Nullability Annotations to Java Classes - Network

[ParaskP7]

Parent #2795

This issue is about adding missing nullability annotations (@Nullable & @NonNull) to as many Java-related Network classes as possible, including fields, arguments, parameters, and return values.

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Tasks (discovery)

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1. DiscoveryRequest.java
2. DiscoveryUtils.java
3. DiscoveryXMLRPCRequest.java
4. SelfHostedEndpointFinder.java
5. WPAPIHeadRequest.java

Tasks (rest/wpapi)

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1. WPAPIGsonRequest.java

Tasks (rest/wpcom)

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1. WPComGsonRequest.java
2. GsonRequest.java
3. JsonObjectOrEmptyArrayDeserializer.java
4. JsonObjectOrFalseDeserializer.java

Tasks (wporg)

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1. WPOrgAPIGsonRequest.java

Tasks (xmlrpc)

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1. XMLRPCException.java
2. XMLRPCFault.java
3. XMLRPCRequest.java
4. XMLRPCSerializer.java
5. XMLRPCUtils.java
6. XMLSerializerUtils.java

Tasks (request)

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1. BaseRequest.java
2. BaseRequestFuture.java
3. BaseUploadRequestBody.java

Tasks (http)

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1. HTTPAuthManager.java
2. HTTPAuthModel.java
3. OkHttpStack.java

Tasks (manager)

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1. MemorizingTrustManager.java
2. OpenJdkCookieManager.java

Tasks (other)

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1. RetryOnRedirectBasicNetwork.java
2. UserAgent.java