Main.java

package ua.kiev.prog;

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        var scanner = new Scanner(System.in);

        System.out.println("Введите выражение:");
        var s = scanner.nextLine(); // 1 + 2

        var parts = s.split(" ");
        if (parts.length != 3) {
            System.out.println("Ошибка!");
            return;
        }

        var x = Double.parseDouble(parts[0]); // 1
        var y = Double.parseDouble(parts[2]); // 2
        var operator = parts[1]; // +

        if ("+".equals(operator))
            System.out.println(x + y);
        else if ("-".equals(operator))
            System.out.println(x - y);
        else if ("*".equals(operator))
            System.out.println(x * y);
    }
}