Added Calculator.java with basic methods

Author: RezuanMustafaHasan

src/Calculator.java

@@ -0,0 +1,6 @@
+public class Calculator {
+    public int add(int a, int b) { return a + b; }
+    public int subtract(int a, int b) { return a - b; }
+    public int multiply(int a, int b) { return a * b; }
+    public int divide(int a, int b) { return a / b; }
+}