Stack exercises

1. 4/26 Exercises: Download Stacks.jar and use a decompression program such as 7-zip to extract the files to a directory, then open the directory as a project in BlueJ. Review the StackInterface to see the methods that can be called on a stack object (which can be constructed using StackArrayBased or StackReferenceBased). Read and understand the following examples:
   • Run the main method of StackTest3 which calls the method merge (as described below) and prints the contents of the stack returned by merge. How would you modify the header of the merge method 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.
        */
     

   • 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. Read and understand the Palindrome class which has a method to check whether a given String is a palindrome, and returns true if it is a palindrome, false otherwise. It uses a stack of characters.

     To extract individual characters from a String, it calls the charAt method that is predefined in the String class, as follows:
     char charAt(int index)
     // Returns the char value at the specified index.

     Modify the checkPalindrome method of Palindrome so that it uses the predefined Stack class (a generic class in the Java Collections Framework); see example using this Stack class.

2. 5/1 Exercise: Define a class Calculator with a method evaluatePostfix that receives as a parameter a String (each character of which is a single-digit integer operand or a binary operator: +, -, * or /, in postfix form) and returns an integer representing the result of evaluating the expression. For example, if the input parameter is "234+*" then the method should return 14. Refer your notes/slides or book/handout re: evaluating a postfix expression, and the palindrome example (to see how to work with a stack of characters).
   • This exercise will be one-half of the last assignment (hw7).