doubt in hashMap kathy seirra ocjp 1 & 2 study guide

[Akshay Rawal]

hi folks..i am new in learning collections. Unable to get how Map works with key value pair...i am unable to get 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"));
m.put("k2",Pets.DOG);

m.put(Pets.CAT , "CAT key");

Dog d1 = new Dog("clover");
m.put(d1,"dog key");

m.put(new Cat() , "cat key");


System.out.println(m.get("c1"));
String c2 = "c2";
	
System.out.println(m.get("c2"));

Pets p = Pets.CAT;

System.out.println(m.get(p));
System.out.println(m.get(d1));
System.out.println(m.get(new Cat()));
System.out.println(m.size());

i am unable to get how values are retrieved using hashcode..somehow i was able to get answer..but it was guess..I am not sure correct way how get method finds value..
i understood that new Cat() gives null because equals and hashcode not overridden...

i am also unable to get..how this code when added at the end of main method produces following output

d1.name = "magnolia";
System.out.println(m.get(d1));//null

d1.name = "clover";
System.out.println(m.get(new Dog("clover")));//dog key

d1.name = "arthur";
System.out.println(m.get(new Dog("clover")));//null

i just know use hashcode to find correct bucket and equals to find object.but practically how it works unable to get..

thanks

regards akshay

[Henry Wong]

Akshay Rawal wrote:
i am unable to get how values are retrieved using hashcode..somehow i was able to get answer..but it was guess..I am not sure correct way how get method finds value..

It would help if you tell us, what were the results, what were your guesses, and what is still confusing you. We can't read your mind, hence, we can't say if you are right or wrong.

Akshay Rawal wrote:
i am also unable to get..how this code when added at the end of main method produces following output

d1.name = "magnolia";
System.out.println(m.get(d1));//null

i just know use hashcode to find correct bucket and equals to find object.but practically how it works unable to get..

In this case, the key of a hashmap must not be mutated. If it is, the behavior is undefined (as it is a broken scenario). Since the behavior is undefined, we can't really say what should happen.

Henry