Discussion
What will be the output of the program?
import java.util.*;
public class Program100
{
public static void main(String[] args)
{
Map<String, Integer> m = new HashMap<String, Integer>();
String key = new String("k");
m.put(key, 1);
System.out.println(m.get("k") + " " + m.get(new String("k")));
}
}
Answer: D
HashMap finds a key with hashCode and equals, never with reference identity, and String implements both by value. Every one of these three String objects therefore agrees, so both lookups return 1. This is exactly why a mutable key, or one without those methods, breaks lookups.
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