Stage 4

src/repl/Main.java

@@ -1,7 +1,54 @@
 package repl;
 
-public class Main {
+import java.util.Scanner;
+
+public class aMain {
     public static void main(String[] args) {
-        System.out.print("Hello world!");
+        Scanner scanner = new Scanner(System.in);
+        boolean yes = true;
+        do {
+           int result = 0;
+            String operation = scanner.nextLine();
+            if (operation.equals("/exit")) {
+                System.out.println("Bye!");
+                break;
+            } else if (operation.equals("/help")) {
+                System.out.println("This program calculate numbers ");
+                continue;
+            } else if (operation.matches("/.+")) {
+                System.out.println("Unknown command");
+                continue;
+            } else if (operation.matches("[a-zA-Z]+")) {
+                System.out.println("Invalid expression");
+                continue;
+            }else if (operation.matches("\\d+[-+]")){
+                System.out.println("Invalid expression");
+                continue;
+            }else if (operation.matches("\\d+\\s+\\d+")){
+                System.out.println("Invalid expression");
+                continue;
+            }
+            System.out.println(calculate(operation));
+            }
+            while (yes) ;
+        }
+        public static int calculate(String operation){
+        int result = 0;
+            String[] arr = operation.split("\\s+");
+            String mOperation = "";
+            for (int i = 0; i < arr.length; i++) {
+                if (arr[i].matches("\\+")) {
+                    mOperation += " ";
+                } else if (arr[i].matches("\\-")) {
+                    arr[i + 1] = '-' + arr[i + 1];
+                    arr[i] = " ";
+                    mOperation += " ";
+                } else mOperation += arr[i];
+            }
+            String[] arr2 = mOperation.split("\\s+");
+            for (String ch : arr2) {
+                result += Integer.parseInt(ch);
+            }
+            return result;
+        }
     }
-}