HashMap

[Shiva Mohan]

    
import java.util.*;
class MyClass{
String name;
MyClass(String name){
this.name=name;
}
 
 
public class Chapter7{
public static void main(String[] args){
MyClass m1=new MyClass("Gowher");
MyClass m2=new MyClass("Gowher");
MyClass m3=new MyClass("King");
 
HashMap h=new HashMap();
h.put(m1,"m1 value");              
h.put(m1,"m2 value");
h.put(m3,"m3 value");
h.put(m2,"m4 value");
System.out.println(h.get(m1));
System.out.println(h.get(m2));
System.out.println(h.get(m3));
System.out.println("Size: "+h.size());
}
}

Result:
m2 value
m4 value
m3 value
Size: 3

The book says HashMap is unordered and unsorted.But for me ,it looks like based on the keys,values are sorted.I am consfused on HashMap.please help me to get out.

[Meena R. Krishnan]

Not sure, what made you to think it is sorted. In your code, the values are printed in the order of system.out.println(). You overwrote the value of key m1 and therefore the latest value of m1 is being printed.

[Ulf Dittmer]

I'm not sure how you can infer anything about whether a Map is sorted or not by adding elements to it, and then retrieving those elements by key?

[Meena R. Krishnan]

Here is the modified code for better understanding

import java.util.*;
class MyClass{
String name;
MyClass(String name){
this.name=name;
}
public String toString(){
return(this.name);
}
}
public class Chapter7{
public static void main(String[] args){
MyClass m1=new MyClass("aGowher");
MyClass m2=new MyClass("bGowher");
MyClass m3=new MyClass("eKing");

HashMap h=new HashMap();
h.put(null,"test");
h.put(m2,"m2 value");
h.put(m3,"m3 value");
h.put(m1,"m1 value");


System.out.println(h.get(m2));
System.out.println(h.get(m3));
System.out.println(h.get(m1));
System.out.println("Size: "+h.size());


MyClass[] m = (MyClass[])h.keySet().toArray(new MyClass[0]);
for ( MyClass mm:m){
if(mm != null)
System.out.println(mm.toString());
else 
System.out.println("null");
}



}
}

The above prints

m2 value
m3 value
m1 value
Size: 4
null
bKing
aGowher
eGowher


[ April 14, 2007: Message edited by: M Krishnan ]

[Srinivasan thoyyeti]

Hi Krishnan,

1)First of all you have defined MyClass without a hashCode or equals.
2)Secondly you are passing MyClass a String and assuming some order.

Are you getting what you are doing ?

[Shiva Mohan]

Thanks for the replies.

1.i just meant,based on the keys (referencevariable m1,m2,m3) that order ,values are sorted.

2.I am trying to work on krishnans modified program.i changed the program in 1.4 format.

    
import java.util.*;
class MyClass{
String name;
MyClass(String name){
this.name=name;
}
public String toString(){
return(this.name);
}
}
public class Chapter7{
public static void main(String[] args){
MyClass m1=new MyClass("aGowher");
MyClass m2=new MyClass("bGowher");
MyClass m3=new MyClass("eKing");

HashMap h=new HashMap();
h.put(null,"test");
h.put(m2,"m2 value");
h.put(m3,"m3 value");
h.put(m1,"m1 value");


System.out.println(h.get(m2));
System.out.println(h.get(m3));
System.out.println(h.get(m1));
System.out.println("Size: "+h.size());


MyClass[] m = (MyClass[])h.keySet().toArray(new MyClass[0]);
for ( int mm=1;mm<h.size();mm++){
if(m != null)
System.out.println(m.toString());
else 
System.out.println("null");
}



}
}

But please help me to get the output

m2 value
m3 value
m1 value
Size: 4
null
bKing
aGowher
eGowher

Mine is just printing address(not values)

[Meena R. Krishnan]

Try this:

import java.util.*;
class MyClass{
 String name;
 MyClass(String name){
 this.name=name;
 }
 public String toString(){
  return(this.name);
 }
}
public class Chapter7{
 public static void main(String[] args){
 MyClass m1=new MyClass("aGowher");
 MyClass m2=new MyClass("bGowher");
 MyClass m3=new MyClass("eKing");
 
 HashMap h=new HashMap();
 h.put(null,"test");
 h.put(m2,"m2 value");
 h.put(m3,"m3 value");
 h.put(m1,"m1 value");
 

 System.out.println(h.get(m2));
 System.out.println(h.get(m3));
 System.out.println(h.get(m1));
 System.out.println("Size: "+h.size());
 
 //printing by converting HashMap into an array
 MyClass[] m = (MyClass[])h.keySet().toArray(new MyClass[0]);
 
 for ( int mm=0;mm<m.length;mm++){
  if(m[mm] != null)
   System.out.println(m[mm].toString()); //print the array element
  else
   System.out.println("null");
 }
 }
}

[Chandra Bhatt]

Hi Shiv Mohan,

Originally posted by Shiv Mohan
(i) just meant,based on the keys (referencevariable m1,m2,m3) that order ,values are sorted.

1- You have not overridden equals() and hashCode() methods of the
Obejct class in your class named MyClass. It is recommended that
when you are using your own class as a key, you should give
definition to both the methods to compare when two objects should
be treated equals. What default implementation of the equals() says,
two objects are equal if their reference variable are referring to
the same object on the heap.

So far as your confusion of ordering is concerned: there is no
magic with get() method:
get method takes the object reference(key) and returns the value,
you confusion is running around the same;
you may be thinking that get is returning the objects in the same way
you have stored. But it is nothing like that,
you can try by changing the order:

hashmap.get(m3);
hashmap.get(m1);
hashmap.get(m2);
 

There is nothing like order in the HashMap. And no order means no sorting
of course.

Regards,
cmbhatt