how to add new elements to a String[] array?

Question

I have a Java code defined as:

String[] where;
/**/where.append(ContactsContract.Contacts.HAS_PHONE_NUMBER+"=1");
/**/where.append(ContactsContract.Contacts.IN_VISIBLE_GROUP+"=1");

Those two appends are not working in this form, how would that work correctly?

Answer 1

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 2

Use a List<Array>, 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

Answer 3

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 4

I'm not that experienced in Java but I have always been told that arrays are static structures that have a predefined size. You have to use an ArrayList or a Vector or any other dynamic structure.

Answer 5

You need to use a Collection List. You cannot re-dimension an array.

Answer 6

There is no method append() on arrays. Instead as already suggested a List object can service the need for dynamically inserting elements eg.

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

Or if you are really keen to use an array:

String[] where = new String[]{
    ContactsContract.Contacts.HAS_PHONE_NUMBER + "=1",
    ContactsContract.Contacts.IN_VISIBLE_GROUP + "=1"
};

but then this is a fixed size and no elements can be added.