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 ensures that the second occurrence of the string literal "Mickey" in our program refers to the same String instance as the first, so our use of an IdentityHashMap in place of a general-purpose Map implementation, such as HashMap, does not affect the program's behavior. Our naive analysis neglects two details, but these details effectively cancel each other out.

The important lesson of this puzzle is: Don't use IdentityHashMap unless you need its identity-based semantics; it is not a general-purpose Map implementation. These semantics are useful for implementing topology-preserving object graph transformations, such as serialization or deep-copying. A secondary lesson is that string constants are interned. As mentioned in Puzzle 13, programs should rarely, if ever, depend on this behavior for their correct operation.