diff --git a/go/04-design-pattern/go.mod b/go/04-design-pattern/go.mod
new file mode 100644
index 0000000..40b6024
--- /dev/null
+++ b/go/04-design-pattern/go.mod
@@ -0,0 +1,3 @@
+module gitty.informatik.hs-mannheim.de/steger/pr3-code/go/04-design-pattern
+
+go 1.25.0
diff --git a/go/04-design-pattern/main.go b/go/04-design-pattern/main.go
new file mode 100644
index 0000000..777d855
--- /dev/null
+++ b/go/04-design-pattern/main.go
@@ -0,0 +1,85 @@
+package main
+
+import (
+	"fmt"
+
+	"gitty.informatik.hs-mannheim.de/steger/pr3-code/go/04-design-pattern/patterns"
+)
+
+type Counter struct {
+	patterns.Subject
+	Count int
+}
+
+func NewCounter() Counter {
+	return Counter{patterns.NewSubject(), 0}
+}
+
+func (c *Counter) Inc() {
+	c.Count++
+	c.Notify()
+}
+
+func (c *Counter) Dec() {
+	c.Count--
+	c.Notify()
+}
+
+type CountPrinter struct {
+	counter *Counter
+}
+
+func (cp *CountPrinter) Update() {
+	fmt.Println("count updated to ", cp.counter.Count)
+}
+
+type CounterHistory struct {
+	counter *Counter
+	history []int
+}
+
+func (ch *CounterHistory) Update() {
+	ch.history = append(ch.history, ch.counter.Count)
+}
+
+func main() {
+	counter := NewCounter()
+
+	history := CounterHistory{&counter, []int{}}
+	counter.Register(&history)
+
+	printer := CountPrinter{&counter}
+	counter.Register(&printer)
+	printerRegistered := true
+
+	var input string
+	for {
+		fmt.Print("Enter a command (+: increment, -: decrement, q: quit): ")
+		fmt.Scanln(&input)
+
+		if len(input) != 1 {
+			fmt.Println("Please enter a single character.")
+			continue
+		}
+
+		switch input[0] {
+		case '+':
+			counter.Inc()
+		case '-':
+			counter.Dec()
+		case 'p':
+			if printerRegistered {
+				counter.Unregister(&printer)
+			} else {
+				counter.Register(&printer)
+			}
+			printerRegistered = !printerRegistered
+		case 'q':
+			fmt.Println("Exiting...")
+			fmt.Println(history.history)
+			return
+		default:
+			fmt.Println("Unknown command. Use '+', '-', or 'q'.")
+		}
+	}
+}
diff --git a/go/04-design-pattern/patterns/observer.go b/go/04-design-pattern/patterns/observer.go
new file mode 100644
index 0000000..8e674ff
--- /dev/null
+++ b/go/04-design-pattern/patterns/observer.go
@@ -0,0 +1,35 @@
+package patterns
+
+type Observer interface {
+	Update()
+}
+
+type Subject interface {
+	Register(o Observer)
+	Unregister(o Observer)
+	Notify()
+}
+
+type subject struct {
+	observers map[Observer]bool
+}
+
+func NewSubject() Subject {
+	return &subject{
+		observers: make(map[Observer]bool),
+	}
+}
+
+func (s *subject) Register(o Observer) {
+	s.observers[o] = true
+}
+
+func (s *subject) Unregister(o Observer) {
+	delete(s.observers, o)
+}
+
+func (s *subject) Notify() {
+	for o := range s.observers {
+		o.Update()
+	}
+}
diff --git a/go/04-design-pattern/patterns/observer_test.go b/go/04-design-pattern/patterns/observer_test.go
new file mode 100644
index 0000000..d4582f0
--- /dev/null
+++ b/go/04-design-pattern/patterns/observer_test.go
@@ -0,0 +1,66 @@
+package patterns_test
+
+import (
+	"testing"
+
+	"gitty.informatik.hs-mannheim.de/steger/pr3-code/go/04-design-pattern/patterns"
+)
+
+type MockObserver struct {
+	callCount int
+}
+
+func (mo *MockObserver) Update() {
+	mo.callCount++
+}
+
+func TestSubject_Notify(t *testing.T) {
+	type observer struct {
+		MockObserver
+		toRegister        bool
+		expectedCallcount int
+	}
+	tests := []struct {
+		name      string
+		observers []*observer
+	}{
+		{"No observers", []*observer{}},
+		{"Single unregistered observer", []*observer{&observer{MockObserver{0}, false, 0}}},
+		{"Single registered observer", []*observer{&observer{MockObserver{0}, true, 1}}},
+		{"Two registered observers", []*observer{&observer{MockObserver{0}, true, 1}, &observer{MockObserver{0}, true, 1}}},
+		{"One registered and one unregistered observers", []*observer{&observer{MockObserver{0}, true, 1}, &observer{MockObserver{0}, false, 0}}},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			s := patterns.NewSubject()
+
+			for _, o := range tt.observers {
+				if o.toRegister {
+					s.Register(o)
+				}
+			}
+
+			s.Notify()
+
+			for _, o := range tt.observers {
+				if o.callCount != o.expectedCallcount {
+					t.Errorf("%s: expected a callcount of %v, actual: %v", tt.name, o.expectedCallcount, o.callCount)
+				}
+
+				//reset callcount & unregister
+				o.callCount = 0
+				s.Unregister(o)
+			}
+
+			//call notify when all observers are unregistered
+			s.Notify()
+
+			//expect a callcount of 0
+			for _, o := range tt.observers {
+				if o.callCount != 0 {
+					t.Errorf("%s: expected a callcount of 0, actual: %v", tt.name, o.callCount)
+				}
+			}
+		})
+	}
+}