Learning Java, 4th Edition

Casts

We’ve now talked about relationships between generic types and even between generic types and raw types. But we haven’t brought up the concept of a cast yet. No cast was necessary when we interchanged generics with their raw types. Instead, we just crossed a line that triggers unchecked warnings from the compiler:

    List list = new ArrayList<Date>();
    List<Date> dl = list;  // unchecked warning

Normally, we use a cast in Java to work with two types that could be assignable. For example, we could attempt to cast an Object to a Date because it is plausible that the Object is a Date value. The cast then performs the check at runtime to see if we are correct. Casting between unrelated types is a compile-time error. For example, we can’t even try to cast an Integer to a String. Those types have no inheritance relationship. What about casts between compatible generic types?

    Collection<Date> cd = new ArrayList<Date>();
    List<Date> ld = (List<Date>)cd; // Ok!

This code snippet shows a valid cast from a more general Collection<Date> to a List<Date>. The cast is plausible here because a Collection<Date> is assignable from and could actually be a List<Date>. Similarly, the following cast catches our mistake where we have aliased a TreeSet<Date> as a Collection<Date> and tried to cast it to a List<Date>:

    Collection<Date> cd = new TreeSet<Date>();
    List<Date> ld = (List<Date>)cd; // Runtime ClassCastException!
    ld.add( new Date() );

There is one case where casts are not effective with generics, ...