how to append values to a StringBuilder array

Question

I am parsing a grid coordinate in degree decimal format. I'd like to store pattern matches in a StringBuilder array for further manipulation. However, I am get a null pointer when calling append() in each while loop.

String coordinateExample = "N14585928W084144340";
this.parseCoordinate(coordinateExample);
...

public void parseCoordinate(String coordinateString) {

    int i = 0;

    //extract NSEW leading characters
    Pattern pL = Pattern.compile("[A-Z]");
    Matcher m = pL.matcher(coordinateString);        
    StringBuilder[] hemisphere = new StringBuilder[]{};

    while (m.find()) {
        hemisphere[i].append(m.group());
        //System.out.println("m.group(): " + m.group());
        i++;
    }        
    // reset i
    i = 0;

    //extract decimal degree digits
    Pattern pN = Pattern.compile("[0-9]++");
    Matcher n = pN.matcher(coordinateString);
    StringBuilder[] coordinate = new StringBuilder[]{};

    while (n.find()) {            
        coordinate[i].append(n.group());
        //System.out.println("q.group(): " + n.group());
        i++;
    }
}

Deadron · Accepted Answer · 2013-04-02 20:02:22Z

6

StringBuilder[] coordinate = new StringBuilder[]{};

This is creating an empty array that can contain instances of type StringBuilder. What you most likely want to do is:

StringBuilder coordinate = new StringBuilder();

which creates an actual StringBuilder object that can be appended to.

edited Apr 2, 2013 at 20:02

answered Apr 2, 2013 at 18:16

Deadron

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4 Comments

Ted Over a year ago

I'll need two coordinate StringBuilder objects and two hemisphere objects. Do I have to create them separately? I'd rather loop through an array...

Ted Over a year ago

I thought that StringBuilder's mutability would allow me to create/initialize a StringBuilder array and then set the values later...

Deadron Over a year ago

The problem is you are confusing an array with the actual StringBuilder Object. An array is an array and in no way encompasses or reflects the functionality of String Builder, it just happens to be able to contain instances of String Builder. If you want an array containing two different instances of string builder then you should do the following: StringBuilder[] coordinate = new StringBuilder[2]; coordinate[0] = new StringBuilder(); coordinate[1] = new StringBuilder(); You would then access each instance as coordinate[0].append("")

Vitaliy Terziev Over a year ago

or with the literal syntax from you answer, StringBuilder [] s = new StringBuilder []{new StringBuilder(),new StringBuilder()};

Mufim · Accepted Answer · 2013-06-20 04:42:00Z

3

You are creating array of string builders but your not instantiating it specify the length when you are using arrays

Solution // StringBuilder[] hemisphere = new StringBuilder[length];

for(int i=0;i<length;i++){
hemisphere [i]=new StringBuilder();
}

Then you wont get any null pointer exception I hope this helps

answered Jun 20, 2013 at 4:42

Mufim

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Comments

tmwanik · Accepted Answer · 2013-04-02 18:14:30Z

0

You are creating an array of StringBuilder without specifying the length i.e StringBuilder[] hemisphere = new StringBuilder[]{}; so you have an empty array that you are trying to populate hence the NullPointerException

answered Apr 2, 2013 at 18:14

tmwanik

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Comments

tmwanik · Accepted Answer · 2013-04-02 18:37:27Z

0

I am really new to Java but why are you using the {} brackets after the instantiation of the builders? Shouldn't it simply be StringBuilder [] name = new StringBuilder []? Or perhaps StringBuilder [][] name = new StringBuilder [][]?

edited Apr 2, 2013 at 18:37

tmwanik

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answered Apr 2, 2013 at 18:25

Michael I

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