Convert ArrayList to Array

Question

I'm having a little trouble converting an array list to an array in java.

I have a class called Table which contains an array list called applicants which takes Strings. When I use my toArray() function it doesn't seem to convert the array list into an array.

I know this because when i run the listArray() function it gives the error:

java.lang.NullPointerException.

Here is the code. Any help would be greatly appreciated.

public class Table {
    public ArrayList<String> applicants;
    public String appArray[];

    public Table() {
        applicants = new ArrayList<String>();
    }

    public void addApplicant(String app) {
        applicants.add(app);
    }

    public void toArray() {
        int x = applicants.size();
        String[] appArray = new String[x];
        appArray = applicants.toArray(appArray);
    }

    public void list() {
        for (int i = 0; i < applicants.size(); i++) {
            System.out.println(applicants.get(i));
        }
    }

    public void listArray() {
        for (int i = appArray.length; i > 0; i--) {
            System.out.println(appArray[i]);
        }
    }
}

Sotirios Delimanolis · Accepted Answer · 2016-06-30 16:53:24Z

16

This is because you are shadowing the appArray field in your method here:

 int x = applicants.size();
 String[] appArray = new String[x];
 appArray = applicants.toArray(appArray);

You create a new local variable called appArray when you should be using the appArray field you already have declared in your class.

You should do

public void toArray()
{
    appArray = applicants.toArray(new String[0]);
}

edited Jun 30, 2016 at 16:53

answered Feb 19, 2013 at 20:19

Sotirios Delimanolis

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5 Comments

Andrew Martin Over a year ago

Can I ask as someone interested in this answer, why do you need to cast the ArrayList to a String[] type? As it is already this type, why won't applicants.toArray() work?

Luiggi Mendoza Over a year ago

@AndrewMartin the function returns Object[] so you should cast it to your class type array.in that way the function will know the type of array to return. Another alternative would be using List#toArray(T[] array), look at the ArrayList#toArray code for an example: grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/…

Sotirios Delimanolis Over a year ago

Or here: stackoverflow.com/questions/11007998/…

krico Over a year ago

This code throws java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [Ljava.lang.String; I think you need to say appArray = applicants.toArray(new String[application.size()]);

Sotirios Delimanolis Over a year ago

@krico You're correct. I've fixed my code to use the overloaded toArray(T[]) method. I thought you were referring to the first snippet.

Khaled Lela · Accepted Answer · 2013-02-19 20:37:37Z

1

appArray = new String[applicants.size()];
int index = 0;
for(String app:applicants){
    appArray [index] = app;
    index++;
}

answered Feb 19, 2013 at 20:37

Khaled Lela

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Comments

Arvind Sridharan · Accepted Answer · 2014-03-14 07:53:01Z

1

Because both your ArrayList and Array should only have strings you can do this.

appArray = applicants.toArray(new String[0]);

More information in the javadoc.

answered Mar 14, 2014 at 7:53

Arvind Sridharan

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