Im trying to write a code where the user inputs an expression and then prints the answer and the postfix expression my problem is the parenthesis are all off and I'm not sure how to get them to print normal or even do without the parenthesis at all
For instance, 53 must be printed ((5)(3)) or it wont work and if I just print 5 3 it gives me an error:
Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: -2
And (5
3) prints -1 ???
Can you explain what I can do with the parenthesis to fix my problem?

import java.util.Scanner;
import java.util.Stack;

class ExprTreeNode {

    ExprTreeNode left, right;   // The usual pointers.
    boolean isLeaf;             // Is this a leaf?
    int value;                  // If so, we'll store the number here.
    char op;                    // If not, we need to know which operator.

}


class ExpressionTree {

    ExprTreeNode root;


    public void parseExpression (String expr)
    {
        root = parse (expr);
    }


    ExprTreeNode parse (String expr)
    {
        ExprTreeNode node = new ExprTreeNode ();

    // Note: expr.charAt(0) is a left paren. 
        // First, find the matching right paren.
       int m = findMatchingRightParen (expr, 1);

       String leftExpr = expr.substring (1, m+1);

        // Bottom-out condition:
        if (m == expr.length()-1) {
            // It's at the other end => this is a leaf.
            String operandStr = expr.substring (1, expr.length()-1);
            node.isLeaf = true;
            node.value = getValue (operandStr);
            return node;
        }

        // Otherwise, there's a second operand and an operator.

    // Find the left paren to match the rightmost right paren.
       int n = findMatchingLeftParen (expr, expr.length()-2);
       String rightExpr = expr.substring (n, expr.length()-1);

    // Recursively parse the left and right substrings.
        node.left = parse (leftExpr);
        node.right = parse (rightExpr);
        node.op = expr.charAt (m+1);

        return node;
    }


    int findMatchingRightParen (String s, int leftPos)
    {
        Stack<Character> stack = new Stack<Character>();
        stack.push (s.charAt(leftPos));
        for (int i=leftPos+1; i<s.length(); i++) {
            char ch = s.charAt (i);
            if (ch == '(') {
                stack.push (ch);
            }
            else if (ch == ')') {
               stack.pop ();
                if ( stack.isEmpty() ) {
                    // This is the one.
                    return i;
                }
            }
        }
        // If we reach here, there's an error.
        System.out.println ("ERROR: findRight: s=" + s + " left=" + leftPos);
        return -1;
    }


    int findMatchingLeftParen (String s, int rightPos)
    {
        Stack<Character> stack = new Stack<Character>();
        stack.push (s.charAt(rightPos));
        for (int i=rightPos-1; i>=0; i--) {
            char ch = s.charAt (i);
            if (ch == ')') {
                stack.push (ch);
            }
            else if (ch == '(') {
               stack.pop ();
                if ( stack.isEmpty() ) {
                    // This is the one.
                    return i;
                }
            }
        }
        // If we reach here, there's an error.
        System.out.println ("ERROR: findLeft: s=" + s + " right=" + rightPos);
        return -1;
    }

    int getValue (String s)
    {
        try {
            int k = Integer.parseInt (s);
            return k;
        }
        catch (NumberFormatException e) {
            return -1;
        }

    }


    public int computeValue ()
    {
        return compute (root);
    }

    int compute (ExprTreeNode node)
    {
        if (node.isLeaf) {
            return node.value;
        }

        // Otherwise do left and right, and add.
        int leftValue = compute (node.left);
        int rightValue = compute (node.right);

        if (node.op == '+') {
            return leftValue + rightValue;
        }
        else if (node.op == '-') {
            return leftValue - rightValue;
        }
        else if (node.op == '*') {
            return leftValue * rightValue;
        }
        else if(node.op=='^'){
            return (int) Math.pow(leftValue, rightValue);
        }
        else {
            return (int)leftValue / rightValue;
        }

    }


    public String convertToPostfix ()
    {
        String str = postOrder (root);
        return str;
    }

    String postOrder (ExprTreeNode node)
    {
        String result = "";
        if (node.left != null) {
            result += postOrder (node.left);
        }
        if (node.right != null) {
            result += " " + postOrder (node.right);
        }
        if (node.isLeaf) {
            result += " " + node.value;
        }
        else {
            result += " " + node.op;
        }
        return result;
    }


}


class PostfixEvaluator {

    public int compute (String postfixExpr)
    {
    // Create a stack: all our operands are integers.
        Stack<Integer> stack = new Stack<Integer>();

    // Use the Scanner class to help us extract numbers or operators:
        Scanner scanner = new Scanner (postfixExpr);

        while (scanner.hasNext()) {

            if (scanner.hasNextInt()) {
        // It's an operand => push it on the stack.
                int N = scanner.nextInt();
                stack.push (N);
            }
            else {
                // It's an operator => apply the operator to the top two operands
                String opStr = scanner.next();
                int b = stack.pop(); // Note: b is popped first.
                int a = stack.pop();
                if (opStr.equals("+")) {
                    stack.push (a+b);
                }
                else if (opStr.equals("-")) {
                    stack.push (a-b);
                }
                else if (opStr.equals("*")) {
                    stack.push (a*b);
                }
                else if (opStr.equals("/")) {
                    stack.push (a/b);
                }
                else if(opStr.equals("^")){
                    stack.push((int) Math.pow(a, b));
                }
            }

        } // end-while

        // Result is on top.
        return stack.pop();
    }

}



public class Test {

    public static void main (String[] argv)
    {
        System.out.println("Enter an expression: ");
        Scanner scan=new Scanner(System.in);
        String s=scan.nextLine();
        test(s);
        System.out.println("Enter another expression:");
        s=scan.nextLine();
         test(s);
         System.out.println("Enter another expression: ");
         s=scan.nextLine();
        test(s);
      //  test (s);

      //  s = "(((1)+(2))*(3))";
      //  test (s);

      //  s = "(((35)-((3)*((3)+(2))))/(4))";
     //   test (s);
    }


    static void test (String s)
    {
        ExpressionTree expTree = new ExpressionTree ();
        expTree.parseExpression (s);
        int v = expTree.computeValue ();
        System.out.println (s + " = " + v);
        String postStr = expTree.convertToPostfix ();
        PostfixEvaluator postfixEval = new PostfixEvaluator();
        int p = postfixEval.compute (postStr);
        System.out.println ("Postfix: " + postStr + "    postfix eval=" + p);
    }

For instance, 5x3 must be printed ((5)(3)) or it wont work and if I just print 5 x3 it gives me an error:
Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: -2
And (5x3) prints -1 ???

ERROR: findRight: s=5 * 3 left=1
Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: -1
    at java.lang.String.substring(String.java:1937)
    at ExpressionTree.parse(Test.java:33)
    at ExpressionTree.parseExpression(Test.java:21)
    at Test.test(Test.java:255)
    at Test.main(Test.java:235)

The problem starts at line 33 (the first line in your code that appears in the stack trace). Looking there. and just before it, you call findMatchingRightParen on line 31. That method returns -1 if there's an error (as well as displaying the error message in the first line you posted). Then at line 33 you use the returned value to index into a String without first checking for error (-1). Using the -1 causes the StringIndexOutOfBoundsException on line 33 (line 4 of the stack trace)

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