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SCJP1.4 : Confusion in equals() method

 
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Hi!
I'm preparing for SCJP1.4 certification.

I learned that I need to override the System.Object class's 'boolean equals(Object o)' method in my Class if I want to use my Class object(say myObj) as the key in a Hashtable.If i do not do the same, then I can not search the HashTable unless I have the same reference:

I wrote the following code, but I am getting the NullPointerException on the last statement:

import java.util.Hashtable;

public class Equals {
private int id;
Equals(int id){this.id=id;}

public boolean equals(Object obj)
{
if((obj instanceof Equals)&&(this.id==((Equals)obj).id))
{return true;}
else
{return false;}
}
public static void main(String[] args) {
Hashtable tbl=new Hashtable();

Equals eq1=new Equals(1);

tbl.put(new Equals(1),"1");
System.out.println(tbl.get(eq1).toString());
}
}

Please tell me if I understood the concept wrong.

Thanks and Regards.
Kuldeep.
 
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Did you learn about overriding the hashCode method too ?
You'll need to return the same hashCode to make to objects equal.
 
Kuldeep Tewari
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Thanks! Satou

I haven't gone thru the hashCode() topic yet.I came across equals() topic first in Bates and Sierra's certification book.

Gonna take the hashCode() topic now.

Regards.
Kuldeep.
 
Greenhorn
Posts: 25
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Hi there .. according to the offical java contract whenever u override equals u need to override hashcode as well. But in case u dont do it, the hashtable of the Object class .. the granddaddy of all the classes..would be inherited.. n it would be used..

the problem with ur code are as follows:
1. ur not using "tbl.put(new Equals(1),"1");" properly ur using a new object of Equlas as a key ....which needs hashcode (n u dont have it.. rt?)

2 "tbl.get(eq1).toString());" here ur trying to get the stored value using the older object created in "Equals eq1=new Equals(1);"..


So since both are different objects (though equal. but without hashcode).. u wont get ur stored value in the used HashTable.

Try this may be u get somthin here string is used as a key which has hashcode overiden

import java.util.Hashtable;

public class Equals {

private int id;

Equals(int id){
this.id=id;
}

public boolean equals(Object obj){
if((obj instanceof Equals)&&(this.id==((Equals)obj).id))return true;
else return false;
}

public static void main(String[] args) {
Hashtable tbl=new Hashtable();
Equals eq1=new Equals(1);
tbl.put("1",eq1);
System.out.println(tbl.get("1").toString());
}


}
 
Kuldeep Tewari
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Thanks Akash!
 
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