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  1. import java.util.*;
  2. import java.lang.*;
  3. import java.io.*;
  4.  
  5. class Ideone
  6. {
  7.     // These 3 choices are the only type-safe things we can do
  8.     private static <T> Map<T, List<T>> getStuff() {
  9.         // return new HashMap<>();
  10.  
  11.         // return null;
  12.  
  13.         Map<T, List<T>> map = new HashMap<>();
  14.         map.put(null, null);
  15.         return map;
  16.     }
  17.  
  18.     // If we want a non-empty map, we have to cast it. But then we can't guarantee it matches
  19.     // the type that was inferred 
  20.     private static <T> Map<T, List<T>> getStuffBad() {
  21.         Map map = new HashMap();
  22.         map.put("a", Collections.emptyList());
  23.         return (Map<T, List<T>>) map;
  24.     }
  25.  
  26.     public static void main (String[] args) throws java.lang.Exception {
  27.         // The same getStuff() call works regardless of whether we want strings or integers
  28.         // The only way this is provable type safe is an empty map or null.
  29.         Map<String, List<String>> strMap = getStuff();
  30.         Map<Integer, List<Integer>> intMap = getStuff();
  31.  
  32.         strMap.put("a", Collections.emptyList());
  33.         System.out.println(strMap);
  34.         intMap.put(1, Collections.emptyList());
  35.         System.out.println(intMap);
  36.  
  37.         // We ask for an int map, which is allowed
  38.         Map<Integer, List<Integer>> badIntMap = getStuffBad();
  39.         try {
  40.             for (Integer i : badIntMap.keySet()) {
  41.                 // But it has a string in it, which throws when we access a non-string method
  42.                 i.intValue();
  43.             }
  44.         }
  45.         catch (Exception e) {
  46.             System.out.println(e.getMessage());
  47.         }
  48.     }
  49. }
Success #stdin #stdout 0.12s 50876KB
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stdin
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Standard input is empty
stdout
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{null=null, a=[]}
{null=null, 1=[]}
class java.lang.String cannot be cast to class java.lang.Integer (java.lang.String and java.lang.Integer are in module java.base of loader 'bootstrap')
https://ideone.com/l6TaxI
language:
Java (HotSpot 12)
created:
4 years ago
visibility:
public

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