Java/Collections Data Structure/Infix Postfix

Converts infix arithmetic expressions to postfix

import java.io.IOException;
public class InToPost {
  private Stack theStack;
  private String input;
  private String output = "";
  public InToPost(String in) {
    input = in;
    int stackSize = input.length();
    theStack = new Stack(stackSize);
  }
  public String doTrans() {
    for (int j = 0; j < input.length(); j++) {
      char ch = input.charAt(j);
      switch (ch) {
      case "+": 
      case "-":
        gotOper(ch, 1); 
        break; //   (precedence 1)
      case "*": // it"s * or /
      case "/":
        gotOper(ch, 2); // go pop operators
        break; //   (precedence 2)
      case "(": // it"s a left paren
        theStack.push(ch); // push it
        break;
      case ")": // it"s a right paren
        gotParen(ch); // go pop operators
        break;
      default: // must be an operand
        output = output + ch; // write it to output
        break;
      }
    }
    while (!theStack.isEmpty()) {
      output = output + theStack.pop();
    }
    System.out.println(output);
    return output; // return postfix
  }
  public void gotOper(char opThis, int prec1) {
    while (!theStack.isEmpty()) {
      char opTop = theStack.pop();
      if (opTop == "(") {
        theStack.push(opTop);
        break;
      }// it"s an operator
      else {// precedence of new op
        int prec2;
        if (opTop == "+" || opTop == "-")
          prec2 = 1;
        else
          prec2 = 2;
        if (prec2 < prec1) // if prec of new op less
        { //    than prec of old
          theStack.push(opTop); // save newly-popped op
          break;
        } else
          // prec of new not less
          output = output + opTop; // than prec of old
      }
    }
    theStack.push(opThis);
  }
  public void gotParen(char ch){ 
    while (!theStack.isEmpty()) {
      char chx = theStack.pop();
      if (chx == "(") 
        break; 
      else
        output = output + chx; 
    }
  }
  public static void main(String[] args) throws IOException {
    String input = "1+2*4/5-7+3/6";
    String output;
    InToPost theTrans = new InToPost(input);
    output = theTrans.doTrans(); 
    System.out.println("Postfix is " + output + "\n");
  }
  class Stack {
    private int maxSize;
  
    private char[] stackArray;
  
    private int top;
  
    public Stack(int max) {
      maxSize = max;
      stackArray = new char[maxSize];
      top = -1;
    }
  
    public void push(char j) {
      stackArray[++top] = j;
    }
  
    public char pop() {
      return stackArray[top--];
    }
  
    public char peek() {
      return stackArray[top];
    }
  
    public boolean isEmpty() {
      return (top == -1);
    }
  }
}

Parse postfix arithmetic expressions

import java.io.IOException;
public class ParsePost {
  private Stack theStack;
  private String input;
  public ParsePost(String s) {
    input = s;
  }
  public int doParse() {
    theStack = new Stack(20);
    char ch;
    int j;
    int num1, num2, interAns;
    for (j = 0; j < input.length(); j++) {
      ch = input.charAt(j); 
      theStack.displayStack("" + ch + " ");
      if (ch >= "0" && ch <= "9") // if a number push it
        theStack.push((int) (ch - "0"));   
      else // it"s an operator
      {
        num2 = theStack.pop();
        num1 = theStack.pop();
        switch (ch) {
        case "+":
          interAns = num1 + num2;
          break;
        case "-":
          interAns = num1 - num2;
          break;
        case "*":
          interAns = num1 * num2;
          break;
        case "/":
          interAns = num1 / num2;
          break;
        default:
          interAns = 0;
        }
        theStack.push(interAns);
      }
    }
    interAns = theStack.pop();
    return interAns;
  }
  public static void main(String[] args) throws IOException {
    String input = "1-2+3*4+5/6-7+8*9";
    int output;
    ParsePost aParser = new ParsePost(input);
    output = aParser.doParse();
    System.out.println("Evaluates to " + output);
  }
  class Stack {
    private int maxSize;
  
    private int[] stackArray;
  
    private int top;
  
    public Stack(int size) {
      maxSize = size;
      stackArray = new int[maxSize];
      top = -1;
    }
  
    public void push(int j) {
      stackArray[++top] = j;
    }
  
    public int pop() {
      return stackArray[top--];
    }
  
    public int peek() {
      return stackArray[top];
    }
  
    public boolean isEmpty() {
      return (top == -1);
    }
  
    public boolean isFull() {
      return (top == maxSize - 1);
    }
  
    public int size() {
      return top + 1;
    }
  
    public int peekN(int n) {
      return stackArray[n];
    }
  
    public void displayStack(String s) {
      System.out.print(s);
      System.out.print("Stack (bottom>top): ");
      for (int j = 0; j < size(); j++) {
        System.out.print(peekN(j));
        System.out.print(" ");
      }
      System.out.println("");
    }
  }
}