How do you implement an abstract type pattern in Go using interfaces and constructors?

Go has no abstract class. The idiomatic equivalent is: define an interface for the contract, provide a factory function that returns the interface, and keep the concrete implementation unexported. Callers depend only on the interface — the implementation is hidden.

// database/db.go — abstract the database driver

// Exported interface — the 'abstract type'
type DB interface {
    Query(sql string, args ...any) (Rows, error)
    Exec(sql string, args ...any) (Result, error)
    Close() error
}

// Unexported concrete implementation — callers never see this
type postgresDB struct {
    conn *pg.Conn
}

func (p *postgresDB) Query(sql string, args ...any) (Rows, error) {
    return p.conn.Query(sql, args...)
}
func (p *postgresDB) Exec(sql string, args ...any) (Result, error) {
    return p.conn.Exec(sql, args...)
}
func (p *postgresDB) Close() error { return p.conn.Close() }

// Exported factory — returns the interface, hides the concrete type
func NewPostgresDB(dsn string) (DB, error) {
    conn, err := pg.Connect(dsn)
    if err != nil { return nil, fmt.Errorf("NewPostgresDB: %w", err) }
    return &postgresDB{conn: conn}, nil
}

// In main/service code:
db, err := database.NewPostgresDB("postgres://localhost/mydb")
if err != nil { log.Fatal(err) }
defer db.Close()

// Caller has no knowledge of postgresDB — depends only on DB interface
// Swap the factory call to use a different DB implementation
// db, _ = database.NewSQLiteDB(":memory:")

This pattern achieves the same goals as abstract class in Java: the implementation is hidden, the interface is the contract, and the factory controls creation. The advantage: the concrete type can be swapped by changing only the factory call — zero impact on the rest of the codebase.

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