How to add new elements to an array?

Question

I have the following code:

String[] where;
where.append(ContactsContract.Contacts.HAS_PHONE_NUMBER + \"=1\");
where.append(ContactsContract.Contacts.IN_VISIB         


        

Answer 1

you can create a arraylist, and use Collection.addAll() to convert the string array to your arraylist

Answer 2

As tangens said, the size of an array is fixed. But you have to instantiate it first, else it will be only a null reference.

String[] where = new String[10];

This array can contain only 10 elements. So you can append a value only 10 times. In your code you're accessing a null reference. That's why it doesnt work. In order to have a dynamically growing collection, use the ArrayList.

Answer 3

If one really want to resize an array you could do something like this:

String[] arr = {"a", "b", "c"};
System.out.println(Arrays.toString(arr)); 
// Output is: [a, b, c]

arr = Arrays.copyOf(arr, 10); // new size will be 10 elements
arr[3] = "d";
arr[4] = "e";
arr[5] = "f";

System.out.println(Arrays.toString(arr));
// Output is: [a, b, c, d, e, f, null, null, null, null]

Answer 4

The size of an array can't be modified. If you want a bigger array you have to instantiate a new one.

A better solution would be to use an ArrayList which can grow as you need it. The method ArrayList.toArray( T[] a ) gives you back your array if you need it in this form.

List<String> where = new ArrayList<String>();
where.add( ContactsContract.Contacts.HAS_PHONE_NUMBER+"=1" );
where.add( ContactsContract.Contacts.IN_VISIBLE_GROUP+"=1" );

If you need to convert it to a simple array...

String[] simpleArray = new String[ where.size() ];
where.toArray( simpleArray );

But most things you do with an array you can do with this ArrayList, too:

// iterate over the array
for( String oneItem : where ) {
    ...
}

// get specific items
where.get( 1 );

Answer 5

Size of array cannot be modified. If you have to use an array, you can use:

System.arraycopy(src, srcpos, dest, destpos, length); 

Answer 6

Use a List<String>, such as an ArrayList<String>. It's dynamically growable, unlike arrays (see: Effective Java 2nd Edition, Item 25: Prefer lists to arrays).

import java.util.*;
//....

List<String> list = new ArrayList<String>();
list.add("1");
list.add("2");
list.add("3");
System.out.println(list); // prints "[1, 2, 3]"

If you insist on using arrays, you can use java.util.Arrays.copyOf to allocate a bigger array to accomodate the additional element. This is really not the best solution, though.

static <T> T[] append(T[] arr, T element) {
    final int N = arr.length;
    arr = Arrays.copyOf(arr, N + 1);
    arr[N] = element;
    return arr;
}

String[] arr = { "1", "2", "3" };
System.out.println(Arrays.toString(arr)); // prints "[1, 2, 3]"
arr = append(arr, "4");
System.out.println(Arrays.toString(arr)); // prints "[1, 2, 3, 4]"

This is O(N) per append. ArrayList, on the other hand, has O(1) amortized cost per operation.

See also

• Java Tutorials/Arrays
  • An array is a container object that holds a fixed number of values of a single type. The length of an array is established when the array is created. After creation, its length is fixed.
• Java Tutorials/The List interface