CODEWITHSUNDEEP
javaloopsfor-loopforeach
George Richardson
George Richardson

Reputation: 1268

Why does this for each loop not work?

I have this code that does not work: 

Panel[] panelArr = new Panel[5];

for (Panel p:panelArr) {
    p = new Panel();
}
Label lblName = new Label("Name:");
panelArr[0].add(lblName);

It comes up with the error:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
        at GUIVehicles$NewSportsCarDialog.<init>(GUIVehicles.java:65)

However if I replace the for-each loop with this for loop it works.

for (int i = 0; i < 5; i++) {
    panelArr[i] = new Panel();
}

As far as I can tell looking through the documentation for the for each loop both for loops should be equivalent. Clearly this is not the case and I was wondering why?

Upvotes: 2

Views: 156

Answers (3)

sdf3qrxewqrxeqwxfew3123
sdf3qrxewqrxeqwxfew3123

Reputation: 1295

In first loop you don't put objects to array. You just iterate over array which has 5 nulls, create local variable Panel p (which points to null) and assign a new Panel object. After this loop array still contains 5 nulls.

If you don't have to stick with array, use list instead:

List<Panel> panelList = new ArrayList<>(5); //Java7 diamond operator
panelList.add(new Panel());
panelList.add(new Panel());
panelList.add(new Panel());
panelList.add(new Panel());
panelList.add(new Panel());

Of course this is not the most elegant way but you will not get NPE.

Upvotes: 0

peter.petrov
peter.petrov

Reputation: 39457

Because p is a local variable, it is not panelArr[i] directly.

In your first loop you basically assign the new Panel to p, not to panelArr[i].

Think of p and panelArr[i] as two variables pointing to objects of type Panel. The fact that you initialize p does not mean you also initialized panelArr[i].

Upvotes: 0

arshajii
arshajii

Reputation: 129507

Your for-each is analogous to this:

for (int i = 0; i < 5; i++) {
    Panel p = panelArr[i];
    p = new Panel();
}

Hence the assignment to p clearly has no effect on the array itself -- you're just redirecting p to point somewhere else, which has no bearing on the array. So at the end of the for-each loop, all the array elements are still null. Your second snippet is the appropriate way to fill the array.

Upvotes: 8

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