hashmap with duplicate key.

[jamil lusa]

Hi, good day everyone,

i am reading the K&b scjp 6 guide book, in this example:

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
}
}

after that i have read this section:

d1.name = "magnolia";
System.out.println(m.get(d1)); // #1
d1.name = "clover";
System.out.println(m.get(new Dog("clover"))); // #2
d1.name = "arthur";
System.out.println(m.get(new Dog("clover"))); // #3

so obviously changing the hashcode-factor will affect the hashmap collection object (from the example above), now my question is
if the hashcode is changed, it is possible to get duplicated key, and what will happen since hashmap does not allow dulplicated key?

I have checked the hashmap API , the function 'put' will override/replace the duplicated value with the previous value. but how about the question taht i am asking? the collection has distinct key, but suddenly its hashcode has been changed. what happen to the collection?

Thanks , appreciate your reply!

[Ankit Garg]

Jamil did you try it out? What was the output when you tried it?

[jamil lusa]

Hi, sorry i do have compiler in my office pc right now. if got, i have tried to compile it already.

i am using my spare time in office to learn scjp. hoho , hope not be caught by my boss.

this question is too tricky, i really cannot figure out.

hope this question is interesting enough, so there is someone may test it.

Good day,
Jamil.

[Alexandr Khomichenko]

There will be duplication of keys. See the sample:

import java.util.*;
class A {
    public String name;    
    public String toString() {
        return name;
    }
    public int hashCode() {
        return name.length();
    }
    public boolean equals(Object o) {        
        if ((o instanceof A) && (((A) o).name == name)) {
            return true;
        } else {
            return false;
        }
    }
}
public class Test {
    public static void main(String[] args) {
        Map<Object,Object> m = new HashMap<Object,Object>();
        A a = new A(); a.name = "x";        
        m.put(a, "value1");
        A a2 = new A(); a2.name = "y";
        m.put(a2,10);
        a.name = "y";
        for (Iterator i = m.keySet().iterator();i.hasNext();System.out.print(((A)i.next()).name+" "));
    }
}

output: y y