Add the Sum class to test as an example.

src/main/java/space/kklochko/simple_jbdd/test_examples/Sum.java

@@ -0,0 +1,8 @@
+package space.kklochko.simple_jbdd.test_examples;
+
+public class Sum {
+    static int sum(int a, int b) {
+        return a + b;
+    }
+}
+

src/test/groovy/space/kklochko/simple_jbdd/test_examples/SumSpec.groovy

@@ -0,0 +1,11 @@
+package space.kklochko.simple_jbdd.test_examples
+
+import spock.lang.Specification
+
+class SumSpec extends Specification {
+    def "Adding two numbers"() {
+        expect: "Two plus three is five."
+        5 == Sum.sum(2, 3);
+    }
+}
+