Interfaces Cheatsheet

Basic Definition

type Name interface {
    Method(param Type) ReturnType
}

Key Rules

• Implicit Implementation: No implements keyword. If the methods exist, the interface is fulfilled.
• Empty Interface: interface{} (or any) is implemented by every type.
• Methods ONLY: Interfaces cannot contain fields, only method signatures.

Type Conversions

• Assertion: v, ok := i.(Type) (Safely access concrete type)
• Switch:

  switch v := i.(type) {
  case TypeA: // ...
  case TypeB: // ...
  }

Design Principles

• Small: Aim for 1–3 methods.
• Anonymous Parameters: Not recommended in interfaces (name them for clarity).
• Embedded: You can embed one interface into another to combine their requirements.

Standard Library Examples

• fmt.Stringer: String() string
• io.Reader: Read(p []byte) (n int, err error)
• error: Error() string