How can I dynamically add items to a Java array?

Question

In PHP, you can dynamically add elements to arrays by the following:

$x = new Array();
$x[] = 1;
$x[] = 2;

After this, $x would be an array like this: {1,2}.

Is there a way to do something similar in Java?

Answer 1

I have seen this question very often in the web and in my opinion, many people with high reputation did not answer these questions properly. So I would like to express my own answer here.

First we should consider there is a difference between array and arraylist.

The question asks for adding an element to an array, and not ArrayList

---

The answer is quite simple. It can be done in 3 steps.

1. Convert array to an arraylist
2. Add element to the arrayList
3. Convert back the new arrayList to the array

Here is the simple picture of it

And finally here is the code:

Step 1:

public List<String> convertArrayToList(String[] array){
        List<String> stringList = new ArrayList<String>(Arrays.asList(array));
        return stringList;
    }

Step 2:

public  List<String> addToList(String element,List<String> list){

            list.add(element);
            return list;
    }

Step 3:

public String[] convertListToArray(List<String> list){
           String[] ins = (String[])list.toArray(new String[list.size()]);
           return ins;
    } 

Step 4

public String[] addNewItemToArray(String element,String [] array){
        List<String> list = convertArrayToList(array);
        list= addToList(element,list);
        return  convertListToArray(list);
}

Answer 2

Use an ArrayList or juggle to arrays to auto increment the array size.

Answer 3

Look at java.util.LinkedList or java.util.ArrayList

List<Integer> x = new ArrayList<Integer>();
x.add(1);
x.add(2);

Answer 4

In Java size of array is fixed , but you can add elements dynamically to a fixed sized array using its index and for loop. Please find example below.

package simplejava;

import java.util.Arrays;

/**
 *
 * @author sashant
 */
public class SimpleJava {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        // TODO code application logic here

        try{
            String[] transactions;
            transactions = new String[10];

            for(int i = 0; i < transactions.length; i++){
                transactions[i] = "transaction - "+Integer.toString(i);            
            }

            System.out.println(Arrays.toString(transactions));

        }catch(Exception exc){
            System.out.println(exc.getMessage());
            System.out.println(Arrays.toString(exc.getStackTrace()));
        }
    }

}

Answer 5

keep a count of where you are in the primitive array

class recordStuff extends Thread
{
    double[] aListOfDoubles;
    int i = 0;

    void run()
    {
        double newData;
        newData = getNewData(); // gets data from somewhere

        aListofDoubles[i] = newData; // adds it to the primitive array of doubles
        i++ // increments the counter for the next pass

        System.out.println("mode: " + doStuff());
    }

    void doStuff()
    {
        // Calculate the mode of the double[] array

        for (int i = 0; i < aListOfDoubles.length; i++) 
        {
            int count = 0;
            for (int j = 0; j < aListOfDoubles.length; j++)
            {
                if (a[j] == a[i]) count++;
            }
            if (count > maxCount) 
            {
                maxCount = count;
                maxValue = aListOfDoubles[i];
            }
        }
        return maxValue;
    }
}

Answer 6

You can use an ArrayList and then use the toArray() method. But depending on what you are doing, you might not even need an array at all. Look into seeing if Lists are more what you want.

See: Java List Tutorial