Write a java class (mystackapp) to emulate a stack structure using an array. provide methods forand use the postfix expression in the text (given below) to test your logic for evaluating the postfix form of the expression.. infix: : (3 * 4 - (2 + 5) ) * 4 / 2 postfix: 3 4 * 2 5 + - 4 * 2 / be sure to avoid error situations such as popping an empty stack, and the stack holding more than one item at the completion of the program. (note: you should pop() he last value on the stack and display it as the final result, this should result in an empty stack).

Here you go,

Program:

1. MyStackApp class: To be defined in MyStackApp.java

package stackdemo;

/**

* Class emulating stack

*/

public class MyStackApp {

   private int top; // will point to the topmost element of   the stack

   private Object[] storage; //array to hold elements of stack

   private int size; //maximum size of stack

   /**

    * Constructor

    */

   public MyStackApp(int n) {

       // TODO Auto-generated constructor stub

       size = n;

       storage = new Object[size];

       top = -1;

   }

   /**

    * Clears the stack

    */

   public void clear()

   {

       top = -1;

   }

   /**

    * @return true is stack is full.

    */

   public boolean isFull()

   {

       return (top >= storage.length-1);

   }

   /**

    * @return true if stack is empty.

    */

   public boolean isEmpty()

   {

    return (top == -1);

   }

   /**

    * Pushes a new element into stack.

    * @param el - element to push

    * @throws Exception

    */

   public void push(Object el) throws Exception

   {

    if (!isFull())

    {

        top++;

        storage[top] = el;

    }

    else

        throw new Exception("Overflow!");

   }

   /**

    * Pops out top element from stack.

    * @throws Exception

    */

   public Object pop() throws Exception

   {

         if(!isEmpty())

         {

           Object tmp = storage[top];

           top--;

           return tmp;

       }

         else

             throw new Exception("Underflow");

   }

   /**

    * @return top element of stack without popping.

    */

 public Object peek()

 {

         if(!isEmpty())

         {

             return storage[top];

         }

       else

           return null;

 }

 /**

  * @return size of stack.

  */

 public int size()

 {

     return size;

 }

}

2. MyStackAppDemo class: To be defined in MyStackAppDemo.java

package stackdemo;

/**

* Class to show the use of stack through postfix expression evaluation

*

*/

public class MyStackAppDemo {

   //check whether a character is operand

   private static boolean isOperand(char c)

   {

       int operand=c-'0';

       if(operand >= 0 && operand <= 9)

           return true;

       else

           return false;            

   }

   //checks whether a character is operator

   private static boolean isOperator(char c)

   {

       if(c=='+' || c=='-' || c=='*' || c=='/')

           return true;

       else

           return false;            

   }

   /**

    * Evaluates a postfix expression

    * @param postfix - expression in postfix notation

    * @return the result of postfix expression evaluation

    * @throws Exception

    */

   public static int (String postfix) throws Exception

   {

       int result=0;

       MyStackApp stack = new MyStackApp(50); //stack as a buffer for elements of postfic expression

       //loop to parse postfix expression

       for(int i=0;i<postfix.length();i++)

       {

           char el=postfix.charAt(i);

               if (isOperand(el)) //if is operand, push to stack

               {

                  stack.push(el - '0'); // convert char to int

               }

               else if (isOperator(el)) //if is operator, pops two top operands from stack and evaluate. Push result back to stack

               {

                   int opr1 = (int) stack.pop();

                   int opr2 = (int) stack.pop();

                   int value;

                   switch (el)

                   {

                       case '+':

                           value = opr2 + opr1;

                           stack.push(value);

                           break;

                       case '-':

                           value = opr2 - opr1;

                           stack.push(value);

                           break;

                       case '*':

                           value = opr2 * opr1;

                           stack.push(value);

                           break;

                       case '/':

                           value = opr2 / opr1;

                           stack.push(value);

                           break;

                      }

                   }

               }

       result = (int)stack.pop();

       return result;        

   }

   public static void main(String[] args) {

       String postfixExpression="34*25+-4*2/";

       System.out.println("Postfix expression is "+postfixExpression);

       try

       {

           System.out.println("Evaluating postfix expression...");

           int value=(postfixExpression);

           System.out.println("Result:"+value);

       } catch (Exception e) {

           // TODO Auto-generated catch block

           e.printStackTrace();

       }

   }

}

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