Recipe 7.12 Converting a Collection to an Array

Problem

You have a Collection but you need a Java language array.

Solution

Use the Collection method toArray( ) .

Discussion

If you have an ArrayList or other Collection and you need a Java language array, you can get it just by calling the Collection's toArray( ) method. With no arguments, you get an array whose type is Object[]. You can optionally provide an array argument, which is used for two purposes:

1. The type of the array argument determines the type of array returned.

2. If the array is big enough (and you can ensure that it is by allocating the array based on the Collection's size( ) method), then this array is filled and returned. If the array is not big enough, a new array is allocated instead. If you provide an array and objects in the Collection cannot be casted to this type, you get an ArrayStoreException.

Example 7-1 shows code for converting an ArrayList to an array of type Object.

Example 7-1. ToArray.java

import java.util.*; /** ArrayList to array */ public class ToArray {     public static void main(String[] args) {         ArrayList al = new ArrayList( );         al.add("Blobbo");         al.add("Cracked");         al.add("Dumbo");         // al.add(new Date( ));    // Don't mix and match!         // Convert a collection to Object[], which can store objects         // of any type.         Object[] ol = al.toArray( );         System.out.println("Array of Object has length " + ol.length);         // This would throw an ArrayStoreException if the line         // "al.add(new Date( ))" above were uncommented.         String[] sl = (String[]) al.toArray(new String[0]);         System.out.println("Array of String has length " + sl.length);     } }