{"id":1101,"date":"2026-08-17T07:35:34","date_gmt":"2026-08-17T07:35:34","guid":{"rendered":"https:\/\/www.devopsschool.com\/tutorials\/?p=1101"},"modified":"2026-08-17T07:35:35","modified_gmt":"2026-08-17T07:35:35","slug":"go-tutorials-go-methods","status":"publish","type":"post","link":"https:\/\/www.devopsschool.com\/tutorials\/go-tutorials-go-methods\/","title":{"rendered":"Go Tutorials: Go Methods"},"content":{"rendered":"\n

Go methods become much easier once you understand one idea:<\/p>\n\n\n\n

\n

A method is simply a function attached to a type.<\/strong><\/p>\n<\/blockquote>\n\n\n\n

And the important rule is:<\/p>\n\n\n\n

\n

A receiver can be based on almost any type you define in the same package \u2014 not only a struct<\/code>.<\/strong><\/p>\n<\/blockquote>\n\n\n\n

You can create methods on your own defined types based on:<\/p>\n\n\n\n

struct\nint\nstring\nfloat\nslice\nmap\narray\nfunction\nchannel<\/code><\/pre>\n\n\n\n
The struct<\/code> case is simply the most common one.<\/p>\n\n\n\n
\n\n\n\n
1. What is a Method in Go?<\/h1>\n\n\n\n
A normal function looks like this:<\/p>\n\n\n\n
func greet(name string) {\n\tfmt.Println(\"Hello\", name)\n}<\/code><\/pre>\n\n\n\n
You call it like:<\/p>\n\n\n\n
greet(\"Rajesh\")<\/code><\/pre>\n\n\n\n
A method<\/strong> has one extra part called the receiver<\/strong>:<\/p>\n\n\n\n
func (u User) Greet() {\n\tfmt.Println(\"Hello\", u.Name)\n}<\/code><\/pre>\n\n\n\n
The receiver is:<\/p>\n\n\n\n
(u User)<\/code><\/pre>\n\n\n\n
So:<\/p>\n\n\n\n
func (receiver) MethodName(parameters) return-types<\/code><\/pre>\n\n\n\n
Example:<\/p>\n\n\n\n
func (u User) Greet() {\n\tfmt.Println(\"Hello\", u.Name)\n}<\/code><\/pre>\n\n\n\n
You call the method through the value:<\/p>\n\n\n\n
u.Greet()<\/code><\/pre>\n\n\n\n
instead of:<\/p>\n\n\n\n
Greet(u)<\/code><\/pre>\n\n\n\n
\n\n\n\n
2. Why Do We Need Methods?<\/h1>\n\n\n\n
Suppose we have:<\/p>\n\n\n\n
type User struct {\n\tName string\n}<\/code><\/pre>\n\n\n\n
Without a method:<\/p>\n\n\n\n
func greet(u User) {\n\tfmt.Println(\"Hello\", u.Name)\n}<\/code><\/pre>\n\n\n\n
Call:<\/p>\n\n\n\n
greet(user)<\/code><\/pre>\n\n\n\n
With a method:<\/p>\n\n\n\n
func (u User) Greet() {\n\tfmt.Println(\"Hello\", u.Name)\n}<\/code><\/pre>\n\n\n\n
Call:<\/p>\n\n\n\n
user.Greet()<\/code><\/pre>\n\n\n\n
The method version is usually easier to understand because the behavior belongs naturally to User<\/code>.<\/p>\n\n\n\n
Think:<\/p>\n\n\n\n
User\n \u251c\u2500\u2500 Name\n \u251c\u2500\u2500 Greet()\n \u251c\u2500\u2500 Login()\n \u251c\u2500\u2500 Logout()\n \u2514\u2500\u2500 ChangePassword()<\/code><\/pre>\n\n\n\n
Instead of having unrelated-looking functions:<\/p>\n\n\n\n
greet(user)\nlogin(user)\nlogout(user)\nchangePassword(user)<\/code><\/pre>\n\n\n\n
\n\n\n\n
3. Function vs Method<\/h1>\n\n\n\n
Feature<\/th>Function<\/th>Method<\/th><\/tr><\/thead>
Attached to type<\/td>\u274c<\/td>\u2705<\/td><\/tr>
Has receiver<\/td>\u274c<\/td>\u2705<\/td><\/tr>
Called using .<\/code><\/td>Usually no<\/td>\u2705<\/td><\/tr>
Example<\/td>add(10,20)<\/code><\/td>user.Greet()<\/code><\/td><\/tr>
Best for<\/td>General operations<\/td>Behavior belonging to a type<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n
Function:<\/p>\n\n\n\n
func Add(a, b int) int {\n\treturn a + b\n}<\/code><\/pre>\n\n\n\n
Call:<\/p>\n\n\n\n
Add(10, 20)<\/code><\/pre>\n\n\n\n
Method:<\/p>\n\n\n\n
func (c Calculator) Add(a, b int) int {\n\treturn a + b\n}<\/code><\/pre>\n\n\n\n
Call:<\/p>\n\n\n\n
calculator.Add(10, 20)<\/code><\/pre>\n\n\n\n
\n\n\n\n
4. Basic Method Example<\/h1>\n\n\n\n
package main\n\nimport \"fmt\"\n\n\/\/ User is our own defined type.\ntype User struct {\n\tName string\n}\n\n\/\/ Greet is a method attached to User.\n\/\/\n\/\/ u     = receiver variable\n\/\/ User  = receiver type\nfunc (u User) Greet() {\n\tfmt.Println(\"Hello\", u.Name)\n}\n\nfunc main() {\n\n\t\/\/ Create a User value.\n\tuser := User{\n\t\tName: \"Rajesh\",\n\t}\n\n\t\/\/ Call the method.\n\tuser.Greet()\n}<\/code><\/pre>\n\n\n\n
Output:<\/p>\n\n\n\n
Hello Rajesh<\/code><\/pre>\n\n\n\n
The important line is:<\/p>\n\n\n\n
func (u User) Greet()<\/code><\/pre>\n\n\n\n
Break it down:<\/p>\n\n\n\n
func\n \u2502\n \u2502    receiver\n \u2502    \u250c\u2500\u2500\u2500\u2500\u2500\u2500\u2510\nfunc (u User) Greet()\n      \u2502 \u2502     \u2502\n      \u2502 \u2502     \u2514\u2500\u2500 method name\n      \u2502\n      \u2514\u2500\u2500 receiver variable<\/code><\/pre>\n\n\n\n
\n\n\n\n
5. What Is a Receiver?<\/h1>\n\n\n\n
The receiver tells Go:<\/p>\n\n\n\n
\n
“This function belongs to this type.”<\/p>\n<\/blockquote>\n\n\n\n
Example:<\/p>\n\n\n\n
func (u User) Greet() {\n}<\/code><\/pre>\n\n\n\n
