add package calculator

calculator/Factory.java

@@ -0,0 +1,18 @@
+package calculator;
+
+public class Factory {
+    public static Compute getCompute(char op) {
+        switch (op) {
+            case '+':
+                return new Add();
+            case '-':
+                return new Subtract();
+            case '*':
+                return new Multiply();
+            case '/':
+                return new Divide();
+            default:
+                return null;
+        }
+    }
+}