Go: What is the difference between interface and struct?

Interface — defines behavior through method signatures, enabling polymorphism. Struct — defines data through named fields, providing concrete implementations.

Interfaces are implicitly satisfied in Go — if a type has all the required methods, it implements the interface. No declaration needed. This enables loose coupling and easy testing.

1type Cache interface {
2    Get(key string) (interface{}, bool)
3    Set(key string, value interface{})
4}
5
6type LRUCache struct {
7    items map[string]interface{}
8    cap   int
9}
10func (c *LRUCache) Get(key string) (interface{}, bool) {
11    v, ok := c.items[key]
12    return v, ok
13}
14func (c *LRUCache) Set(key string, value interface{}) {
15    c.items[key] = value
16}

Structs are concrete types with named fields. You attach methods to them and can embed other structs for composition.

1type Config struct {
2    Debug bool
3    Port  int
4}

When to use:

• Interface — abstraction, DI, testing.
• Struct — concrete data, models, configs.

Common mistakes:

• Large interfaces — keep small and focused.
• Defining interfaces where implementations live.
• Nil interface trap: typed nil in interface ≠ nil interface.
• Overusing any — loses type safety.