Here:<\/p>\n\n\n\n
u<\/code><\/pre>\n\n\n\n
is the receiver variable.<\/p>\n\n\n\n
And:<\/p>\n\n\n\n
User<\/code><\/pre>\n\n\n\n
is the receiver type.<\/p>\n\n\n\n
Inside the method, u<\/code> gives you access to that particular User<\/code>.<\/p>\n\n\n\n
Example:<\/p>\n\n\n\n
func (u User) Greet() {\n\tfmt.Println(u.Name)\n}<\/code><\/pre>\n\n\n\n
If:<\/p>\n\n\n\n
user := User{Name: \"Rajesh\"}<\/code><\/pre>\n\n\n\n
and you call:<\/p>\n\n\n\n
user.Greet()<\/code><\/pre>\n\n\n\n
then inside the method:<\/p>\n\n\n\n
u.Name<\/code><\/pre>\n\n\n\n
is:<\/p>\n\n\n\n
Rajesh<\/code><\/pre>\n\n\n\n
\n\n\n\n
6. The Most Important Receiver Rule<\/h1>\n\n\n\n
A simplified rule that is excellent for learning is:<\/p>\n\n\n\n
\n
You can define methods on your own defined types in the same package.<\/strong><\/p>\n<\/blockquote>\n\n\n\n
The underlying type does not<\/strong> have to be a struct.<\/p>\n\n\n\n
For example:<\/p>\n\n\n\n
type Age int\ntype Name string\ntype Numbers []int\ntype Scores map[string]int\ntype Coordinates [2]int\ntype Task func()\ntype MessageChannel chan string<\/code><\/pre>\n\n\n\n
All of these are types you defined.<\/p>\n\n\n\n
Therefore, methods can be attached to them.<\/p>\n\n\n\n
\n\n\n\n
7. Struct Receiver<\/h1>\n\n\n\n
This is by far the most common case.<\/p>\n\n\n\n
package main\n\nimport \"fmt\"\n\ntype User struct {\n\tName string\n}\n\nfunc (u User) Greet() {\n\tfmt.Println(\"Hello\", u.Name)\n}\n\nfunc main() {\n\n\tu := User{\n\t\tName: \"Rajesh\",\n\t}\n\n\tu.Greet()\n}<\/code><\/pre>\n\n\n\n
Output:<\/p>\n\n\n\n
Hello Rajesh<\/code><\/pre>\n\n\n\n
When should you use it?<\/h3>\n\n\n\n
Struct methods are excellent when modeling things such as:<\/p>\n\n\n\n
User\nEmployee\nCustomer\nOrder\nProduct\nServer\nDatabase\nCar\nBankAccount<\/code><\/pre>\n\n\n\n
For example:<\/p>\n\n\n\n
type BankAccount struct {\n\tBalance float64\n}<\/code><\/pre>\n\n\n\n
Possible methods:<\/p>\n\n\n\n
Deposit()\nWithdraw()\nBalance()<\/code><\/pre>\n\n\n\n
\n\n\n\n
8. Custom int<\/code> Receiver<\/h1>\n\n\n\n
Methods can also belong to your own integer type.<\/p>\n\n\n\n
package main\n\nimport \"fmt\"\n\n\/\/ Age is a new defined type\n\/\/ whose underlying type is int.\ntype Age int\n\n\/\/ IsAdult is a method on Age.\nfunc (a Age) IsAdult() bool {\n\treturn a >= 18\n}\n\nfunc main() {\n\n\tvar age Age = 25\n\n\tfmt.Println(age.IsAdult())\n}<\/code><\/pre>\n\n\n\n
Output:<\/p>\n\n\n\n
true<\/code><\/pre>\n\n\n\n
Notice:<\/p>\n\n\n\n
type Age int<\/code><\/pre>\n\n\n\n
This does not<\/strong> mean Age<\/code> is merely a variable.<\/p>\n\n\n\n
It creates a new defined type.<\/p>\n\n\n\n
Then:<\/p>\n\n\n\n
func (a Age) IsAdult() bool<\/code><\/pre>\n\n\n\n
attaches behavior to it.<\/p>\n\n\n\n
This can make code very readable:<\/p>\n\n\n\n
age.IsAdult()<\/code><\/pre>\n\n\n\n
\n\n\n\n
9. Custom string<\/code> Receiver<\/h1>\n\n\n\n
You can create methods on your own string type.<\/p>\n\n\n\n
package main\n\nimport (\n\t\"fmt\"\n\t\"strings\"\n)\n\ntype Name string\n\nfunc (n Name) Upper() string {\n\treturn strings.ToUpper(string(n))\n}\n\nfunc main() {\n\n\tname := Name(\"rajesh\")\n\n\tfmt.Println(name.Upper())\n}<\/code><\/pre>\n\n\n\n
Output:<\/p>\n\n\n\n
RAJESH<\/code><\/pre>\n\n\n\n
Notice this:<\/p>\n\n\n\n
string(n)<\/code><\/pre>\n\n\n\n
Because Name<\/code> is our own type:<\/p>\n\n\n\n
type Name string<\/code><\/pre>\n\n\n\n
we convert it back to string<\/code> when calling a standard library function expecting a string.<\/p>\n\n\n\n
\n\n\n\n
10. Slice Receiver<\/h1>\n\n\n\n
A slice can also be the underlying type of your own defined type.<\/p>\n\n\n\n
package main\n\nimport \"fmt\"\n\ntype Numbers []int\n\nfunc (n Numbers) Sum() int {\n\n\ttotal := 0\n\n\tfor _, value := range n {\n\t\ttotal += value\n\t}\n\n\treturn total\n}\n\nfunc main() {\n\n\tnums := Numbers{10, 20, 30}\n\n\tfmt.Println(nums.Sum())\n}<\/code><\/pre>\n\n\n\n
Output:<\/p>\n\n\n\n
60<\/code><\/pre>\n\n\n\n
This is quite useful.<\/p>\n\n\n\n
Instead of writing:<\/p>\n\n\n\n
Sum(nums)<\/code><\/pre>\n\n\n\n
you can write:<\/p>\n\n\n\n
nums.Sum()<\/code><\/pre>\n\n\n\n
It reads naturally:<\/p>\n\n\n\n
\n
Numbers, calculate your sum.<\/p>\n<\/blockquote>\n\n\n\n
\n\n\n\n
11. Map Receiver<\/h1>\n\n\n\n
Maps can also have methods when you create your own defined map type.<\/p>\n\n\n\n
package main\n\nimport \"fmt\"\n\ntype Scores map[string]int\n\nfunc (s Scores) Get(name string) int {\n\treturn s[name]\n}\n\nfunc main() {\n\n\tscores := Scores{\n\t\t\"Rajesh\": 90,\n\t\t\"Amit\":   80,\n\t}\n\n\tfmt.Println(scores.Get(\"Rajesh\"))\n}<\/code><\/pre>\n\n\n\n
Output:<\/p>\n\n\n\n
90<\/code><\/pre>\n\n\n\n
You could also create methods such as:<\/p>\n\n\n\n
