Calculator.java

package calculator;
import sun.applet.Main;
import sun.rmi.runtime.Log;

import java.io.*;


public class New {
    public static final String OUT_FILE_PATH = "./output.txt";

    public static void main(String[] args) {

        MyScanner scanner = new MyScanner();

        while (true) {
            System.out.println("Please, enter Your choice 1(read from file) or 2(manual input #x + y#): ");
            String num1 = scanner.scanString();

            if (num1.equals("2")) {
                System.out.println("Please, enter the whole operation ");
                String num2 = scanner.scanString();
                precondition(num2);
            } else if (num1.equals("1")) {
                String line = null;

                try {
                    BufferedReader reader = new BufferedReader(
                            new FileReader(OUT_FILE_PATH));
                    line = reader.readLine();
                    reader.close();
                } catch (FileNotFoundException e) {
                    e.printStackTrace();
                } catch (IOException e) {
                    e.printStackTrace();
                } finally {
                    System.out.println(" ");
                }
                precondition(line);
            } else {
                continue;
            }
        }
    }

    public enum Actions {
        SUM,
        SUBTRACTION,
        MULTIPLY,
        DIVIDE
    }

    public static char getActionSymbol(Actions action) {
        switch (action) {
            case SUM:
                return '+';
            case SUBTRACTION:
                return '-';
            case MULTIPLY:
                return '*';
            case DIVIDE:
                return '/';
        }
        return ' ';
    }

    public static double precondition(String valueToEvaluate) {
        try {
            if (valueToEvaluate.contains("*") || valueToEvaluate.contains("/") ||
                    valueToEvaluate.contains("+") || valueToEvaluate.contains("-")) {
                Actions currentAction = null;
                int indexOfMultiply = valueToEvaluate.indexOf("*");
                int indexOfDivide = valueToEvaluate.indexOf("/");
                int indexOfSum = valueToEvaluate.indexOf("+");
                int indexOfSubtraction = valueToEvaluate.indexOf("-");

                if (indexOfMultiply > 0) {
                    currentAction = Actions.MULTIPLY;
                } else if (indexOfDivide > 0) {
                    currentAction = Actions.DIVIDE;
                } else if (indexOfSum > 0) {
                    currentAction = Actions.SUM;
                } else if (indexOfSubtraction > 0) {
                    currentAction = Actions.SUBTRACTION;
                } else {
                    System.err.println("");
                }

                char currentActionChar = getActionSymbol(currentAction);
                int indexOfAction = valueToEvaluate.indexOf(currentActionChar);
                String firstPart = valueToEvaluate.substring(0, indexOfAction).trim();
                String secondPart = valueToEvaluate.substring(indexOfAction + 1).trim();

                System.out.println("FIRST: " + currentActionChar + "|" + firstPart + "|" + currentAction);
                System.out.println("SECOND: |" + secondPart + "|");
                System.out.println("---------------------------------------");

                double value1 = Double.parseDouble(firstPart);
                double value2 = Double.parseDouble(secondPart);
                double result = 0;
                if ('*' == currentActionChar) {
                    result = value1 * value2;
                }
                if ('/' == currentActionChar) {
                    result = value1 / value2;
                }
                if ('+' == currentActionChar) {
                    result = value1 + value2;
                }
                if ('-' == currentActionChar) {
                    result = value1 - value2;
                }
                System.out.println("RESULT: " + result);
                System.out.println();
                return result;

            } else {
                System.err.println("Cannot recognize expression. Please try again(press ENTER).");
            }
        } catch (Exception e) {
            System.out.println("Перехвачено исключение # некорректный ввод выражения #");
        }
        return 0;
    }

}

package calculator;

import java.util.Scanner;

public class MyScanner {

public String scanString() {
    Scanner sign = new Scanner(System.in);
    String mySign = sign.nextLine();
    return mySign;
}



}