convert an infix expression to postfix

infixToPostfix.java

package test;

import java.util.Stack;

public class InfixToPostfix {
	
	//checks for the priority of an operator
	public static int priority(char ch){
		
		switch(ch){
			case '^':
				return 5;
			case '*':
				return 4;
			case '/':
					return 3;
			case '+':
				return 2;
			case '-':return 1;
		}
		return -1;
		
	}
	
	//method to check if a character is an operand 
	public static boolean isOperand(char op){
		return (op>='a' && op<='z'  || op>='A' && op<='Z');
	}
	
	//solution
	public static String solution(String infix){
		
		if(infix == null || infix == ""){
			return "invalif data";
		}
		
		Stack<Character> stack = new Stack<Character>();
		String output = "";
		
		//scan through list and use each character
		for(int i=0; i < infix.length(); i++){
			
			//checks if character is an operand, if it is
			//it puts it in the output string
			if(isOperand(infix.charAt(i))){
				output+=infix.charAt(i);
			}
			
			// if character is ( , push it into stack
			else if(infix.charAt(i) == '('){
				stack.push(infix.charAt(i));
			}
			
			//if character is ) it pops every character in
			//stack uptil ( to the ouput string
			//then removes (
			else if(infix.charAt(i) == ')'){
				while(!stack.isEmpty() && stack.peek()!='('){
					
					output += stack.pop();
					
				}
				if(stack.isEmpty()){
					
					return "ivalid expression";
				}
				else{
					stack.pop();
				}
				
			}
			
			else{
				while(!stack.isEmpty() && priority(infix.charAt(i))<priority(stack.peek())){
					output+=stack.pop();
					//System.out.println("dff");
				}
				stack.push(infix.charAt(i));
				//output +=stack.pop();
				//System.out.println(stack.peek());
			}
		
		}
		
		 while(!stack.isEmpty()){
			 output+=stack.pop();
		 }
		
		return output;
		
	}
	
	public static void main(String[]args){
		
		String test = "a+b*(c^d-e)^(f+g*h)-i";
		//solution(test);
		System.out.println(solution(test));
		
	}
	
}