HashMap and hashCode()

[saqib sarwar]

[B]

import java.util.*;
class MapEQ {
public static void main(String[] args) {
        Map<ToDos, String> m = new HashMap<ToDos, String>();
        
            ToDos t1 = new ToDos("Monday");      
            ToDos t2 = new ToDos("Monday");        
            ToDos t3 = new ToDos("Tuesday");
        
        m.put(t1, "doLaundry");
        m.put(t2, "payBills");
        m.put(t3, "cleanAttic");
        
        System.out.println(m.size());
} }
class ToDos{
String day;
 
    ToDos(String d) { day = d; }
    
    public boolean equals(Object o) {
        return ((ToDos)o).day == this.day;
    }
     public int hashCode() { return 9; }
}

[/B]

IT OUTPUTS
2

Why if hashCode is same 9;

[Keith Lynn]

Remember that the main requirement of the hashCode contract is that if two objects are equal according to the equals method, then they should have the same hashCode.

If you make the hashCode always return 9, and properly override equals, then you are fulfilling the contract.

It's not an error, it's just inefficienct.
[ April 04, 2007: Message edited by: Keith Lynn ]

[Henry Wong]

IT OUTPUTS
2

Why if hashCode is same 9;

Just because two objects have the same hashcode, doesn't meant that they are equal. If the hashcode are the same, then the map uses the equals() method to further test for equality.

Henry

[Srinivasan thoyyeti]

Hi sarwar,

You can refer to Map API to see what put method does.

Always refer API.

[saqib sarwar]

1st thanks for prompt replies

NOW. as we know

[B]

String s1 = new String("Monday");
String s2 = new String("Monday");
System.out.println(s1==s2);

[/B]

Outputs = false

SO

[B]public boolean equals(Object o) {        return ((ToDos)o).day == this.day;    }[/B]

will return false for t1 and t2.

and obviously return false for t2 and t3.

than how

m.put(t1, "doLaundry"); // added to map OK
m.put(t2, "payBills"); // how can added to map if same hash as t1
m.put(t3, "cleanAttic"); // and how can t3 added to map if same hash as t1

kindly elaborate

[Keith Lynn]

Notice you aren't creating new String objects, you are using String literals.

[Srinivasan thoyyeti]

Hi Sarwar,

you are smart. i got you.

ToDos t1 = new ToDos("Monday"); // "Monday" created in pool
ToDos t2 = new ToDos("Monday"); // "Monday" refers to above in pool

Hence both the keys are equal by definition

public boolean equals(Object o) { return ((ToDos)o).day == this.day; }

If you don't want them to be overwritten try like this

ToDos t1 = new ToDos(new String("Monday"));
ToDos t2 = new ToDos(new String("Monday"));

Hope it clear.
Lets Rock

[saqib sarwar]

thanks thoyyeti .

thanks a lot . i got it.