Tip: Understand Golang methods and interface

Methods:

• methods are functions with a special receiver ( a normal or define type)
• so we define a type and create a method based on this type,  so the logic is the first type  ---> define method.
• As we have lots of common types, we use a type to group all sorts of methods
• So the type is the centrepiece to think thorough

Interface:

• Golang interface is still a type, but not normal concrete type like string, int ....etc. Instead, it is an abstract type built on top of those concrete types.
•  It has two elements:  a concrete type + value of the concrete type
• According to the interface definition, which concrete types implements this interface, Golang automatically matches which concrete types are bond to this interface.
• So in the same program, the same interface type can be different concrete types and values.
• Concrete types have methods. We can call these methods like my-type.my-methods(). It is the same for the interface. We can call these methods like my-interface.my-methods()
• A method can be called by both interface and its concrete type associated.
• Use type assertion to get the value from the interface, then use the value to invoke other methods which are not defined in the interface.
• One of the reasons why we have an interface is: since we also have lots of common methods, i.e. print string, play sports... all sorts of actions are considered as methods, so the genius part is we define an interface as a common signature of methods, so the centrepiece is not a TYPE, but methods,  it uses a common method (i.e. print string)  to group all sorts of types...