working of map?

[venkatesh badrinathan]

    
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
}
}

can anyone please explain how the 'map' works?? ofcourse we explicitly do not call equals and hashcode methods, but i do not know when are those methods called and how are the data stored in the map?? i think knowing these details could help me get the concepts..

[Deepak Chopra]

/* This is the simple concept of equal and hash code. I prefer if you can read the bucket example give in
K&B. When you put a key~value pair. Hash code value of key is find out and it is kept in the bucket and value is
associated with it.
Now when you try to get out the value using key. for eg : m.get(k2)
here it first find out the hashcode of k2, then it goes to specific bucket and pull out
all the keys related to that bucket.compare each key with k2, first match and value is return to you.

Also do remember the hashcode and equals contract.
if two objects are equal, means they must belong to same bucket. that means their hash code must be equal.
but if two object have equal hashcodes, they may or may not be equal.*/

[Arijit Daripa]

Originally posted by venkatesh badrinathan:

can anyone please explain how the 'map' works?? ofcourse we explicitly do not call equals and hashcode methods, but i do not know when are those methods called and how are the data stored in the map?? i think knowing these details could help me get the concepts..

This is an example from K&B. It is explained there. And darling! a very few in this world can make you understand the core JAVA better than Kathy & Bert do. So stick there. I repeat, stick there. Go through the chapter after and after until you grasp it. I went through that chapter thrice and still going through.
-Best of luck............................

[Nabila Mohammad]

I had exactly the same problem..

Got somepart of it ..but still not crystal clear..

Any way.. Check this link out.

I assure you it 'll definetly help...

https://coderanch.com/t/268571/java-programmer-SCJP/certification/equals-hashCode


https://coderanch.com/t/268751/java-programmer-SCJP/certification/hashCode-equals

[venkatesh badrinathan]

Thanks Nabila

[venkatesh badrinathan]

i have seen programs where
put(arg1,arg2)
arg1 is used as a key and arg2 as value and, vise-versa. is it possible to use
arg2 as key and arg1 as value??? if so, while we retrieve the value from buckets will not there be any prob???

[Nabila Mohammad]

What kind of problem do you think it 'll come across....?