Add()\nRemove()\nExists()\nAverage()\nHighest()<\/code><\/pre>\n\n\n\n
Example conceptually:<\/p>\n\n\n\n
scores.Get(\"Rajesh\")\nscores.Add(\"John\", 95)\nscores.Remove(\"Amit\")<\/code><\/pre>\n\n\n\n
\n\n\n\n
12. Array Receiver<\/h1>\n\n\n\n
Arrays can also be used.<\/p>\n\n\n\n
package main\n\nimport \"fmt\"\n\ntype Coordinates [2]int\n\nfunc (c Coordinates) Print() {\n\tfmt.Println(c[0], c[1])\n}\n\nfunc main() {\n\n\tc := Coordinates{10, 20}\n\n\tc.Print()\n}<\/code><\/pre>\n\n\n\n
Output:<\/p>\n\n\n\n
10 20<\/code><\/pre>\n\n\n\n
Here:<\/p>\n\n\n\n
type Coordinates [2]int<\/code><\/pre>\n\n\n\n
creates a new defined type based on an array.<\/p>\n\n\n\n
Then:<\/p>\n\n\n\n
func (c Coordinates) Print()<\/code><\/pre>\n\n\n\n
adds behavior to it.<\/p>\n\n\n\n
\n\n\n\n
13. Function Receiver<\/h1>\n\n\n\n
This is surprising when first learning Go.<\/p>\n\n\n\n
Even a function can be the underlying type of a type with methods.<\/p>\n\n\n\n
package main\n\nimport \"fmt\"\n\ntype Task func()\n\nfunc (t Task) Run() {\n\n\tfmt.Println(\"Starting task...\")\n\n\tt()\n}\n\nfunc main() {\n\n\ttask := Task(func() {\n\t\tfmt.Println(\"Task running\")\n\t})\n\n\ttask.Run()\n}<\/code><\/pre>\n\n\n\n
Output:<\/p>\n\n\n\n
Starting task...\nTask running<\/code><\/pre>\n\n\n\n
Here:<\/p>\n\n\n\n
type Task func()<\/code><\/pre>\n\n\n\n
defines a function type.<\/p>\n\n\n\n
Then:<\/p>\n\n\n\n
func (t Task) Run()<\/code><\/pre>\n\n\n\n
adds a method to it.<\/p>\n\n\n\n
And because t<\/code> itself is a function, this runs it:<\/p>\n\n\n\n
t()<\/code><\/pre>\n\n\n\n
\n\n\n\n
14. Channel Receiver<\/h1>\n\n\n\n
Channels can also be used.<\/p>\n\n\n\n
package main\n\nimport \"fmt\"\n\ntype MessageChannel chan string\n\nfunc (m MessageChannel) Send(message string) {\n\tm <- message\n}\n\nfunc main() {\n\n\tch := make(MessageChannel, 1)\n\n\tch.Send(\"Hello\")\n\n\tfmt.Println(<-ch)\n}<\/code><\/pre>\n\n\n\n
Output:<\/p>\n\n\n\n
Hello<\/code><\/pre>\n\n\n\n
Instead of writing:<\/p>\n\n\n\n
ch <- \"Hello\"<\/code><\/pre>\n\n\n\n
you created behavior:<\/p>\n\n\n\n
ch.Send(\"Hello\")<\/code><\/pre>\n\n\n\n
This can sometimes make APIs easier to understand.<\/p>\n\n\n\n
\n\n\n\n
15. Float Receiver<\/h1>\n\n\n\n
The same idea applies to floating-point types.<\/p>\n\n\n\n
package main\n\nimport \"fmt\"\n\ntype Price float64\n\nfunc (p Price) WithTax() Price {\n\treturn p * 1.18\n}\n\nfunc main() {\n\n\tprice := Price(100)\n\n\tfmt.Println(price.WithTax())\n}<\/code><\/pre>\n\n\n\n
Output:<\/p>\n\n\n\n
118<\/code><\/pre>\n\n\n\n
Or approximately that value depending on formatting.<\/p>\n\n\n\n
This gives you expressive code:<\/p>\n\n\n\n
price.WithTax()<\/code><\/pre>\n\n\n\n
\n\n\n\n
16. You Cannot Attach a Method Directly to int<\/code><\/h1>\n\n\n\n
This is invalid:<\/p>\n\n\n\n
func (x int) Double() int {\n\treturn x * 2\n}<\/code><\/pre>\n\n\n\n
Go will reject it.<\/p>\n\n\n\n
Why?<\/p>\n\n\n\n
Because:<\/p>\n\n\n\n
int<\/code><\/pre>\n\n\n\n
is a built-in type defined by Go.<\/p>\n\n\n\n
You did not define it in your package.<\/p>\n\n\n\n
Instead, define your own type:<\/p>\n\n\n\n
type MyInt int<\/code><\/pre>\n\n\n\n
Then:<\/p>\n\n\n\n
func (x MyInt) Double() MyInt {\n\treturn x * 2\n}<\/code><\/pre>\n\n\n\n
Complete example:<\/p>\n\n\n\n
package main\n\nimport \"fmt\"\n\ntype MyInt int\n\nfunc (x MyInt) Double() MyInt {\n\treturn x * 2\n}\n\nfunc main() {\n\n\tnumber := MyInt(10)\n\n\tfmt.Println(number.Double())\n}<\/code><\/pre>\n\n\n\n
Output:<\/p>\n\n\n\n
20<\/code><\/pre>\n\n\n\n
\n\n\n\n
17. Same Package Rule<\/h1>\n\n\n\n
Suppose package people<\/code> defines:<\/p>\n\n\n\n
package people\n\ntype User struct {\n\tName string\n}<\/code><\/pre>\n\n\n\n
From another package you cannot do this:<\/p>\n\n\n\n
package main\n\nimport \"example.com\/people\"\n\nfunc (u people.User) Greet() {\n}<\/code><\/pre>\n\n\n\n
\u274c Not allowed.<\/p>\n\n\n\n
Why?<\/p>\n\n\n\n
Because User<\/code> belongs to package people<\/code>.<\/p>\n\n\n\n
Methods for User<\/code> must be declared in the package where the receiver base type is defined.<\/p>\n\n\n\n
So conceptually:<\/p>\n\n\n\n
package people\n\ntype User struct{}\n\nfunc (u User) Greet() {}<\/code><\/pre>\n\n\n\n
\u2705 Correct.<\/p>\n\n\n\n
\n\n\n\n
18. Why Does Go Have This Rule?<\/h1>\n\n\n\n
It keeps ownership clear.<\/p>\n\n\n\n
Imagine package A creates:<\/p>\n\n\n\n
User<\/code><\/pre>\n\n\n\n
and package B could secretly add:<\/p>\n\n\n\n
User.Delete()<\/code><\/pre>\n\n\n\n
and package C adds:<\/p>\n\n\n\n
User.Save()<\/code><\/pre>\n\n\n\n
Then it would become difficult to know what methods actually belong to User<\/code>.<\/p>\n\n\n\n
Go avoids this.<\/p>\n\n\n\n
