This code convert Infix notation to Postfix notation strings for a calculator. You can use whatever way to introduces the Strings but this time I introduce it manually. Hope this can work for you! //Unfortunately this doesn´t validate some facts of a calculator yet.

Infix to Postfix Java Code

import java.util.Stack;

public class InfijoAPosfijo {

	public static void main(String[] args) {

		String myString = "2.3 + 1.2 * ( ( 3.2 + 12 ) - 12 )"; //You can put your entrance here
	    String[] expression = myString.split(" ");	    
	    String postfix = inf_posf(expression);
	    System.out.println(postfix);
	    
	}
  
public static String inf_posf(String[] exp) {
        Stack<String> pila = new Stack<String>();
        String postfix = "";                                // Inicializar el String postfijo final
        for (int i=0;i<exp.length;i++){                   //Buscar en todos los elementos del array exp
           
        	if (!esOperando(exp[i]) && !esParentesisDer(exp[i])){                 
                while (!pila.empty()
                		&& !pila.peek().equals("(")
                		&& tieneMayorPrioridad(pila.peek(), exp[i])){
		                    postfix += " " + pila.peek(); 
		                    pila.pop();
                }
                pila.push(exp[i]);
            }
            else if (esOperando(exp[i])) {
                postfix += " " + exp[i];
            }
            else if (esParentesisDer(exp[i])){
                while (!pila.empty() && !pila.peek().equals("(")){
                    postfix += " " + pila.peek(); pila.pop();
                }
                pila.pop();
            }
        }
        while (!pila.empty()){
            postfix += " " + pila.peek(); pila.pop();
        }
        return postfix;                                            
    }
   
   public static boolean esParentesisDer(String Parent){
	   String parentesis = ")";
	   if (Parent.compareTo(parentesis) == 0)
		   return true;
	   else return false;
   }
 
    public static boolean esOperando(String Op){
    	//Float flotante = new Float(Op);
    	try
    	{
    	  Float.parseFloat(Op);
    	  return true;
    	}
    	catch(NumberFormatException e)
    	{
    	  //not a Float
    		return false;
    	}
    }

    public static boolean tieneMayorPrioridad(String op1, String op2){
        int op1Weight = getPesoOperador(op1);
        int op2Weight = getPesoOperador(op2);
        if ((op1Weight == op2Weight) && (op1Weight != 3)){
            return true;
        }else if ((op1Weight == 3) && (op2Weight == 3)) {
        	return false;
        }
        return op1Weight>op2Weight? true : false;
    }

    public static int getPesoOperador(String op){
        int weight = -1;
        switch (op)
        {
            case "+":
            case "-":
                weight = 1;
                break;
            case "*":
            case "/":
            case "sin":
            case "cos":
            case "tan":
            case "cot":
            case "sec":
            case "csc":
                weight = 2;
                break;
            case "^":
            	weight = 3;	 
            	break;
        }
        return weight;
    }
}