Re: null you have, ... it null you don't

Subject: Re: null you have, ... it null you don't
From: Wilhelm Fitzpatrick <email@hidden>
Date: Tue, 2 Mar 2004 16:57:45 -0800

On Mar 2, 2004, at 3:58 PM, Terrance Davis wrote:

for(int x=0; x<s1.length; x++) {
HashMap hm = new HashMap();
v.add( hm.put(o, s1[x]) );

if (v.get(x) == null) System.out.println("wowzers");
}

If you read the JavaDoc for Map.put(key,value) you will see that it is defined to return the object that is CURRENTLY stored in the Map under supplied key. Since this is a brand new Map, obviously nothing is currently stored.

Consider this code:

Map m = new HashMap();
m.put( "key", "value1" );
System.out.println( m.put( "key", "value2" ) );

This will print "value1".

for(int x=0; x<s1.length; x++) {

HashMap hm = new HashMap();
hm.put(o, s1[x]);
v.add( hm );

if (v.get(x) == null) System.out.println("wowzers");
}

This time through, you've filled your vector with s1.length references to the same instance of HashMap, and NOT the objects that you are putting in to your HashMap as I imagine you want. If after this code you said:

System.out.println( v.get(0) );

You might be surprised by the result!

I'd love to know if this is a bug or by design!

by design :)

-wilhelm
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References:
	>null you have, ... it null you don't (From: Terrance Davis <email@hidden>)

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