The package defining the type controls its methods.<\/p>\n\n\n\n
\n\n\n\n
19. Value Receiver<\/h1>\n\n\n\n
So far, we have mostly written:<\/p>\n\n\n\n
func (u User) Greet()<\/code><\/pre>\n\n\n\n
This is called a value receiver<\/strong>.<\/p>\n\n\n\n
Example:<\/p>\n\n\n\n
package main\n\nimport \"fmt\"\n\ntype User struct {\n\tName string\n}\n\nfunc (u User) ChangeName() {\n\n\tu.Name = \"Amit\"\n\n\tfmt.Println(\"Inside:\", u.Name)\n}\n\nfunc main() {\n\n\tuser := User{\n\t\tName: \"Rajesh\",\n\t}\n\n\tuser.ChangeName()\n\n\tfmt.Println(\"Outside:\", user.Name)\n}<\/code><\/pre>\n\n\n\n
Output:<\/p>\n\n\n\n
Inside: Amit\nOutside: Rajesh<\/code><\/pre>\n\n\n\n
Why?<\/p>\n\n\n\n
Because:<\/p>\n\n\n\n
(u User)<\/code><\/pre>\n\n\n\n
receives a copy of the User<\/code> value.<\/p>\n\n\n\n
Conceptually:<\/p>\n\n\n\n
Original User\n     |\n     | copy\n     v\nMethod receiver<\/code><\/pre>\n\n\n\n
Changing the copy does not change the original struct.<\/p>\n\n\n\n
\n\n\n\n
20. Pointer Receiver<\/h1>\n\n\n\n
If you want the method to modify the original value, you commonly use a pointer receiver:<\/p>\n\n\n\n
func (u *User) ChangeName()<\/code><\/pre>\n\n\n\n
Example:<\/p>\n\n\n\n
package main\n\nimport \"fmt\"\n\ntype User struct {\n\tName string\n}\n\nfunc (u *User) ChangeName() {\n\tu.Name = \"Amit\"\n}\n\nfunc main() {\n\n\tuser := User{\n\t\tName: \"Rajesh\",\n\t}\n\n\tuser.ChangeName()\n\n\tfmt.Println(user.Name)\n}<\/code><\/pre>\n\n\n\n
Output:<\/p>\n\n\n\n
Amit<\/code><\/pre>\n\n\n\n
Now the original struct was modified.<\/p>\n\n\n\n
\n\n\n\n
21. Value Receiver vs Pointer Receiver<\/h1>\n\n\n\n
This is one of the most important method concepts in Go.<\/p>\n\n\n\n
Value receiver:<\/p>\n\n\n\n
func (u User) Method()<\/code><\/pre>\n\n\n\n
Pointer receiver:<\/p>\n\n\n\n
func (u *User) Method()<\/code><\/pre>\n\n\n\n
Main difference:<\/p>\n\n\n\n
Receiver<\/th>Gets<\/th>Can modify original?<\/th>Typical use<\/th><\/tr><\/thead>
User<\/code><\/td>Copy<\/td>Usually \u274c<\/td>Read-only behavior<\/td><\/tr>
*User<\/code><\/td>Pointer<\/td>\u2705<\/td>Modify the value<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n
Easy memory rule:<\/p>\n\n\n\n
(User)\n   \u2193\ncopy\n\n(*User)\n   \u2193\noriginal<\/code><\/pre>\n\n\n\n
\n\n\n\n
22. Complete Value vs Pointer Example<\/h1>\n\n\n\n
package main\n\nimport \"fmt\"\n\ntype Counter struct {\n\tValue int\n}\n\n\/\/ Value receiver gets a copy.\nfunc (c Counter) Show() {\n\tfmt.Println(\"Current:\", c.Value)\n}\n\n\/\/ Pointer receiver can change original.\nfunc (c *Counter) Increment() {\n\tc.Value++\n}\n\nfunc main() {\n\n\tcounter := Counter{\n\t\tValue: 10,\n\t}\n\n\tcounter.Show()\n\n\tcounter.Increment()\n\n\tcounter.Show()\n}<\/code><\/pre>\n\n\n\n
Output:<\/p>\n\n\n\n
Current: 10\nCurrent: 11<\/code><\/pre>\n\n\n\n
\n\n\n\n
23. Why Don’t We Write &counter<\/code>?<\/h1>\n\n\n\n
You may notice:<\/p>\n\n\n\n
counter.Increment()<\/code><\/pre>\n\n\n\n
works even though:<\/p>\n\n\n\n
Increment()<\/code><\/pre>\n\n\n\n
expects:<\/p>\n\n\n\n
*Counter<\/code><\/pre>\n\n\n\n
Go automatically handles this in many ordinary method calls.<\/p>\n\n\n\n
You could think of:<\/p>\n\n\n\n
counter.Increment()<\/code><\/pre>\n\n\n\n
roughly as:<\/p>\n\n\n\n
(&counter).Increment()<\/code><\/pre>\n\n\n\n
when counter<\/code> is addressable.<\/p>\n\n\n\n
This is one reason Go pointer receiver syntax stays relatively clean.<\/p>\n\n\n\n
\n\n\n\n
24. Methods Can Have Parameters<\/h1>\n\n\n\n
A receiver is not a replacement for parameters.<\/p>\n\n\n\n
You can have both.<\/p>\n\n\n\n
Example:<\/p>\n\n\n\n
package main\n\nimport \"fmt\"\n\ntype Calculator struct{}\n\nfunc (c Calculator) Add(a int, b int) int {\n\treturn a + b\n}\n\nfunc main() {\n\n\tcalc := Calculator{}\n\n\tresult := calc.Add(10, 20)\n\n\tfmt.Println(result)\n}<\/code><\/pre>\n\n\n\n
Output:<\/p>\n\n\n\n
30<\/code><\/pre>\n\n\n\n
Here:<\/p>\n\n\n\n
(c Calculator)<\/code><\/pre>\n\n\n\n
is the receiver.<\/p>\n\n\n\n
And:<\/p>\n\n\n\n
a int, b int<\/code><\/pre>\n\n\n\n
are normal function parameters.<\/p>\n\n\n\n
\n\n\n\n
25. Methods Can Return Values<\/h1>\n\n\n\n
Exactly like functions.<\/p>\n\n\n\n
package main\n\nimport \"fmt\"\n\ntype Rectangle struct {\n\tWidth  float64\n\tHeight float64\n}\n\nfunc (r Rectangle) Area() float64 {\n\treturn r.Width * r.Height\n}\n\nfunc main() {\n\n\tr := Rectangle{\n\t\tWidth:  10,\n\t\tHeight: 5,\n\t}\n\n\tfmt.Println(r.Area())\n}<\/code><\/pre>\n\n\n\n
Output:<\/p>\n\n\n\n
50<\/code><\/pre>\n\n\n\n
\n\n\n\n
26. Methods Can Return Multiple Values<\/h1>\n\n\n\n
Methods have the same return capabilities as normal functions.<\/p>\n\n\n\n
