JavaCalculator.java

import java.util.Scanner;

public class JavaCalculator {
    // ((5 + 4) + 4) * 2
    public static void main(String[] args) {
        int stopper = 9;
        Scanner expressionScanner = new Scanner(System.in);
        Scanner stopperScanner = new Scanner(System.in);
        while (stopper != 0) {
            System.out.println("Arithmetic Calculator");
            System.out.println("0:\tExit");
            System.out.println("1:\tRead Expression From File");
            System.out.println("2:\tEnter Expression From Keyboard");
            stopper = stopperScanner.nextInt();
            if (stopper == 2) {
                System.out.println("Enter Expression\t");
                String expression = expressionScanner.nextLine();
                System.out.println("---------------------------------------------");
                System.out.println("Answer: " + evaluateExpression(expression));
                System.out.println("---------------------------------------------");
                System.out.println();
            } else if (stopper == 0) {
                stopperScanner.close();
                expressionScanner.close();
            }
        }
    }

    public static double evaluateExpression(final String str) {
        return new Object() {
            int pos = -1, ch;

            void nextChar() {
                ch = (++pos < str.length()) ? str.charAt(pos) : -1;
            }

            boolean eat(int charToEat) {
                while (ch == ' ')
                    nextChar();
                if (ch == charToEat) {
                    nextChar();
                    return true;
                }
                return false;
            }

            double parse() {
                nextChar();
                double x = parseExpression();
                if (pos < str.length())
                    throw new RuntimeException("Unexpected: " + (char) ch);
                return x;
            }

            double parseExpression() {
                double x = parseTerm();
                for (;;) {
                    if (eat('+'))
                        x += parseTerm(); // addition
                    else if (eat('-'))
                        x -= parseTerm(); // subtraction
                    else
                        return x;
                }
            }

            double parseTerm() {
                double x = parseFactor();
                for (;;) {
                    if (eat('*'))
                        x *= parseFactor(); // multiplication
                    else if (eat('/'))
                        x /= parseFactor(); // division
                    else
                        return x;
                }
            }

            double parseFactor() {
                if (eat('+'))
                    return parseFactor(); // unary plus
                if (eat('-'))
                    return -parseFactor(); // unary minus

                double x;
                int startPos = this.pos;
                if (eat('(')) { // parentheses
                    x = parseExpression();
                    eat(')');
                } else if ((ch >= '0' && ch <= '9') || ch == '.') { // numbers
                    while ((ch >= '0' && ch <= '9') || ch == '.')
                        nextChar();
                    x = Double.parseDouble(str.substring(startPos, this.pos));
                } else if (ch >= 'a' && ch <= 'z') { // functions
                    while (ch >= 'a' && ch <= 'z')
                        nextChar();
                    String func = str.substring(startPos, this.pos);
                    x = parseFactor();
                    if (func.equals("sqrt"))
                        x = Math.sqrt(x);
                    else if (func.equals("sin"))
                        x = Math.sin(Math.toRadians(x));
                    else if (func.equals("cos"))
                        x = Math.cos(Math.toRadians(x));
                    else if (func.equals("tan"))
                        x = Math.tan(Math.toRadians(x));
                    else
                        throw new RuntimeException("Unknown function: " + func);
                } else {
                    throw new RuntimeException("Unexpected: " + (char) ch);
                }

                if (eat('^'))
                    x = Math.pow(x, parseFactor()); // exponentiation

                return x;
            }
        }.parse();
    }

}