Hello Java experts
I am learning Java 7 generics, reading Cay Horstmann, Core Java7, Volume I, on page 716.
I dont understand why the run time error (cast illegal) occurs, please see code below.
Can anyone explain it to me better than Cay does?
Thanks!
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public class ProcessArgs {
public static <T extends Comparable> T[] minmax(T... a)
{
Object[] mm = new Object[2];
mm[0] = a[0];
mm[1] = a[1];
if (mm[0] instanceof Comparable)
{
System.out.println("Comparable"); // this is True, prints Comparable at run-time
}
return (T[]) mm; // run-time error here
/* Run-Time ERROR as below:
ComparableException in thread "main"
java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [Ljava.lang.Comparable;
at ProcessArgs.minmax(ProcessArgs.java:13)
at ProcessArgs.main(ProcessArgs.java:18)
*/
}
public static void main(String[] args)
{
String[] sa = minmax("Hello","World"); // ERROR, illegal cast
System.out.println(sa[0] + sa[1]);
Object o = "Hello World"; //works - if we comment out the method call to minmax above
Comparable<String> s = (Comparable) o; // works
Comparable s2 = (Comparable) o; // works
System.out.println(s + " " + (String) s2); // works
return;
}
}