Interfaces

Interfaces are how Go does polymorphism — writing one function that works with many different types. By the end of this lesson you'll define an interface, satisfy it implicitly, store mixed types in one slice, and pull concrete values back out with type assertions and type switches.

Learn Interfaces in our free Go course — a beginner-friendly interactive lesson with worked examples, a practice exercise and a quick recall.

Part of the free Go course at LearnCodingFast — hands-on lessons with examples you run in your browser, plus practice exercises and a quick quiz.

What You'll Learn in This Lesson

Since Go 1.18 you can write any as a built-in alias for {'interface '} . They mean exactly the same thing.

1️⃣ Defining and Satisfying an Interface

An interface lists the methods a type must have. The crucial Go idea: satisfaction is implicit . You never write implements Speaker — if your type has a Speak() string method, it is a Speaker , full stop. This lets you write a function once and have it accept types that didn't even exist when you wrote it.

Your turn. Give French the one method it needs to satisfy Greeter , then run it.

2️⃣ A Slice of an Interface

Because every type that satisfies Shape can stand in for it, you can put different concrete types into a single and treat them uniformly. This is the everyday payoff of interfaces: collections of "things that can do X" without caring exactly what each one is.

3️⃣ Type Assertions & Type Switches

Sometimes you have an interface value and need the real type back. A type assertion v, ok := i.(T) gives you the value and a boolean telling you whether the assertion held — always use the two-value form so a wrong guess doesn't panic. When you have several possibilities, a type switch reads far more cleanly.

4️⃣ The Empty Interface & Stringer

The empty interface {'interface '} (or any ) is satisfied by everything, which is exactly why fmt.Println can accept any value. One interface you'll meet constantly is fmt.Stringer : give your type a String() string method and fmt will use it automatically when printing.

These lines define a Speaker interface, a type that satisfies it, and use it. Put them in the correct order:

C, F, B, D, E, A, G. Declare the interface, then the concrete type, then the method that satisfies the contract; inside main create a value and call its method. The interface and type definitions live at package level (outside main ).

Predict the output before revealing the answer.

"" false (an empty string then false). The dynamic type is int , not string , so the assertion fails. The two-value form returns the zero value and ok = false instead of panicking.

2. Does this compile, and what does it print?

It compiles and prints money . Coin has a String() string method, so it implicitly satisfies Stringer — no declaration needed.

3. What does the type switch print for describe(3.5) ?

other — 3.5 is a float64 , which matches neither the int nor the string case, so the default branch runs.

Q: Why doesn't Go have an implements keyword?

Go uses structural typing for interfaces: if the methods match, the type satisfies the interface automatically. This decouples your types from the interfaces other packages define, so a type can satisfy an interface that was written long after the type itself.

Q: What's the difference between any and {'interface '} ?

None — any is a built-in alias for {'interface '} added in Go 1.18. Use any for readability in new code.

Q: When should I use a type switch instead of separate interfaces?

Prefer interfaces and polymorphism when behaviour varies by type. Reach for a type switch only when you genuinely must branch on the concrete type — for example, when decoding unknown JSON or formatting values for display.

Yes. type ReadWriter interface {' '} combines both method sets. A type satisfies it only if it has every method from both.

No blanks this time — just a brief. Define a one-method interface, two types that satisfy it, then range over a slice holding both. This "list of things that share a behaviour" shape appears in almost every real Go codebase.

Practice quiz

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