Why Does Gson Throw \'Expected BEGIN_ARRAY but was BEGIN_OBJECT\' and How to Fix It?

Expected BEGIN_ARRAY but was BEGIN_OBJECT at Line 1, Column 2

When attempting to parse JSON using Gson, an error can occur indicating: "com.google.gson.JsonSyntaxException: java.lang.IllegalStateException: Expected BEGIN_ARRAY but was BEGIN_OBJECT." This error signifies a mismatch between the expected format of the JSON data and the actual structure of the received data.

The root cause of this issue lies in the discrepancy between the following two lines of code:

List<Post> postsList = Arrays.asList(gson.fromJson(reader, Post[].class));

This code attempts to deserialize the JSON into an array of Post objects, expecting the JSON to be formatted as [{"timeZoneId": ...}, ...]. However, the JSON provided is an object, not an array, as evidenced by the presence of "{" and "}" delimiters.

To rectify this error, it is necessary to modify the code to correctly parse the JSON as a single Post object. Replace the line mentioned above with the following:

Post post = gson.fromJson(reader, Post.class);

With this adjustment, Gson will deserialize the JSON into a single Post object, resolving the Expected BEGIN_ARRAY but was BEGIN_OBJECT error. Note that it assumes that the JSON conforms to the structure defined in the Post class, with a timeZoneId field. If the JSON contains additional or different fields, the code may need further modification to handle them appropriately.

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