@@ -11,8 +11,8 @@ func main() {
 const country, code = "India", 91
 
 const (
-employeeId string = "E101"
-salary float64 = 50000.0
+employeeId string = "E101"
+salary float64 = 50000.0
 )
 
 // Typed Constant

@@ -0,0 +1,106 @@
+package main
+
+import (
+"fmt"
+"math"
+)
+
+// Go Interface - `Shape`
+type Shape interface {
+Area() float64
+Perimeter() float64
+}
+
+// ==================================================================
+// Struct type `Circle` - implements the `Shape` interface implicitly
+type Circle struct {
+Radius float64
+}
+
+func (c Circle) Area() float64 {
+return math.Pi * c.Radius * c.Radius
+}
+
+func (c Circle) Perimeter() float64 {
+return 2 * math.Pi * c.Radius
+}
+
+func (c Circle) Diameter() float64 {
+return 2 * c.Radius
+}
+
+// ====================================================================
+// Struct type `Rectangle` - implements the `Shape` interface implicitly
+type Rectangle struct {
+Length, Width float64
+}
+
+func (r Rectangle) Area() float64 {
+return r.Length * r.Width
+}
+
+func (r Rectangle) Perimeter() float64 {
+return 2 * (r.Length + r.Width)
+}
+
+// ==================================================================================
+// Generic function to calculate the total area of multiple shapes of different types
+func CalculateTotalArea(shapes ...Shape) float64 {
+totalArea := 0.0
+for _, s := range shapes {
+totalArea += s.Area()
+}
+return totalArea
+}
+
+// ====================================================================
+// Interfaces can also be used as fields
+type MyDrawing struct {
+shapes []Shape
+bgColor string
+fgColor string
+}
+
+func (drawing MyDrawing) Area() float64 {
+totalArea := 0.0
+for _, s := range drawing.shapes {
+totalArea += s.Area()
+}
+return totalArea
+}
+
+// ==================================================================
+func main() {
+var s Shape = Circle{5.0}
+fmt.Printf("Shape Type = %T, Shape Value = %v\n", s, s)
+fmt.Printf("Area = %f, Perimeter = %f\n\n", s.Area(), s.Perimeter())
+
+var s1 Shape = Rectangle{4.0, 6.0}
+fmt.Printf("Shape Type = %T, Shape Value = %v\n", s1, s1)
+fmt.Printf("Area = %f, Perimeter = %f\n", s1.Area(), s1.Perimeter())
+
+totalArea := CalculateTotalArea(Circle{2}, Rectangle{4, 5}, Circle{10})
+fmt.Println("Total area = ", totalArea)
+
+drawing := MyDrawing{
+shapes: []Shape{
+Circle{2},
+Rectangle{3, 5},
+Rectangle{4, 7},
+},
+bgColor: "red",
+fgColor: "white",
+}
+
+fmt.Println("Drawing", drawing)
+fmt.Println("Drawing Area = ", drawing.Area())
+
+s = Circle{5}
+fmt.Printf("(%v, %T)\n", s, s)
+fmt.Printf("Shape area = %v\n", s.Area())
+
+s = Rectangle{4, 7}
+fmt.Printf("(%v, %T)\n", s, s)
+fmt.Printf("Shape area = %v\n", s.Area())
+
+}

@@ -46,4 +46,6 @@
 
 ### Methods and Interfaces
 
-+ [Golang Methods Tutorial with Examples](https://www.callicoder.com/golang-methods-tutorial/)
++ [Golang Methods Tutorial with Examples](https://www.callicoder.com/golang-methods-tutorial/)
+
++ [Golang Interfaces Tutorial with Examples](https://www.callicoder.com/golang-interfaces/)