package main\n\nimport \"fmt\"\n\ntype Calculator struct{}\n\nfunc (c Calculator) Divide(a, b float64) (float64, bool) {\n\n\tif b == 0 {\n\t\treturn 0, false\n\t}\n\n\treturn a \/ b, true\n}\n\nfunc main() {\n\n\tcalc := Calculator{}\n\n\tresult, ok := calc.Divide(10, 2)\n\n\tfmt.Println(result)\n\tfmt.Println(ok)\n}<\/code><\/pre>\n\n\n\n
Output:<\/p>\n\n\n\n
5\ntrue<\/code><\/pre>\n\n\n\n
\n\n\n\n
27. One Type Can Have Many Methods<\/h1>\n\n\n\n
This is where methods become especially useful.<\/p>\n\n\n\n
package main\n\nimport \"fmt\"\n\ntype BankAccount struct {\n\tOwner   string\n\tBalance float64\n}\n\nfunc (b BankAccount) ShowBalance() {\n\tfmt.Println(\"Balance:\", b.Balance)\n}\n\nfunc (b *BankAccount) Deposit(amount float64) {\n\tb.Balance += amount\n}\n\nfunc (b *BankAccount) Withdraw(amount float64) {\n\tb.Balance -= amount\n}\n\nfunc main() {\n\n\taccount := BankAccount{\n\t\tOwner:   \"Rajesh\",\n\t\tBalance: 1000,\n\t}\n\n\taccount.ShowBalance()\n\n\taccount.Deposit(500)\n\n\taccount.Withdraw(200)\n\n\taccount.ShowBalance()\n}<\/code><\/pre>\n\n\n\n
Output:<\/p>\n\n\n\n
Balance: 1000\nBalance: 1300<\/code><\/pre>\n\n\n\n
Now the type feels like one logical object:<\/p>\n\n\n\n
BankAccount\n     \u2502\n     \u251c\u2500\u2500 Owner\n     \u251c\u2500\u2500 Balance\n     \u2502\n     \u251c\u2500\u2500 ShowBalance()\n     \u251c\u2500\u2500 Deposit()\n     \u2514\u2500\u2500 Withdraw()<\/code><\/pre>\n\n\n\n
\n\n\n\n
28. When Should I Create a Method?<\/h1>\n\n\n\n
Create a method when the operation logically belongs to the type.<\/p>\n\n\n\n
For example:<\/p>\n\n\n\n
type User struct{}<\/code><\/pre>\n\n\n\n
Good candidates:<\/p>\n\n\n\n
user.Login()\nuser.Logout()\nuser.FullName()\nuser.IsAdmin()<\/code><\/pre>\n\n\n\n
Because all these describe behavior of a User<\/code>.<\/p>\n\n\n\n
For:<\/p>\n\n\n\n
type Order struct{}<\/code><\/pre>\n\n\n\n
Methods might be:<\/p>\n\n\n\n
order.Total()\norder.Cancel()\norder.Pay()\norder.Ship()<\/code><\/pre>\n\n\n\n
For:<\/p>\n\n\n\n
type Numbers []int<\/code><\/pre>\n\n\n\n
methods might be:<\/p>\n\n\n\n
numbers.Sum()\nnumbers.Average()\nnumbers.Max()<\/code><\/pre>\n\n\n\n
\n\n\n\n
29. When Should I Use a Normal Function Instead?<\/h1>\n\n\n\n
Not every function needs to be a method.<\/p>\n\n\n\n
Suppose:<\/p>\n\n\n\n
func Add(a, b int) int<\/code><\/pre>\n\n\n\n
There may be no meaningful object that owns the operation.<\/p>\n\n\n\n
Then a normal function is perfectly appropriate.<\/p>\n\n\n\n
Use a method when:<\/p>\n\n\n\n
\"This operation belongs to this type.\"<\/code><\/pre>\n\n\n\n
Use a function when:<\/p>\n\n\n\n
\"This is a general operation.\"<\/code><\/pre>\n\n\n\n
Example:<\/p>\n\n\n\n
user.Save()<\/code><\/pre>\n\n\n\n
makes sense.<\/p>\n\n\n\n
But:<\/p>\n\n\n\n
Math.Add()<\/code><\/pre>\n\n\n\n
may be unnecessary in Go when:<\/p>\n\n\n\n
Add(a, b)<\/code><\/pre>\n\n\n\n
is simpler.<\/p>\n\n\n\n
\n\n\n\n
30. Where Are Methods Declared?<\/h1>\n\n\n\n
Methods are usually declared in the same package as their receiver type.<\/p>\n\n\n\n
You might have:<\/p>\n\n\n\n
project\/\n\u2502\n\u251c\u2500\u2500 go.mod\n\u2502\n\u251c\u2500\u2500 main.go\n\u2502\n\u2514\u2500\u2500 user\/\n    \u251c\u2500\u2500 user.go\n    \u2514\u2500\u2500 methods.go<\/code><\/pre>\n\n\n\n
For example:<\/p>\n\n\n\n
user\/user.go<\/code><\/p>\n\n\n\n
package user\n\ntype User struct {\n\tName string\n}<\/code><\/pre>\n\n\n\n
user\/methods.go<\/code><\/p>\n\n\n\n
package user\n\nimport \"fmt\"\n\nfunc (u User) Greet() {\n\tfmt.Println(\"Hello\", u.Name)\n}<\/code><\/pre>\n\n\n\n
These methods don’t have to physically be in the same .go<\/code> file.<\/p>\n\n\n\n
They just need to belong to the same package as the receiver’s defined type.<\/p>\n\n\n\n
That distinction is important:<\/p>\n\n\n\n
\n
Same package does not mean same file.<\/strong><\/p>\n<\/blockquote>\n\n\n\n
\n\n\n\n
31. Receiver Name Can Be Anything<\/h1>\n\n\n\n
This:<\/p>\n\n\n\n
func (u User) Greet()<\/code><\/pre>\n\n\n\n
could technically be:<\/p>\n\n\n\n
func (x User) Greet()<\/code><\/pre>\n\n\n\n
or:<\/p>\n\n\n\n
func (user User) Greet()<\/code><\/pre>\n\n\n\n
But Go commonly uses short receiver names.<\/p>\n\n\n\n
For example:<\/p>\n\n\n\n
func (u User)\nfunc (c Customer)\nfunc (o Order)\nfunc (p Product)\nfunc (s Server)<\/code><\/pre>\n\n\n\n
This is common Go style.<\/p>\n\n\n\n
\n\n\n\n
32. Avoid this<\/code> and self<\/code><\/h1>\n\n\n\n
Coming from Java, C++, Python, etc., you may expect:<\/p>\n\n\n\n
this<\/code><\/pre>\n\n\n\n
or:<\/p>\n\n\n\n
self<\/code><\/pre>\n\n\n\n
Go does not have a special this<\/code> or self<\/code> keyword.<\/p>\n\n\n\n
Instead you explicitly choose the receiver name:<\/p>\n\n\n\n
func (u User) Greet() {\n\tfmt.Println(u.Name)\n}<\/code><\/pre>\n\n\n\n
