CopyOfTest.java

package arrays;

import java.lang.reflect.Array;
import java.util.Arrays;

/**
 * This program demonstrates the use of reflection for manipulating arrays.
 *
 * @author Cay Horstmann
 * @version 1.2 2012-05-04
 */
public class CopyOfTest {
    public static void main(String[] args) {
        int[] a = {1, 2, 3};
        a = (int[]) goodCopyOf(a, 10);
        System.out.println(Arrays.toString(a));

        String[] b = {"Tom", "Dick", "Harry"};
        b = (String[]) goodCopyOf(b, 10);
        System.out.println(Arrays.toString(b));

        /*
        System.out.println("The following call will generate an exception.");
        b = (String[]) badCopyOf(b, 10);
        */
    }

    /**
     * This method attempts to grow an array by allocating a new array and copying all elements.
     *
     * @param a         the array to grow
     * @param newLength the new length
     *                  return a larger array that contains all elements of a. However, the returned array has
     *                  type ObjectQ , not the same type as a
     */
    /*
    @NotNull
    private static Object badCopyOf(Object a, int newLength) {
        Object[] newArray = new Object[newLength];
        System.arraycopy(a, 0, newArray, 0, Math.min(a.length, newLength)); // not useful
        return newArray;
    }
    */

    /**
     * This method grows an array by allocating a new array of the same type and
     * copying all elements.
     *
     * @param a the array to grow. This can be an object array or a primitive
     *          type array
     * @return a larger array that contains all elements of a.
     */
    private static Object goodCopyOf(Object a, int newLength) {
        Class cl = a.getClass();
        if (!cl.isArray())
            return null;
        Class componentType = cl.getComponentType();
        int length = Array.getLength(a);
        Object newArray = Array.newInstance(componentType, newLength);
        System.arraycopy(a, 0, newArray, 0, Math.min(length, newLength));
        return newArray;
    }
}