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

Subject: Re: null you have, ... it null you don't
From: email@hidden
Date: Tue, 02 Mar 2004 17:00:01 -0800

Terrance Davis wrote:
| Here's a fun one. Using java 1.4.2 on my nifty new ibook this fills the
| Vector v with nulls:
|
| String s1[] =
| {"g","gt","f","a","z","e","qw","th","lslslslsl","this or that","this and
| that","1123","#4lskdjf","asdf","fdsa","zxq","zyx"};
|
| String o = "somekey";
| Vector v = new Vector();
|
| 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");
| }
| But this doesn't:
|
| String s1[] =
| {"g","gt","f","a","z","e","qw","th","lslslslsl","this or that","this and
| that","1123","#4lskdjf","asdf","fdsa","zxq","zyx"};
|
| String o = "somekey";
| Vector v = new Vector();
|
| 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");
| }
|
|
| I'd love to know if this is a bug or by design!

What you describe is exactly what I'd expect to see happen. The statement

v.add( hm );

adds the HashMap itself (guaranteed non-null). while

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

adds whatever HashMap.put() returns, which might or might not be null.

What *does* HashMap.put() return? The javadocs tell all:

public Object put(Object key,
Object value)

...

Returns:
previous value associated with specified key, or null if there was no
mapping for key. A null return can also indicate that the HashMap
previously associated null with the specified key.

In other words, each call returns the value that the added value replaced. Since the HashMap is created anew each time around the loop, there is never a previous value to return, so put() always returns null.

| Perhaps my iBook is overheating....

Your brain is the more likely victim, I think.

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

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