So:<\/p>\n\n\n\n
u<\/code><\/pre>\n\n\n\n
plays the role that this<\/code> or self<\/code> often plays in other languages.<\/p>\n\n\n\n
\n\n\n\n
33. Methods Are Important for Interfaces<\/h1>\n\n\n\n
This is a major reason methods matter in Go.<\/p>\n\n\n\n
Suppose we define an interface:<\/p>\n\n\n\n
type Speaker interface {\n\tSpeak()\n}<\/code><\/pre>\n\n\n\n
And a type:<\/p>\n\n\n\n
type Dog struct{}<\/code><\/pre>\n\n\n\n
with:<\/p>\n\n\n\n
func (d Dog) Speak() {\n\tfmt.Println(\"Woof\")\n}<\/code><\/pre>\n\n\n\n
Then Dog<\/code> automatically satisfies Speaker<\/code>.<\/p>\n\n\n\n
Complete example:<\/p>\n\n\n\n
package main\n\nimport \"fmt\"\n\ntype Speaker interface {\n\tSpeak()\n}\n\ntype Dog struct{}\n\nfunc (d Dog) Speak() {\n\tfmt.Println(\"Woof\")\n}\n\nfunc makeSound(s Speaker) {\n\ts.Speak()\n}\n\nfunc main() {\n\n\tdog := Dog{}\n\n\tmakeSound(dog)\n}<\/code><\/pre>\n\n\n\n
Output:<\/p>\n\n\n\n
Woof<\/code><\/pre>\n\n\n\n
Notice there is no:<\/p>\n\n\n\n
implements Speaker<\/code><\/pre>\n\n\n\n
statement.<\/p>\n\n\n\n
Go checks the methods.<\/p>\n\n\n\n
Because Dog<\/code> has:<\/p>\n\n\n\n
Speak()<\/code><\/pre>\n\n\n\n
it satisfies:<\/p>\n\n\n\n
Speaker<\/code><\/pre>\n\n\n\n
\n\n\n\n
34. Methods Create the Method Set<\/h1>\n\n\n\n
Every type has a set of methods associated with it.<\/p>\n\n\n\n
Suppose:<\/p>\n\n\n\n
type User struct{}<\/code><\/pre>\n\n\n\n
and:<\/p>\n\n\n\n
func (u User) Greet() {}\n\nfunc (u User) Name() {}\n\nfunc (u User) IsAdmin() {}<\/code><\/pre>\n\n\n\n
Then conceptually:<\/p>\n\n\n\n
User Method Set\n\u2502\n\u251c\u2500\u2500 Greet()\n\u251c\u2500\u2500 Name()\n\u2514\u2500\u2500 IsAdmin()<\/code><\/pre>\n\n\n\n
Interfaces check these methods to determine whether the type satisfies them.<\/p>\n\n\n\n
\n\n\n\n
35. Pointer Receivers and Interfaces<\/h1>\n\n\n\n
This is a slightly more advanced but very important topic.<\/p>\n\n\n\n
Consider:<\/p>\n\n\n\n
type Speaker interface {\n\tSpeak()\n}\n\ntype Dog struct{}\n\nfunc (d *Dog) Speak() {\n\tfmt.Println(\"Woof\")\n}<\/code><\/pre>\n\n\n\n
Because Speak()<\/code> has a pointer receiver:<\/p>\n\n\n\n
*Dog<\/code><\/pre>\n\n\n\n
implements Speaker<\/code>.<\/p>\n\n\n\n
But the plain Dog<\/code> value does not have that pointer-receiver method in its method set for interface satisfaction.<\/p>\n\n\n\n
So this works:<\/p>\n\n\n\n
dog := &Dog{}\n\nmakeSound(dog)<\/code><\/pre>\n\n\n\n
But this does not:<\/p>\n\n\n\n
dog := Dog{}\n\nmakeSound(dog)<\/code><\/pre>\n\n\n\n
when Speak()<\/code> exists only on *Dog<\/code>.<\/p>\n\n\n\n
This distinction matters especially with interfaces.<\/p>\n\n\n\n
\n\n\n\n
36. One Important Technical Precision<\/h1>\n\n\n\n
Earlier we used the simple rule:<\/p>\n\n\n\n
\n
“A receiver can be almost any named type you define.”<\/p>\n<\/blockquote>\n\n\n\n
The precise Go rule is slightly more specific.<\/p>\n\n\n\n
The receiver base type must be a defined type belonging to the same package<\/strong>, and the base type cannot be an interface or pointer type.<\/p>\n\n\n\n
For normal learning, remember:<\/p>\n\n\n\n
You define a type\n        \u2193\nin your package\n        \u2193\nattach methods to it<\/code><\/pre>\n\n\n\n
Examples:<\/p>\n\n\n\n
type Age int\ntype Name string\ntype User struct{}\ntype Numbers []int<\/code><\/pre>\n\n\n\n
Perfectly valid.<\/p>\n\n\n\n
\n\n\n\n
37. Defined Type vs Alias \u2014 Important<\/h1>\n\n\n\n
Consider:<\/p>\n\n\n\n
type Age int<\/code><\/pre>\n\n\n\n
This creates a new defined type.<\/p>\n\n\n\n
Therefore:<\/p>\n\n\n\n
func (a Age) IsAdult() bool<\/code><\/pre>\n\n\n\n
is valid.<\/p>\n\n\n\n
But this:<\/p>\n\n\n\n
type Age = int<\/code><\/pre>\n\n\n\n
is a type alias<\/strong>.<\/p>\n\n\n\n
It means:<\/p>\n\n\n\n
Age is simply another spelling for int.<\/code><\/pre>\n\n\n\n
It does not create a new local defined type that lets you attach methods to built-in int<\/code>.<\/p>\n\n\n\n
So understand the difference:<\/p>\n\n\n\n
type Age int<\/code><\/pre>\n\n\n\n
New defined type.<\/p>\n\n\n\n
Versus:<\/p>\n\n\n\n
type Age = int<\/code><\/pre>\n\n\n\n
Alias.<\/p>\n\n\n\n
For methods, you usually want:<\/p>\n\n\n\n
type Age int<\/code><\/pre>\n\n\n\n
\n\n\n\n
38. Complete Example: Multiple Receiver Types<\/h1>\n\n\n\n
Here is one program demonstrating several types.<\/p>\n\n\n\n
