5 ms·

For me, it's mainly that you know the runtime type of the type parameter, so you can do things like new T() in your generic class.

• kmiroslav 10y ago
  Pass a factory `createT()` instead, problem solved.
  • adevine 10y ago
    Exactly - except it's gross and results in a lot of annoying boilerplate code for what should be considered a simple operation. Tons of generic APIs essentially do the same thing by passing in the class object in the constructor, e.g. http://stackoverflow.com/a/1090488/1075909 http://stackoverflow.com/a/1090488/1075909 . The question for people not that familiar with the Java type system is often "Why do I have to pass in the class object when I just declared the type parameter in my instance?" That's not the only thing you need to workaround because of type erasure (the "TypeToken" stuff in a lot of deserialization libraries is another one that comes to mind frequently), but it's probably the first annoying example people hit.
    • kmiroslav 10y ago
      Erasure is not the problem here, you need to be able to specify a type constraint that will allow you to invoke T(). What if T is an interface? An abstract class? A class with no default constructor? The only reasonable way to do this is to accept a lambda that tells the compiler exactly what call is legal to create an instance of T. And that's called a factory.
      • adevine 10y ago
        Yes, good point. But if you did have a type constraint, like C#, you would still need the runtime type info to know what to create. This is legal in C#: public static T Factory<T>() where T:new() { return new T(); }