Prefix to postfix conversion using stack in java

Java program for Prefix to postfix conversion using stack. Here problem description and explanation.

// Java program for
// Prefix to postfix conversion

// Stack Node
class StackNode
{
	public String data;
	public StackNode next;
	public StackNode(String data, StackNode top)
	{
		this.data = data;
		this.next = top;
	}
}
class MyStack
{
	public StackNode top;
	public int count;
	public MyStack()
	{
		this.top = null;
		this.count = 0;
	}
	// Returns the number of element in stack
	public int size()
	{
		return count;
	}
	public boolean isEmpty()
	{
		if (this.size() > 0)
		{
			return false;
		}
		else
		{
			return true;
		}
	}
	// Add a new element in stack
	public void push(String data)
	{
		// Make a new stack node
		// And set as top
		this.top = new StackNode(data, this.top);
		// Increase node value
		this.count++;
	}
	// Add a top element in stack
	public String pop()
	{
		String temp = "";
		if (this.isEmpty() == false)
		{
			// Get remove top value
			temp = this.top.data;
			this.top = this.top.next;
			// Reduce size
			this.count--;
		}
		return temp;
	}
	// Used to get top element of stack
	public String peek()
	{
		if (!this.isEmpty())
		{
			return top.data;
		}
		else
		{
			return "";
		}
	}
}
public class Conversion
{
	// Check operator
	public boolean isOperator(char text)
	{
		if (text == '+' || text == '-' || 
            text == '*' || text == '/' || 
            text == '^' || text == '%')
		{
			return true;
		}
		return false;
	}
	// Check operands
	public boolean isOperands(char text)
	{
		if ((text >= '0' && text <= '9') || 
            (text >= 'a' && text <= 'z') || 
            (text >= 'A' && text <= 'Z'))
		{
			return true;
		}
		return false;
	}
	// Converting the given prefix expression to 
	// postfix expression
	public void prefixToPostfix(String prefix)
	{
		// Get the size
		int size = prefix.length();
		// Create stack object
		MyStack s = new MyStack();
		// Some useful variables which is using 
		// of to storing current result
		String auxiliary = "";
		String op1 = "";
		String op2 = "";
		boolean isValid = true;
		for (int i = size - 1; i >= 0; i--)
		{
			// Check whether given prefix location
			// at [i] is an operator or not
			if (isOperator(prefix.charAt(i)))
			{
				// When operator exist
				// Check that two operands exist or not
				if (s.size() > 1)
				{
					op1 = s.pop();
					op2 = s.pop();
					auxiliary = op1 + op2 + prefix.charAt(i);
					s.push(auxiliary);
				}
				else
				{
					isValid = false;
				}
			}
			else if (isOperands(prefix.charAt(i)))
			{
				// When get valid operands
				auxiliary = "" + prefix.charAt(i);
				s.push(auxiliary);
			}
			else
			{
				// Invalid operands or operator
				isValid = false;
			}
		}
		if (isValid == false)
		{
			// When have something wrong
			System.out.println("Invalid Prefix : " + prefix);
		}
		else
		{
			// Display calculated result
			System.out.println(" Prefix : " + prefix);
			System.out.println(" Postfix  : " + s.pop());
		}
	}
	public static void main(String[] args)
	{
		Conversion task = new Conversion();
		String prefix = "+*ab^cd";
		task.prefixToPostfix(prefix);
		prefix = "-+*^%adcex*y^ab";
		task.prefixToPostfix(prefix);
	}
}

Output

 Prefix : +*ab^cd
 Postfix  : ab*cd^+
 Prefix : -+*^%adcex*y^ab
 Postfix  : ad%c^e*x+yab^*-