Respuesta :
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 evaluatePostfixExpression(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=evaluatePostfixExpression(postfixExpression);
System.out.println("Result:"+value);
} catch (Exception e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
