For Each loop problem.

[Nigel Browne]

I am currently playing around with some of the new features in Java 1.5. I get a null pointer exception at runtime with the following code.

JTextField term[] = new JTextField[5];
System.out.print(term.length)  // This prints 5 as expected.
for (JTextField txtf: term){
   txtf.setToolTipText("Enter term in quarters.");
}

Why is the loop throwing the exception

[Rick Beaver]

The five elements in your array are all null as you have not populated them yet so you will get NPE's

[Lionel Badiou]

Hello Nigel,

To get a array of objects you must :

create the array (what you did actually)

create each object

In you case you may use something like

  JTextField term[] = {new JTextField(), new JTextField(), new JTextField(), new JTextField(), new JTextField());

Hope that helps,

[Nigel Browne]

There was a big DOH when I read the first reply. However with the old for loop (Which is tried and tested) I can do the following

JTextField term[] = new JTextField[5];
for (int i = 0; i<term.length; i++){
   term[i] = new JTextField(" ", 5);
   term[i].setToolTipText("Enter term in quarters.");
}

But with the so called Enhanced for loop I can't do the following

JTextField term[] = new JTextField[5];
for (JTextField txtf: term){
   txtf = new JTextField(" ", 5);
   txtf.setToolTipText("Enter term in quarters.");
}

Maybe this will be sorted for the next really enhanced for loop

[Dan Patterson]

I think it should be.

JTextField term[] = new JTextField[5];
for (JTextField term: txtf){   
txtf = new JTextField(" ", 5);   
txtf.setToolTipText("Enter term in quarters.");}  

[Nigel Browne]

Originally posted by Dan Patterson:
I think it should be.

JTextField term[] = new JTextField[5];
for (JTextField term: txtf){   
txtf = new JTextField(" ", 5);   
txtf.setToolTipText("Enter term in quarters.");}  

No the : is read as 'in'. So it has to be

for (JTextField txtf:term){...}

I can of course achieve what I wanted using a mix of both the old and new for loops but that hardly leads to a consistant approach.

[Ko Ko Naing]

Guys,
I found this useful info from Sun's Official Page about J2SE 5.0 new features.

So when should you use the for-each loop? Any time you can. It really beautifies your code. Unfortunately, you cannot use it everywhere. Consider, for example, the expurgate method. The program needs access to the iterator in order to remove the current element. The for-each loop hides the iterator, so you cannot call remove. Therefore, the for-each loop is not usable for filtering. Similarly it is not usable for loops where you need to replace elements in a list or array as you traverse it. Finally, it is not usable for loops that must iterate over multiple collections in parallel. These shortcomings were known by the designers, who made a conscious decision to go with a clean, simple construct that would cover the great majority of cases.

Hope this is informative to ranchers out here. The original page can be accessed here!