map question from practice exam

[Rachel Glenn]

Here is the sample code:

import java.util.*;
public class Mangler{
   public static <K,V> Map<V,K> mangle(Map<K,V> in){
      Map<V,K> out = new HashMap<V,K>();
      for (Map.Entry<K,V> entry :in.entrySet()())
          out.put(entry.getValue().entry.getKey());
      return out;
   }


   public static main(String[] args){
      Map m1 = new HashMap();
      m1.put("a",1);
      m1.put("b",2);
      Map m2 = mangle(m1);
      System.out.println(m2.get("a") + " " + m2.get(2));
   }
}

I understand that the call to m2.get("a") returns NULL and the call to m2.get(2) returns "b".

I am surprised it is even allowed to compile. The answer says a warning is generated, but I guess the compiler sees that since a raw map is being passed in, that all type info is lost in the actual mangle function?

[Greg Charles]

I see obvious compile errors at lines 5,6, and 11. Line 11 is an obvious typo, but I'm less sure what you're going for on lines 5 and 6.

The thing to remember about generics is they are a compile time concept. If you use a raw type, the compiler will warn you, but won't error out. The runtime won't even care ... at least, not until you try to use an object as a type it's not.

[Rachel Glenn]

Greg Charles wrote:I see obvious compile errors at lines 5,6, and 11. Line 11 is an obvious typo, but I'm less sure what you're going for on lines 5 and 6.

The thing to remember about generics is they are a compile time concept. If you use a raw type, the compiler will warn you, but won't error out. The runtime won't even care ... at least, not until you try to use an object as a type it's not.

yes I do have some typos...but I see what you are saying. Your response made things clear for me in that if we do use the typed collections (as in Map<T>), the compiler will do type checking for us and issue errors if needed. But if we do NOT use the typed collections, but rather untyped version, like a plain old Map, then the most the compiler can do is issue warnings.

[Himai Minh]

Let me type up the code :

import java.util.*;   
public class Mangler{   
   public static <K,V> Map<V,K> mangle(Map<K,V> in){   
      Map<V,K> out = new HashMap<V,K>();   
      for (Map.Entry<K,V> entry :in.entrySet())   
          out.put(entry.getValue(),entry.getKey());   
      return out;   
   }   
  
  
   public static void main(String[] args){   
      Map m1 = new HashMap();   
      m1.put("a",1);   
      m1.put("b",2);   
      Map m2 = mangle(m1);   
      System.out.println(m2.get("a") + " " + m2.get(2));   
   }   
}