Handling "java.lang.NullPointerException"

Question

so I'm learning Java for the first time. The following code throws a NullPointer exception when square.toString() is called in another class (where square is an instance of the object defined in this class), and I'm a little foggy about why this doesn't work. Can someone explain that to me?

public class SquareBuilder { String box; String[] parts; final private String beam = "----"; final private String collumn = "|"; private int size; public SquareBuilder(int firstSize) { size = firstSize; } public static String repeatString(String s, int n) { StringBuilder sb = new StringBuilder(); for (int i = 0; i < n; i++) { sb.append(s); } return sb.toString(); } public void changeSize(int newSize) { size = newSize; } public String toString( ) { parts[0] = repeatString(beam,size) + "\n"; parts[1] = collumn + repeatString(" ",4*size-2) + collumn + "\n"; box = parts[0] + repeatString(parts[1],size-2) + parts[0]; return box; } }

What's the full stack trace of your exception?

Jeffrey
– Jeffrey

2013-03-02 21:56:55 +00:00
Commented Mar 2, 2013 at 21:56 — Jeffrey
– Jeffrey, Commented Mar 2, 2013 at 21:56

Jon Skeet · Accepted Answer · 2013-03-02 21:58:04Z

It's unclear why you've declared an instance variable called parts which you then dereference (parts[0] = ...) in toString() - but you're never initializing it, so the value is null... hence the exception.

Why is your implementation not just:

public String toString( ) { String ends = repeatString(beam,size) + "\n"; String middle = collumn + repeatString(" ",4*size-2) + collumn + "\n"; return ends + middle + ends; }

It's very unusual to want to modify an object in toString().

I had arrays on the brain for some reason, obviously no reason to use them. Thanks.

PermGenError · Accepted Answer · 2013-03-02 21:58:14Z

Your question is not framed properly, however i guess that you are getting an NPE in your toString() method. Reason is you never really initialized your array String[] parts; in your code.

You have to initialize it first:

public SquareBuilder(int firstSize) { size = firstSize; this.parts = new int[give the size here]; }

ncmathsadist · Accepted Answer · 2013-03-02 22:07:34Z

A NullPointerException comes when you attempt to use methods on a null object. You should, in your constructor, initialize the parts array using the expression

parts = new String[whateverSizeYouWant];

It is not clear to me what you are doing with the array.

Collectives™ on Stack Overflow

Handling "java.lang.NullPointerException"

3 Answers 3

1 Comment

Comments

Comments

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

1 Comment

Comments

Comments

Related