package main\n\nimport (\n\t\"fmt\"\n\t\"strings\"\n)\n\n\/\/ ---------- Struct ----------\n\ntype User struct {\n\tName string\n}\n\nfunc (u User) Greet() {\n\tfmt.Println(\"Hello\", u.Name)\n}\n\n\/\/ ---------- Integer ----------\n\ntype Age int\n\nfunc (a Age) IsAdult() bool {\n\treturn a >= 18\n}\n\n\/\/ ---------- String ----------\n\ntype Name string\n\nfunc (n Name) Upper() string {\n\treturn strings.ToUpper(string(n))\n}\n\n\/\/ ---------- Slice ----------\n\ntype Numbers []int\n\nfunc (n Numbers) Sum() int {\n\n\ttotal := 0\n\n\tfor _, number := range n {\n\t\ttotal += number\n\t}\n\n\treturn total\n}\n\n\/\/ ---------- Map ----------\n\ntype Scores map[string]int\n\nfunc (s Scores) Get(name string) int {\n\treturn s[name]\n}\n\n\/\/ ---------- Array ----------\n\ntype Coordinates [2]int\n\nfunc (c Coordinates) Print() {\n\tfmt.Println(c[0], c[1])\n}\n\n\/\/ ---------- Function ----------\n\ntype Task func()\n\nfunc (t Task) Run() {\n\tfmt.Println(\"Starting task...\")\n\tt()\n}\n\n\/\/ ---------- Channel ----------\n\ntype MessageChannel chan string\n\nfunc (m MessageChannel) Send(message string) {\n\tm <- message\n}\n\nfunc main() {\n\n\t\/\/ Struct\n\tuser := User{Name: \"Rajesh\"}\n\tuser.Greet()\n\n\t\/\/ Integer\n\tage := Age(25)\n\tfmt.Println(\"Adult:\", age.IsAdult())\n\n\t\/\/ String\n\tname := Name(\"rajesh\")\n\tfmt.Println(\"Upper:\", name.Upper())\n\n\t\/\/ Slice\n\tnumbers := Numbers{10, 20, 30}\n\tfmt.Println(\"Sum:\", numbers.Sum())\n\n\t\/\/ Map\n\tscores := Scores{\n\t\t\"Rajesh\": 90,\n\t}\n\tfmt.Println(\"Score:\", scores.Get(\"Rajesh\"))\n\n\t\/\/ Array\n\tcoordinates := Coordinates{10, 20}\n\tcoordinates.Print()\n\n\t\/\/ Function\n\ttask := Task(func() {\n\t\tfmt.Println(\"Task running\")\n\t})\n\ttask.Run()\n\n\t\/\/ Channel\n\tchannel := make(MessageChannel, 1)\n\n\tchannel.Send(\"Hello from channel\")\n\n\tfmt.Println(<-channel)\n}<\/code><\/pre>\n\n\n\n
Expected output:<\/p>\n\n\n\n
Hello Rajesh\nAdult: true\nUpper: RAJESH\nSum: 60\nScore: 90\n10 20\nStarting task...\nTask running\nHello from channel<\/code><\/pre>\n\n\n\n
\n\n\n\n
39. Receiver Types Summary<\/h1>\n\n\n\n
Underlying type<\/th>Example<\/th>Method allowed?<\/th><\/tr><\/thead>
Struct<\/td>type User struct{}<\/code><\/td>\u2705<\/td><\/tr>
Integer<\/td>type Age int<\/code><\/td>\u2705<\/td><\/tr>
String<\/td>type Name string<\/code><\/td>\u2705<\/td><\/tr>
Float<\/td>type Price float64<\/code><\/td>\u2705<\/td><\/tr>
Slice<\/td>type Numbers []int<\/code><\/td>\u2705<\/td><\/tr>
Map<\/td>type Scores map[string]int<\/code><\/td>\u2705<\/td><\/tr>
Array<\/td>type Point [2]int<\/code><\/td>\u2705<\/td><\/tr>
Function<\/td>type Task func()<\/code><\/td>\u2705<\/td><\/tr>
Channel<\/td>type Messages chan string<\/code><\/td>\u2705<\/td><\/tr>
Built-in int<\/code> directly<\/td>int<\/code><\/td>\u274c<\/td><\/tr>
Built-in string<\/code> directly<\/td>string<\/code><\/td>\u274c<\/td><\/tr>
Type from another package<\/td>http.Client<\/code><\/td>\u274c<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n
\n\n\n\n
40. Method Anatomy<\/h1>\n\n\n\n
Take:<\/p>\n\n\n\n
func (u *User) ChangeName(name string) bool {\n\tu.Name = name\n\treturn true\n}<\/code><\/pre>\n\n\n\n
Breakdown:<\/p>\n\n\n\n
func\n \u2502\n \u2502    receiver\n \u2502    \u250c\u2500\u2500\u2500\u2500\u2500\u2500\u2500\u2510\n \u2502    \u2502\nfunc (u *User) ChangeName(name string) bool {\n      \u2502 \u2502       \u2502          \u2502          \u2502\n      \u2502 \u2502       \u2502          \u2502          \u2514\u2500\u2500 return type\n      \u2502 \u2502       \u2502          \u2502\n      \u2502 \u2502       \u2502          \u2514\u2500\u2500 parameter\n      \u2502 \u2502       \u2502\n      \u2502 \u2502       \u2514\u2500\u2500 method name\n      \u2502 \u2502\n      \u2502 \u2514\u2500\u2500 receiver type\n      \u2502\n      \u2514\u2500\u2500 receiver variable<\/code><\/pre>\n\n\n\n
So a method contains:<\/p>\n\n\n\n
Receiver\nMethod name\nParameters\nReturn values\nBody<\/code><\/pre>\n\n\n\n
Almost exactly like a function, except for the receiver.<\/p>\n\n\n\n
\n\n\n\n
41. How to Design a Method<\/h1>\n\n\n\n
When creating a type, ask:<\/p>\n\n\n\n
\n
What actions naturally belong to this value?<\/p>\n<\/blockquote>\n\n\n\n
For:<\/p>\n\n\n\n
type Employee struct {\n\tName   string\n\tSalary float64\n}<\/code><\/pre>\n\n\n\n
Potential methods:<\/p>\n\n\n\n
employee.FullName()\nemployee.GiveRaise()\nemployee.CalculateTax()\nemployee.PrintDetails()<\/code><\/pre>\n\n\n\n
You probably wouldn’t create:<\/p>\n\n\n\n
employee.ConnectDatabase()<\/code><\/pre>\n\n\n\n
unless database connection genuinely belongs to the employee concept.<\/p>\n\n\n\n
Method design should represent logical behavior.<\/p>\n\n\n\n
\n\n\n\n
42. Real-World Example<\/h1>\n\n\n\n
