[Java] Ineffectual code to handle file downloads

Description

The implementation of GenericType.equals() requires that its argument must be an instance of a GenericType, or an instance of a descendant of it, for this method to return true.

However, ApiClient.mustache:489 reads if (returnType.equals(File.class)), where returnType is of type GenericType<T> and File.class is java.lang.Class<java.io.File>.

Thus, this branch will never execute and constitutes dead code. The Eclipse Java compiler flags code containing this fragment as "Unlikely argument type for equals(): Class seems to be unrelated to GenericType".

Similarly, ApiClient.mustache:483 reads if ("byte[]".equals(returnType.toString())), but the implementation of toString shows that the string returned from this method will always have the prefix "GenericType{" regardless of what the template parameter T is.

This fragment is in the ApiClient mustache for multiple libraries.

Swagger-codegen version

At least since 3.0.35, but present in master now.

Suggest a fix/enhancement

Maybe the tests should read something like:

final Class<?> retCls = returnType.getRawType();
if (byte[].class.equals(retCls)) {
    /* ... */
} else if (File.class.equals(retCls)) {

instead. This compiles without warnings, but I haven't tested if the code would now actually do what it is supposed to.

Start with modules/swagger-codegen/src/main/resources/Java/libraries/resteasy/ApiClient.mustache around lines 483 and 489, then check the corresponding ApiClient.mustache files for the other libraries. Inspect how generated clients handle byte-array and File downloads, and verify that the revised checks compile without the reported warning and preserve the intended behavior.