Notes

public class Exp8_42 {
public static Stack<Object> stack = new Stack<Object>();
public static Character op[] = new Character[20];
public static String temp;

public static void main(String[] args) {
Scanner input = new Scanner(System.in);
char y;

while(true){
System.out.println("Input postfix expression: ");
temp = input.next();
for (int i = 0; i < temp.length(); i++) {
op[i]=temp.charAt(i);

}
evaluate();
System.out.println("Again?");
y = input.next().charAt(0);
if(y=='n'||y=='n'){
break;
}
else stack.clear();

}


}
public static void evaluate(){
int a,b;
for (int i = 0; i <temp.length(); i++) {

if(op[i]!='+'&&op[i]!='*'&&op[i]!='/'&&op[i]!='-'&&op[i]!='^')
{
stack.push(op[i]);

}
else if(op[i]=='+'||op[i]=='*'||op[i]=='/'||op[i]=='-'||op[i]=='^')
{
b = Integer.parseInt(stack.pop().toString());
a = Integer.parseInt(stack.pop().toString());
stack.push(operate(a,op[i],b));
System.out.println(""+a+" "+b);

}
}
System.out.println("The value of the expression is "+stack.pop());





}


public static int operate (Integer a,Character b,Integer c)
{
if(b=='+')
return (a+c);
else if(b=='-')
return (a-c);
else if(b=='*')
return (a*c);
else if(b=='/')
return (a/c);
else if (b=='^')
{
for (int i = 1; i < c; i++) {
a=a*a;
}
return a;
}
else
return 0;

}



public static class Stack<T> {
private int top = 0;
private final static int stackMax=100;
// highest index of stk array
private Object[] stk = new Object[stackMax+1];
//Elements must be cast back.
public Stack() { // constructor
}
// other methods

public void push(T el){
if (top == stackMax) // is stack full ?
System.out.println("Stack Push Overflow Error");
else top = top + 1;
stk[top] = el;
}
public T pop(){
if (isEmpty())
{
System.out.println("Stack Pop Underflow Error");
return null;
}
else
top = top-1;
return (T)stk[top + 1];
}
/*
public T top(T s){
if (isEmpty()){
System.out.println("Stack is Empty");
return null;
}
else
return (T)stk[top];
}*/

public boolean isEmpty(){
if (top == 0)
return true;
else
return false;

}

public void clear(){ // in Java, s is passed as the implied parameter “this”
top = 0;
}

}



}