implemented singleton pattern

creational/singleton/singleton.go

@@ -0,0 +1,24 @@
+package singleton
+
+type Singleton interface {
+	AddOne() int
+}
+
+type singleton struct {
+	count int
+}
+
+var instance *singleton
+
+func GetInstance() Singleton {
+	if instance == nil {
+		instance = new(singleton)
+	}
+
+	return instance
+}
+
+func (s *singleton) AddOne() int {
+	s.count++
+	return s.count
+}

creational/singleton/singleton_test.go

@@ -0,0 +1,29 @@
+package singleton
+
+import "testing"
+
+func TestGetInstance(t *testing.T) {
+
+	firstCounter := GetInstance()
+
+	if firstCounter == nil {
+		t.Error("Expected pointer to Singleton after calling GetInstance(), not nil")
+	}
+
+	expectedCounter := firstCounter
+
+	count := firstCounter.AddOne()
+	if count != 1 {
+		t.Errorf("After calling for the first time to count, the count must be 1 but it is %d\n", count)
+	}
+
+	secondCounter := GetInstance()
+	if secondCounter != expectedCounter {
+		t.Error("Expected same counter in secondCounter but got different instance")
+	}
+
+	count = secondCounter.AddOne()
+	if count != 2 {
+		t.Errorf("After calling AddOne in second counter, count must be 2 but was $d\n", count)
+	}
+}