Question

Type erasure is supposed to erase all generic information…
If this is the case how does a library like GSON use generics to determine what type to deserialize to?

e.g.

private Map<String,Date> tenordates;

This will deserialize to <String,Date>
where as

private Map<Date,Date> tenordates;

will deserialize to <Date,Date>

so somehow its using the generic info at runtime.

Answer 1

Type erasure does not erase all type information. It does not delete it from class, field, return type and parameter definitions. The type information in the following examples is retained:

public class Foo extends List<Bar> { ..}

private List<Foo> foos;

public List<Foo> getFoos() {..}

public void doSomething(List<Foo> foos) {..}

This is accesible via reflection – the java.lang.reflect.ParameterizedType. You can check whether a given Type is instanceof that class, cast to it and obtain the type information.