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Putting object in HashMap

 
Rizwan Qadri
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Hi,All,
Can we put() an object as a key in HashMap? I am doing this but, doing get(key) gives me null value. Thanks
 
Paul Sturrock
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Yes. In fact we must use an Object as a key (though auto-boxing may make it appear otherwise). can you post the code that is giving you a problem?
 
Rizwan Qadri
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Hi, Paul,
this is the code:

import java.util.*;
class Collection1
{
static int i=9;
public static void main(String[] args)
{
Map mp=new HashMap();
mp.put(new Collection1(),i);

Collection1 cl=new Collection1();

System.out.println("Hello World!"+mp.get(new Collection1()));

}
}
 
Jesper de Jong
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You must implement an equals() and a hashCode() method in your class Collection1 to make this work properly.

See the API documentation of the equals() and hashCode() methods in class Objects for information on how to implement those methods.
 
Rizwan Qadri
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Thanks Jesper
 
Jan van Mansum
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To get the same value back from a map you must use a key object that is "equal" to the one you used to put it in the map

 
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