Interface Programs

Learn interface programs through an executable example, then connect the small demonstration to production code.

Interfaces describe method sets and are implemented implicitly. They allow functions to work with multiple concrete types that offer the required behavior.

The example has two shapes implementing the same Area method and sums their areas without a type switch.

Follow this diagram before executing the example. Check where a value or decision changes what runs next.

This example is a complete program. Save it as main.go or paste it into the Go Playground.

Go code

package main

import "fmt"

type Shape interface{ Area() int }
type Square struct{ Side int }

func (s Square) Area() int { return s.Side * s.Side }

type Rectangle struct{ W, H int }

func (r Rectangle) Area() int { return r.W * r.H }
func totalArea(shapes []Shape) int {
	sum := 0
	for _, s := range shapes {
		sum += s.Area()
	}
	return sum
}
func main() { fmt.Println("Total area:", totalArea([]Shape{Square{3}, Rectangle{2, 5}})) }

Output

Total area: 19

Square{3} contributes 9 and Rectangle{2,5} contributes 10. The interface-based totalArea function adds them to obtain 19.

• Interfaces are satisfied implicitly; no implements keyword is required.
• Method sets matter: some pointer-receiver methods are unavailable on a value type.

Add a Triangle type with an appropriate area calculation and include it in the slice.

Tip: Edit the example above, predict the output, and run the modified code to test your reasoning.

You have practiced Interface Programs in a small, self-contained program. The important habit is to predict what runs, inspect the actual output and distinguish the language rule from the particular example.

Before continuing, make sure you can explain the program to someone who has not seen it, including the line that changes its result.