(This is with Java 7
)
I was trying to put some JSON string-generating method in my base class rather than having near-identical code in all the subclasses. The first, naive thing I tried was:
public abstract class Base
{
[rest of class...]
final public <T extends Base> String toJsonString() throws IOException {
JacksonRepresentation<T> rep =
new JacksonRepresentation<>(MediaType.APPLICATION_JSON, this);
return rep.getText();
}
}
But that wouldn't compile, giving the error:
error: incompatible types
required: JacksonRepresentation<T>
found: JacksonRepresentation<Base>
where T is a type-variable:
T extends Base declared in method <T>toJsonString()
So I tried this:
public abstract class Base
{
[rest of class...]
final public String toJsonString() throws IOException {
return jsonStringHelper(this);
}
private static <T extends Base> String jsonStringHelper(T object)
throws IOException {
JacksonRepresentation<T> rep =
new JacksonRepresentation<>(MediaType.APPLICATION_JSON, object);
return rep.getText();
}
}
and that worked fine. Why is that? Why can't/doesn't the compiler realize that the type of this
is a type that satisfied T extends Base
and do the necessary resolution?
Because you can have Class1 and Class2 that both extend base, and someone could do this:
Class1 class1 = new Class1();
String result = class1.<Class2>jsonStringHelper();
So while it is guaranteed that 'this' is a subclass of Base, there is no guarantee that 'this' is an instance of T.
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