Chapter 7: Generics and Collections K and B pg no 584

[kiruthigha rajan]

import java.util.*;
class Dog {
public Dog(String n) { name = n; }
public String name;
public boolean equals(Object o) {
if((o instanceof Dog) &&
(((Dog)o).name == name)) {
return true;
} else {
return false;
}
}
public int hashCode() {return name.length(); }
}
class Cat { }
enum Pets {DOG, CAT, HORSE }
class MapTest {
public static void main(String[] args) {
Map<Object, Object> m = new HashMap<Object, Object>();
m.put("k1", new Dog("aiko")); // add some key/value pairs
m.put("k2", Pets.DOG);
m.put(Pets.CAT, "CAT key");
Dog d1 = new Dog("clover"); // let's keep this reference
m.put(d1, "Dog key");
m.put(new Cat(), "Cat key");
System.out.println(m.get("k1")); // #1
String k2 = "k2";
System.out.println(m.get(k2)); // #2
Pets p = Pets.CAT;
System.out.println(m.get(p)); // #3
System.out.println(m.get(d1)); // #4
System.out.println(m.get(new Cat())); // #5
System.out.println(m.size()); // #6
}
}
which produces something like this:
Dog@1c
DOG
CAT key
Dog key
null
5

i have some doubts in this code.
As far as i understand "The Map interface maps unique keys to value means it associate value to unique keys which you use to retrieve value at a later date."
when we see the first output "System.out.println(m.get("k1"));" it printsthe hashcode and not the aiko
But this line prints the System.out.println(m.get(d1)); // #4 the dog key
As hashMap returns the value of the key then why its not returning in the first case..please explain how hashMap works..
thanks in advance

[Tim McGuire]

m.put("k1", new Dog("aiko")); // add some key/value pairs  
System.out.println(m.get("k1")); // #1  

that will call the toString method of the object mapped to the key k1. If toString is not overridden for an object, you will get that "hash" string. So, your Dog object needs to override toString()

m.put(d1, "Dog key"); 

maps the key d1 to a String containing "Dog key" , so no surprise that it prints the string.

m.put("k2", Pets.DOG); 

Pets.DOG is just a string, so it prints the string