Added Calculator.java with basic operations (add , sub , mul , dev)

Author: TusharKumarRoy

Calculator.java

@@ -0,0 +1,9 @@
+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) {
+        if (b == 0) throw new IllegalArgumentException("Cannot divide by zero");
+        return a / b;
+    }
+}