Parentheses Checking

Given an expression string exp, write a program to examine whether the pairs and the orders of “{“,”}”,”(“,”)”,”[","]” are correct in exp. For example, the program should print true for exp = “[a+{b-(c-(d-e)*d)+w)}+q]” and false for exp = “[(])”

 import java.util.*;

public class Parentheses
   {
   public static void main(String[] args)
{
int n,i,pos,posm,t;
boolean flag=true;
char[] symb={'(','{','[',']','}',')'};
char tmp;
Scanner in = new Scanner(System.in);
String expression;
System.out.println(" Enter the expression:");
expression=in.nextLine();
System.out.println(" Entered expression is:"+expression);
n=expression.length();
Stack s = new Stack(n);
System.out.println("");
for(i=0;i<n;i++)
{
pos=0;
posm=0;
if(expression.charAt(i)==symb[0] || expression.charAt(i)==symb[1] || expression.charAt(i)==symb[2] )
{
s.push(expression.charAt(i));

}
if(expression.charAt(i)==symb[3] || expression.charAt(i)==symb[4] || expression.charAt(i)==symb[5] )
{
if(s.isempty()==true)
{
flag=false;
break;
}
else
{
tmp=s.pop();
for(t=0;t<3;t++)
if(symb[t]==tmp)
pos=t;
for(t=3;t<6;t++)
if(symb[t]==expression.charAt(i))
posm=t;
if(pos+posm!=5)
{
flag=false;
break;
}
}
}
 

}
if(s.isempty()==false)
flag=false;
if(flag==true)
System.out.println(" Valid Expression:");
else
System.out.println(" Invalid Expression:");
in.close();
}

  }
class Stack
{
public Stack(int a)
{
top=-1;
n=a;
stack = new char[n];
}
public void push(char x)
{
stack[++top]=x;
}
public char pop()
{
return (stack[top--]);
}
public char stacktop()
{
return (stack[top]);
}
public boolean isempty()
{
return ( top==-1 ? true: false);
}
private int top;
private int n;
private char[] stack;

}