
   }
 
   @Override
-  public JavaOnlyArray getArray(String name) {
+  public JavaOnlyArray getArray(@Nonnull String name) {
     return (JavaOnlyArray) mBackingMap.get(name);
   }
 
   @Override
-  public Dynamic getDynamic(String name) {
+  public @Nonnull Dynamic getDynamic(@Nonnull String name) {
     return DynamicFromMap.create(this, name);
   }
 
   @Override
-  public ReadableType getType(String name) {
+  public @Nonnull ReadableType getType(@Nonnull String name) {
     Object value = mBackingMap.get(name);
     if (value == null) {
       return ReadableType.Null;
@@ -145,7 +147,7 @@ public class JavaOnlyMap implements ReadableMap, WritableMap {
   }
 
   @Override
-  public ReadableMapKeySetIterator keySetIterator() {
+  public @Nonnull ReadableMapKeySetIterator keySetIterator() {
     return new ReadableMapKeySetIterator() {
       Iterator<String> mIterator = mBackingMap.keySet().iterator();
 
@@ -162,47 +164,47 @@ public class JavaOnlyMap implements ReadableMap, WritableMap {
   }
 
   @Override
-  public void putBoolean(String key, boolean value) {
+  public void putBoolean(@Nonnull String key, boolean value) {
     mBackingMap.put(key, value);
   }
 
   @Override
-  public void putDouble(String key, double value) {
+  public void putDouble(@Nonnull String key, double value) {
     mBackingMap.put(key, value);
   }
 
   @Override
-  public void putInt(String key, int value) {
+  public void putInt(@Nonnull String key, int value) {
     mBackingMap.put(key, value);
   }
 
   @Override
-  public void putString(String key, String value) {
+  public void putString(@Nonnull String key, @Nonnull String value) {
     mBackingMap.put(key, value);
   }
 
   @Override
-  public void putNull(String key) {
+  public void putNull(@Nonnull String key) {
     mBackingMap.put(key, null);
   }
 
   @Override
-  public void putMap(String key, WritableMap value) {
+  public void putMap(@Nonnull String key, @Nonnull WritableMap value) {
     mBackingMap.put(key, value);
   }
 
   @Override
-  public void merge(ReadableMap source) {
+  public void merge(@Nonnull ReadableMap source) {
     mBackingMap.putAll(((JavaOnlyMap) source).mBackingMap);
   }
 
   @Override
-  public void putArray(String key, WritableArray value) {
+  public void putArray(@Nonnull String key, @Nonnull WritableArray value) {
     mBackingMap.put(key, value);
   }
 
   @Override
-  public HashMap<String, Object> toHashMap() {
+  public @Nonnull HashMap<String, Object> toHashMap() {
     return new HashMap<String, Object>(mBackingMap);
   }