• Use type (void*) in C

typedef struct Node Node;
struct Node {
  void* item;
  Node* next;
};
void* get_last (Node* xs) {
  while (xs->next != NULL) {
    xs = xs->next;
  }
  return xs->item;
}

Benefits? Downsides?

• No static protection against casts

  int main () {
    int p     = (int*) malloc (sizeof(int));
    *p        = 2123456789;
    Node* xs  = (Node*) malloc (sizeof(Node));
    xs->next  = NULL;
    xs->item  = p;               // store pointer
    double* q = get_last(xs);    // alias of p
    printf ("q=%f\n", *q);       // unsafe access
  }
  

  $ clang -m32 parametric-03.c && ./a.out
  q=96621069057346178268049192388430659584.000000
  

• Generic list using subtype polymorphism

static class Node {
  Object  item;
  Node    next;
}
static Object getLast (Node xs) {
  while (xs.next != null) {
    xs = xs.next;
  }
  return xs.item;
}

• ClassCastException better than unsafe access

  public static void main (String[] args) {
    Integer p = Integer.valueOf(2123456789);
    Node xs   = new Node();
    xs.next   = null;
    xs.item   = p;                      // store Integer
    Double q  = (Double) getLast(xs);   // ClassCastException
    System.out.printf ("d=%f\n", q);    // unsafe access
  }
  

  $ javac Parametric2.java
  $ java Parametric2
  java.lang.ClassCastException: Integer cannot be cast to Double
    at Parametric.main(Parametric2.java:10)
  

• Container base class

  abstract class Node {
    Object item;
    Node next;
    Object getLast() {
      Node xs = this;
      while (xs.next != null) xs = xs.next;
      return xs.item;
    }
  }
  

• Implementations reuse getLast

  class IntNode extends Node {
    int getLastItem() {
      int last = (int)getLast(); // cast
      return last;
    }
  }
  

Benefits? Downsides?

• Need type parameters

  class Node {
    ??? item;
    Node next;
    ??? getLast() {
      Node xs = this;
      while (xs.next != null) xs = xs.next;
      return xs.item;
    }
  }
  

• Generic list using parametric polymorphism

class Node<X> {
  X       item;
  Node<X> next;
  X getLast () {
    Node<X> xs = this;
    while (xs.next != null) xs = xs.next;
    return xs.item;
  }
}

• Compiler errors better than runtime exceptions

  public static void main (String[] args) {
    Integer p = Integer.valueOf(2123456789);
    Node<Integer> xs = new Node<>();
    xs.next   = null;
    xs.item   = p;                      // store Integer
    Double q  = (Double) getLast(xs);   // compiler error
    System.out.printf ("d=%f\n", q);    // unsafe access
  }
  

  $ javac Parametric1.java
  error: incompatible types: Integer cannot be converted to Double
    Double q = (Double) getLast(xs);   // compiler error
                              ^
  

static class ArrayList<X> {
  X[] a;
  ArrayList(int n) {
    a = new X[n];
  }
  void put (int i, X item) { a[i] = item; }
  X    get (int i)         { return a[i]; }
}

$ javac Parametric3.java
error: generic array creation
  a = new X[n];
          ^

static class ArrayList<X> {
  X[] a;
  ArrayList(int n) {
    a = (X[]) new Object[n];
  }
  void put (int i, X item) { a[i] = item; }
  X    get (int i)         { return a[i]; }
}

$ javac -Xlint:unchecked Parametric4.java
warning: [unchecked] unchecked cast
    a = (X[]) new Object[n];
              ^

static class ArrayList<X> {
  X[] a;
  
  @SuppressWarnings("unchecked")
  ArrayList(int n) {
    a = (X[]) new Object[n];
  }
  void put (int i, X item) { a[i] = item; }
  X    get (int i)         { return a[i]; }
}

$ javac -Xlint:unchecked Parametric4.java

ArrayList<String> ss = new ArrayList<>(10);
ArrayList os = ss;
os.put (1, 2123456789);
String s = ss.get (1);

$ javac Parametric6.java
Note: Parametric6.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.

$ java Parametric6                     
ClassCastException: Integer cannot be cast to class String
  at Parametric.main(Parametric6.java:4)

String[] ss = new String[10];
Object[] os = ss;
os[1] = 2123456789;
String s = ss[1];

$ javac Parametric7.java
// no warnings

$ java Parametric7                     
ArrayStoreException: Integer
   at Parametric.main(Parametric7.java:3)