Postfix to Infix Notation

PostFixtoInfix.java

/**
 * Convert from Postfix to Infix Notation 
 * Infix to Postfix :https://gist.github.com/KodeSeeker/f776c5477c3be3f43a77
 * Algo: 
 * 1) Walk through postfix expression
 * 2) If input is a num then  push to stack.
 * 3) If the input is a operator then pop the last two operatonds from stack and perform operation. Push result back onto stack.
 * 
 * Error condition : if you see a operator and there arent 2 operands in the left in the stack.
 * 
 * To reduce the paranthesis: Add a parenthesis ONLY if the operator you encounter has a higher precedence than the current major 
 * operator in the expression(on the stack.)
 * */
 
 public String postFixtoInfix(String infix)
 {
     if (infix==null)
        return null;
    Stack<Expression> ops= new Stack<Expression>();
    
    for(Character c:infix.tocharArray())
    {
        if(!c.isOperator())
        {
            ops.push(c);
        }
        else
        {
            Operator o= new Operator(c); // assume there is an Operator object.
         /**  
           if(ops.peek().isExpression())
           {
            //compare its operator to the current operator and update it if its LESSER precedence than the current operator
            
            Expression e= ops.pop();
            if (e.getOperator().precedence< o.precedence)
            {
             o.precedence=e.getOperator().precedence;
            
             e.ExpValue="("+ e.getExpValue+")"
             
            }
            ops.push(e);
           }
            
           else{**/
            
            if (ops.pop()!=null)
                String op1= ops.pop();
            if (ops.pop()!=null)
                String op2= ops.pop();
            else
            {
                throw new StackUnderFlowException();
            }
           
            Expression interim= new Expression();
            interim.value= op1+o.getValue()+op2;// string concatenation
            interim.opvalue=op1.getValue();
          
           /* if(interim.opvalue < o.getValue())
                interim="("+interim+")"; //Need to handle brackets.
            interim.opvalue=o.getValue();
            */
            ops.push(interim);
        }
          
        }
        return ops.pop().toString();// final expression.
    }
 }