Advanced Puzzlers - Java Puzzlers: Traps, Pitfalls, and Corner Cases - page 269

}
public static void main(String[] args) {
Pair p = new Pair<Object>(23, "skidoo");
System.out.println(p.first() + " " + p.second());
for (String s : p.stringList())
System.out.print(s + " ");
}
}
Solution 88: Raw Deal
This program appears reasonably straightforward. It creates a pair whose first element is the
Integer representing 23 and whose second element is the string "skidoo" . Then the program prints
the first and second elements of the pair, separated by a space. Finally, it iterates over the string
representations of these elements and prints them again, so it ought to print 23 skidoo twice. Sadly,
it doesn't even compile. Worse, the compiler's error message is terribly confusing:
Pair.java:26: incompatible types;
found: Object, required: String
for (String s : p.stringList())
^
This message would make sense if Pair.stringList were declared to return List<Object> , but it
returns List<String> . What on earth is going on?
This rather surprising behavior is caused by the program's use of raw types. A raw type is simply the
name of a generic class or interface without any type parameters. For example, List<E> is a generic
interface, List<String> is a parameterized type, and List is a raw type. In our program, the sole
use of raw types is the declaration of the local variable p in main :
Pair p = new Pair<Object>(23, "skidoo");