Re: A problem regarding generics

From:

Kevin McMurtrie <mcmurtrie@pixelmemory.us>

Newsgroups:

comp.lang.java.programmer

Date:

Sun, 06 Jun 2010 23:11:57 -0700

Message-ID:

<4c0c8dad$0$22162$742ec2ed@news.sonic.net>

In article <hugmeo$5cm$1@news.albasani.net>, Lew <noone@lewscanon.com>
wrote:

Tom Anderson wrote:

See:

http://java.sun.com/docs/books/jls/third_edition/html/typesValues.html#1102
57

Kevin McMurtrie wrote:

tom

Don't quote sigs.

Java sometimes adds a non-generics method like this:

When you say "Java ... adds", do you mean in the java[x].* packages?

public void add (Object foo)
{
add((SomeClass)foo);
}

Can you give an example from the API?

It can lead to ClassCastException on nonexistent lines.

What do you mean?

Doh! I accidentally sent before I finished.

Generics can break inheritance. For example, the generics declaration
below demands that an override of put() take only a String as the key.
At the same time, HashMap without generics must take an Object as a key.
The compiler fixes this by adding a hidden method.

This compiles with a warning:

public class SnoopingMap<V> extends java.util.HashMap<String, V>
{
   @Override
   public V put(String key, V value)
   {
      System.out.println(key + " -> " + value);
      return super.put(key, value);
   }

   public static void main (String args[])
   {
      SnoopingMap m= new SnoopingMap();
      m.put(new Integer(4), new Integer(5));
   }
}

But fails to run with an error on a bogus line number:

Exception in thread "main" java.lang.ClassCastException:
java.lang.Integer cannot be cast to java.lang.String
   at SnoopingMap.put(SnoopingMap.java:1)
   at SnoopingMap.main(SnoopingMap.java:13)

The javap utility shows the hidden method:

public java.lang.Object put(java.lang.Object, java.lang.Object);
  Signature: (Ljava/lang/Object;Ljava/lang/Object;)Ljava/lang/Object;
  Code:
   0: aload_0
   1: aload_1
   2: checkcast #16; //class java/lang/String
   5: aload_2
   6: invokevirtual #17; //Method
        put:(Ljava/lang/String;Ljava/lang/Object;)Ljava/lang/Object;
   9: areturn

  LineNumberTable:
   line 1: 0

That translates to this code, which will not compile if you add it
yourself:

public Object put(Object key, Object value)
{
    return put((String)key, value);
{
--
I won't see Google Groups replies because I must filter them as spam