Data Structures, Algorithms, & Applications in Java
Chapter 12, Exercise 21

We create a class with the static data members a and last. The public method inOrder simply sets these data members and invokes the recursive method theInOrder, which does the actual traversal. The code is given below. The test program and output are in the files ArrayBinaryTreeWithInorder.*. 
public class ArrayBinaryTreeWithInorder
{
   // data members
   static Object [] a;   // array that contains the tree
   static int last;      // position of last element in array a

   /** visit method that prints the element in a[i] */ 
   public static void visit(int i)
      {System.out.print(a[i] + " ");}


   /** inorder traversal */
   public static void inOrder(Object [] theArray, int theLast)
   {
      // set static data members
      a = theArray;
      last = theLast;

      // start the recursive traversal method at the root
      theInOrder(1);
   }

   /** actual method to do the inorder traversal */
   static void theInOrder(int i)
   {// traverse subtree rooted at a[i]
      if (i <= last && a[i] != null)
      {// root exists
         theInOrder(2 * i);         // do left subtree
         visit(i);                  // visit tree root
         theInOrder(2 * i + 1);     // do right subtree
      }
   }
}