Library Puzzlers - Java Puzzlers: Traps, Pitfalls, and Corner Cases - page 180

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Puzzle 62: The Name Game
This program puts two mappings into a map and prints its size. What does it print?
import java.util.*;
public class NameGame {
public static void main(String args[]) {
Map<String, String> m =
new IdentityHashMap<String, String>();
m.put("Mickey", "Mouse");
m.put("Mickey", "Mantle");
System.out.println(m.size());
}
}
Solution 62: The Name Game
A naive analysis of this program suggests that it should print 1 . The program puts two mappings
into the map, but both have the same key ( Mickey ). It's a map, not a multimap, so the baseball
legend (Mickey Mantle) should overwrite the animated rodent (Mickey Mouse), leaving a single
mapping in the map.
A more thorough analysis casts doubt on this prediction. The documentation for IdentityHashMap
says, "this class implements the Map interface with a hash table, using reference-equality in place of
[value]-equality when comparing keys" [Java-API] . In other words, the program will print 2 rather
than 1 if the second occurrence of the string literal "Mickey" evaluates to a different String
instance from the first. So does the program print 1 , does it print 2 , or might its behavior vary from
implementation to implementation?
If you tried running the program, you found that it prints 1 , as suggested by our naive analysis, even
though the analysis is flawed. Why? The language specification guarantees that string constants are
interned . In other words, string constants that are equal will also be identical [JLS 15.28]. This