Stack exercises

Consider an interface StackInterface that is implemented by StackArrayBased and StackReferenceBased. StackInterface specifies methods that can be called on a stack object, which can be constructed using StackArrayBased or StackReferenceBased. You may reference StackInterface, StackArrayBased and StackReferenceBased in our textbook by Carrano.

How would you modify the header of the merge method (below) so that it can receive as parameters (and return) stacks that are either array-based or reference-based?

  /**
   * merge: Given two stacks containing Integer objects in increasing order from the bottom up, 
   * create a third stack such that the Integer objects are in decreasing order from the bottom up.
   * If an item appears n times in the two given stacks, it will appear n times in the new stack.
   *
   * @param s1  the first stack
   * @param s2  the second stack
   * @return    the new stack, with the items from the two given stacks merged.
   */
  public static StackReferenceBased merge(StackArrayBased s1, StackReferenceBased s2)

A palindrome is a String of characters that reads the same way backwards and forwards, e.g., "radar" is a palindrome whereas "cat" is not. The empty String is considered a palindrome, because it is empty backwards and forwards. A String of one character is considered a palindrome, because it is the same backwards and forwards.
To extract individual characters from a String, the charAt method that is predefined in the String class is documented as follows:
char charAt(int index)
// Returns the char value at the specified index.
Design, implement and test a method in Java that creates and uses a stack of characters to determine if a given String is a palindrome, i.e., return true if the given String is a palindrome or false otherwise. Use the predefined Stack class (a generic class in the Java Collections Framework).