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

3

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().

answered Mar 2, 2013 at 21:58

Jon Skeet

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1 Comment

user2127595 Over a year ago

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

1

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];

}

answered Mar 2, 2013 at 21:58

PermGenError

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Comments

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

0

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.

answered Mar 2, 2013 at 22:07

ncmathsadist

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