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  1. import java.io.*;
  2. abstract class ScalarProduct<A> {
  3.     public abstract Integer scalarProduct(A second);
  4. }
  5. class Nil extends ScalarProduct<Nil>{
  6.     Nil(){}
  7.     public Integer scalarProduct(Nil second) {
  8.         return 0;
  9.     }
  10. }
  11. class Cons<A extends ScalarProduct<A>> extends ScalarProduct<Cons<A>>{
  12.     public Integer value;
  13.     public A tail;
  14.     Cons(Integer _value, A _tail) {
  15.         value = _value;
  16.         tail = _tail;
  17.     }
  18.     public Integer scalarProduct(Cons<A> second){
  19.         return value * second.value + tail.scalarProduct(second.tail);
  20.     }
  21. }
  22. class _Test{
  23.     public static Integer main(Integer n){
  24.         return _main(n, 0, new Nil(), new Nil());
  25.     }
  26.     public static <A extends ScalarProduct<A>> Integer _main(Integer n, Integer i, A first, A second){
  27.         if (n == 0) {
  28.             return first.scalarProduct(second);
  29.         } else {
  30.             return _main(n-1, i+1, (A) new Cons<A>(2*i+1,first), second);
  31.             //return _main(n-1, i+1, new Cons<A>(2*i+1,first), new Cons<A>(i*i, second));
  32.             //return _main(n-1, i+1, first, new Cons<A>(i*i, second));
  33.         }
  34.     }
  35. }
  36. class Test{
  37.     public static void main(String [] args){
  38.         System.out.print("Enter a number: ");
  39.         try {
  40.             BufferedReader is = new BufferedReader(new InputStreamReader(System.in));
  41.             String line = is.readLine();
  42.             Integer val = Integer.parseInt(line);
  43.             System.out.println(_Test.main(val));
  44.         } catch (NumberFormatException ex) {
  45.             System.err.println("Not a valid number");
  46.         } catch (IOException e) {
  47.             System.err.println("Unexpected IO ERROR");
  48.         }
  49.     }
  50. }
Runtime error #stdin #stdout #stderr 0.08s 380160KB
comments (?)
stdin 
3
stdout 
Enter a number:
stderr 
Exception in thread "main" java.lang.ClassCastException: Nil cannot be cast to Cons
	at Cons.scalarProduct(Main.java:11)
	at _Test._main(Main.java:28)
	at _Test._main(Main.java:30)
	at _Test._main(Main.java:30)
	at _Test._main(Main.java:30)
	at _Test.main(Main.java:24)
	at Test.main(Main.java:43)
https://ideone.com/N8RiOn
language:
Java (12.0.1)
created:
10 years ago
visibility:
public

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