Use JsonReader.setLenient(true) to accept malformed JSON at line 1 column 1 path $

This is a well-known issue and based on this answer you could add setLenient:

Gson gson = new GsonBuilder()
        .setLenient()
        .create();

Retrofit retrofit = new Retrofit.Builder()
        .baseUrl(BASE_URL)
        .client(client)
        .addConverterFactory(GsonConverterFactory.create(gson))
        .build();

Now, if you add this to your retrofit, it gives you another error:

com.google.gson.JsonSyntaxException: java.lang.IllegalStateException: Expected BEGIN_OBJECT but was STRING at line 1 column 1 path $

This is another well-known error you can find answer here (this error means that your server response is not well-formatted); So change server response to return something:

{
    android:[
        { ver:"1.5", name:"Cupcace", api:"Api Level 3" }
        ...
    ]
}

For better comprehension, compare your response with Github api.

Suggestion: to find out what's going on to your request/response add HttpLoggingInterceptor in your retrofit.

Based on this answer your ServiceHelper would be:

private ServiceHelper() {
        httpClient = new OkHttpClient.Builder();
        HttpLoggingInterceptor interceptor = new HttpLoggingInterceptor();
        interceptor.setLevel(HttpLoggingInterceptor.Level.BODY);
        httpClient.interceptors().add(interceptor);
        Retrofit retrofit = createAdapter().build();
        service = retrofit.create(IService.class);
    }

Also don't forget to add:

compile 'com.squareup.okhttp3:logging-interceptor:3.3.1'

Also this issue occurres when the response contenttype is not application/json. In my case response contenttype was text/html and i faced this problem. I changed it to application/json then it worked.

There was an error in understanding of return Type Just add Header and it will solve your problem

@Headers("Content-Type: application/json")