Java核心技術很優美的代碼3 實現數組的動態分配

/**
   @version 1.01 2004-02-21
   @author Cay Horstmann
*/

import java.lang.reflect.*;
import java.util.*;

public class ArrayGrowTest
{  
   public static void main(String[] args)
   {  
      int[] a = { 1, 2, 3 };
      a = (int[]) goodArrayGrow(a);
      arrayPrint(a);

      String[] b = { "Tom", "Dick", "Harry" };
      b = (String[]) goodArrayGrow(b);
      arrayPrint(b);

      System.out.println("The following call will generate an exception.");
      b = (String[]) badArrayGrow(b);
   }

   /**
      This method attempts to grow an array by allocating a
      new array and copying all elements. 
      @param a the array to grow
      @return a larger array that contains all elements of a.
      However, the returned array has type Object[], not
      the same type as a
   */
   static Object[] badArrayGrow(Object[] a)
   {  
      int newLength = a.length * 11 / 10 + 10;
      Object[] newArray = new Object[newLength];
      System.arraycopy(a, 0, newArray, 0, a.length);
      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 fundamental type array
      @return a larger array that contains all elements of a.

   */
   static Object goodArrayGrow(Object a)
   {  
      Class cl = a.getClass();
      if (!cl.isArray()) return null;
      Class componentType = cl.getComponentType();
      int length = Array.getLength(a);
      int newLength = length * 11 / 10 + 10;

      Object newArray = Array.newInstance(componentType, newLength);
      System.arraycopy(a, 0, newArray, 0, length);
      return newArray;
   }

   /**
      A convenience method to print all elements in an array
      @param a the array to print. can be an object array 
      or a fundamental type array
   */
   static void arrayPrint(Object a)
   {  
      Class cl = a.getClass();
      if (!cl.isArray()) return;
      Class componentType = cl.getComponentType();
      int length = Array.getLength(a);
      System.out.print(componentType.getName()
         + "[" + length + "] = { ");
      for (int i = 0; i < Array.getLength(a); i++)
         System.out.print(Array.get(a, i) + " ");
      System.out.println("}");
   }
}