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 (you can control this with 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 there are objects in the Collection that cannot be casted to this type, then 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 " + ol.length);
    }
}
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