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Re: Why does Java complain if you cast a Map but not an Object?

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On 4/19/2023 11:16 AM, e.d.pro...@gmail.com wrote:
> @SuppressWarnings("unchecked")
> public static Map<MyObject,MyObject> testObjectUnchecked(Object in) {
>    return (Map<MyObject,MyObject>) in; // warning
> }
> public static Map<MyObject,MyObject> testObjectChecked(Object in) {
>    return testMap(in); // no warning
> }
> public static Map<MyObject,MyObject> testMap(Object in) {
>    if (in instanceof Map) {
>      Map<?,?> m1 = (Map<?,?>)in;
>      Map<MyObject,MyObject> m2 = new HashMap<>();
>      for (Entry<?,?> e : m1.entrySet()) {
>        m2.put((MyObject)e.getKey(),(MyObject)e.getValue()); // no warning
>      }
>      return m2;
>    }
>    return Collections.<MyObject,MyObject>emptyMap();
> }

I think it makes sense.

(MyObject)e.getKey() and (MyObject)e.getValue() will work
fine if they are a MyObject and throw a ClassCastException
if they are not a MyObject.

(Map<MyObject,MyObject>)in is worse:
- it will work if it is a Map<MyObject,MyObject>
- it will throw a ClassCastException it is not a Map
- it will seemingly work but likely cause a ClassCastException
   at some later point if it is a Map<X,Y>  but not a
   Map<MyObject,MyObject>

Try this example using List instead of Map:

import java.util.ArrayList;
import java.util.List;

public class ToWarnOrNotToWarn {
     private static String test1(Object o) {
         try {
             return (String)o;
         } catch(ClassCastException ex) {
             System.out.println(o.getClass().getName() + " is not a 
String");
             return null;
         }
     }
     private static List<String> test2(Object o) {
         try {
             return (List<String>)o;
         } catch(ClassCastException ex) {
             System.out.println(o.getClass().getName() + " is not a 
List<String>");
             return null;
         }
     }
     public static void main(String[] args) {
         Object o1 = "ABC";
         System.out.println(test1(o1));
         Object o2 = 123;
         System.out.println(test1(o2));
         System.out.println(test2(true));
         List<String> a1 = new ArrayList<String>();
         a1.add("ABC");
         System.out.println(test2(a1));
         System.out.println(test2(a1).get(0).substring(0, 1));
         List<Integer> a2 = new ArrayList<Integer>();
         a2.add(123);
         System.out.println(test2(a2));
         System.out.println(test2(a2).get(0).substring(0, 1));
     }
}

result:

ABC
java.lang.Integer is not a String
null
java.lang.Boolean is not a List<String>
null
[ABC]
A
[123]
Exception in thread "main" java.lang.ClassCastException: 
java.lang.Integer cannot be cast to java.lang.String
	at ToWarnOrNotToWarn.main(ToWarnOrNotToWarn.java:36)

the warning on test2 is justified because the cast in test2 cause
a ClassCastException in main.

Arne

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Thread

Why does Java complain if you cast a Map but not an Object? "e.d.pro...@gmail.com" <e.d.programmer@gmail.com> - 2023-04-19 08:16 -0700
  Re: Why does Java complain if you cast a Map but not an Object? Arne Vajhøj <arne@vajhoej.dk> - 2023-04-19 13:43 -0400
    Re: Why does Java complain if you cast a Map but not an Object? "e.d.pro...@gmail.com" <e.d.programmer@gmail.com> - 2023-04-19 11:19 -0700
      Re: Why does Java complain if you cast a Map but not an Object? Arne Vajhøj <arne@vajhoej.dk> - 2023-04-19 15:23 -0400
        Re: Why does Java complain if you cast a Map but not an Object? Michael Jung <miju@golem.phantasia.org> - 2023-04-23 14:30 +0200

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