To write a java program that evaluates the postfix expression.

 Postfix Evaluation

Aim: 

To write a java program that evaluates the postfix expression.

Description: 

This Program illustrates the evaluation of postfix expression. The program works for expressions that contain only single-digit integers and the four arithmetic operators. The program uses java.util.Stackclass for creating a stack of integer values.
While evaluating a postfix expression if we encounter an operand then push that operand onto stack otherwise if we encounter an operator Pop the top two operands from the stack, apply the operator to them, and evaluate it. Push this result onto the stack. Initially stack is empty

Program:
import java.io.*;
import java.util.*;
class MyClass
{
int isoperator(char symbol)
{
if(symbol == '+' || symbol == '-' || symbol == '*' || symbol == '/' )
return 1;
else
return 0;
}
double evaluate(String postfix)
{
Stack<Double> stk = new Stack<Double>();
int i;
char symbol;
double oper1,oper2,result;
for(i=0;i<postfix.length();i++)
{
symbol = postfix.charAt(i);
if (isoperator(symbol)==0)
stk.push((double)(symbol-48) );
else
{
oper2 = stk.pop();
oper1 = stk.pop();
result = calculate(oper1,symbol,oper2);
stk.push(result);
}
}//end of for.
result = stk.pop();
return(result);
}
double calculate(double oper1,char symbol,double oper2)
{
switch(symbol)
{
case '+' : return(oper1+oper2);
case '-' : return(oper1-oper2);
case '*' : return(oper1* oper2);
case '/' : return(oper1/oper2);
default : return(0);
}
}
}
class Postfix
{
public static void main(String args[]) throws IOException
{ BufferedReader br = new BufferedReader(new InputStreamReader(
System.in));
System.out.println("Enter postfix expression : ");
String postfix = br.readLine();
MyClass ob = new MyClass();
System.out.println("The result value is : " + ob.evaluate(postfix));
}
}