package main\n\nimport \"fmt\"\n\ntype BankAccount struct {\n\tAccountHolder string\n\tBalance       float64\n}\n\nfunc (a BankAccount) ShowBalance() {\n\tfmt.Println(\"Balance:\", a.Balance)\n}\n\nfunc (a *BankAccount) Deposit(amount float64) {\n\n\tif amount <= 0 {\n\t\tfmt.Println(\"Invalid deposit\")\n\t\treturn\n\t}\n\n\ta.Balance += amount\n}\n\nfunc (a *BankAccount) Withdraw(amount float64) {\n\n\tif amount > a.Balance {\n\t\tfmt.Println(\"Insufficient balance\")\n\t\treturn\n\t}\n\n\ta.Balance -= amount\n}\n\nfunc main() {\n\n\taccount := BankAccount{\n\t\tAccountHolder: \"Rajesh\",\n\t\tBalance:       1000,\n\t}\n\n\taccount.ShowBalance()\n\n\taccount.Deposit(500)\n\n\taccount.ShowBalance()\n\n\taccount.Withdraw(300)\n\n\taccount.ShowBalance()\n}<\/code><\/pre>\n\n\n\n
Output:<\/p>\n\n\n\n
Balance: 1000\nBalance: 1500\nBalance: 1200<\/code><\/pre>\n\n\n\n
This is a natural place for methods because:<\/p>\n\n\n\n
BankAccount\n    \u2502\n    \u251c\u2500\u2500 data\n    \u2502   \u251c\u2500\u2500 AccountHolder\n    \u2502   \u2514\u2500\u2500 Balance\n    \u2502\n    \u2514\u2500\u2500 behavior\n        \u251c\u2500\u2500 ShowBalance()\n        \u251c\u2500\u2500 Deposit()\n        \u2514\u2500\u2500 Withdraw()<\/code><\/pre>\n\n\n\n
\n\n\n\n
43. How Method Calls Work Mentally<\/h1>\n\n\n\n
Given:<\/p>\n\n\n\n
account.Deposit(500)<\/code><\/pre>\n\n\n\n
Read it naturally as:<\/p>\n\n\n\n
\n
“Ask this account to deposit 500.”<\/p>\n<\/blockquote>\n\n\n\n
Given:<\/p>\n\n\n\n
numbers.Sum()<\/code><\/pre>\n\n\n\n
read:<\/p>\n\n\n\n
\n
“Ask these numbers for their sum.”<\/p>\n<\/blockquote>\n\n\n\n
Given:<\/p>\n\n\n\n
age.IsAdult()<\/code><\/pre>\n\n\n\n
read:<\/p>\n\n\n\n
\n
“Ask this age whether it represents an adult.”<\/p>\n<\/blockquote>\n\n\n\n
This mental model makes method APIs much easier to design.<\/p>\n\n\n\n
\n\n\n\n
44. Method vs Struct vs Interface<\/h1>\n\n\n\n
These three concepts are closely connected.<\/p>\n\n\n\n
Struct<\/h3>\n\n\n\n
Stores data.<\/p>\n\n\n\n
type Dog struct {\n\tName string\n}<\/code><\/pre>\n\n\n\n
Method<\/h3>\n\n\n\n
Adds behavior.<\/p>\n\n\n\n
func (d Dog) Speak() {\n\tfmt.Println(\"Woof\")\n}<\/code><\/pre>\n\n\n\n
Interface<\/h3>\n\n\n\n
Defines required behavior.<\/p>\n\n\n\n
type Speaker interface {\n\tSpeak()\n}<\/code><\/pre>\n\n\n\n
Together:<\/p>\n\n\n\n
        STRUCT\n       Dog{Name}\n          \u2502\n          \u2502 has\n          \u25bc\n        METHOD\n        Speak()\n          \u2502\n          \u2502 satisfies\n          \u25bc\n       INTERFACE\n       Speaker<\/code><\/pre>\n\n\n\n
This relationship is central to Go.<\/p>\n\n\n\n
\n\n\n\n
45. The Big Picture<\/h1>\n\n\n\n
Suppose:<\/p>\n\n\n\n
type Dog struct {\n\tName string\n}<\/code><\/pre>\n\n\n\n
Data:<\/p>\n\n\n\n
Name<\/code><\/pre>\n\n\n\n
Method:<\/p>\n\n\n\n
func (d Dog) Speak()<\/code><\/pre>\n\n\n\n
Now Dog<\/code> has behavior:<\/p>\n\n\n\n
Dog\n \u251c\u2500\u2500 Name\n \u2514\u2500\u2500 Speak()<\/code><\/pre>\n\n\n\n
Then:<\/p>\n\n\n\n
type Speaker interface {\n\tSpeak()\n}<\/code><\/pre>\n\n\n\n
Because Dog<\/code> has Speak()<\/code>:<\/p>\n\n\n\n
Dog satisfies Speaker<\/code><\/pre>\n\n\n\n
You do not explicitly declare that relationship.<\/p>\n\n\n\n
Go discovers it from the methods.<\/p>\n\n\n\n
\n\n\n\n
46. Best Practice: Value or Pointer Receiver?<\/h1>\n\n\n\n
A useful beginner rule:<\/p>\n\n\n\n
Use a pointer receiver<\/strong> when the method changes the value.<\/p>\n\n\n\n
func (u *User) ChangeName(name string)<\/code><\/pre>\n\n\n\n
Use a value receiver<\/strong> when it only reads the value and the type is reasonably small.<\/p>\n\n\n\n
func (u User) FullName() string<\/code><\/pre>\n\n\n\n
For larger structs, pointer receivers are also often preferred to avoid copying the entire struct.<\/p>\n\n\n\n
Example:<\/p>\n\n\n\n
type User struct {\n\tName  string\n\tEmail string\n}<\/code><\/pre>\n\n\n\n
Read-only:<\/p>\n\n\n\n
func (u User) DisplayName() string {\n\treturn u.Name\n}<\/code><\/pre>\n\n\n\n
Modification:<\/p>\n\n\n\n
func (u *User) ChangeEmail(email string) {\n\tu.Email = email\n}<\/code><\/pre>\n\n\n\n
\n\n\n\n
47. One Style Recommendation<\/h1>\n\n\n\n
If a type has several methods and some need pointer receivers, it is often cleaner to use pointer receivers consistently for that type.<\/p>\n\n\n\n
For example:<\/p>\n\n\n\n
func (u *User) ChangeName() {}\nfunc (u *User) Save() {}\nfunc (u *User) Validate() {}<\/code><\/pre>\n\n\n\n
rather than randomly mixing receiver styles without reason.<\/p>\n\n\n\n
This becomes especially important when interfaces and method sets are involved.<\/p>\n\n\n\n
\n\n\n\n
48. Common Beginner Mistakes<\/h1>\n\n\n\n
Mistake 1: Thinking methods require structs<\/h3>\n\n\n\n
Wrong:<\/p>\n\n\n\n
Method = struct function<\/code><\/pre>\n\n\n\n
Better:<\/p>\n\n\n\n
Method = function attached to your own defined type<\/code><\/pre>\n\n\n\n
Structs are just the most common receiver types.<\/p>\n\n\n\n
\n\n\n\n
Mistake 2: Trying to attach a method to int<\/code><\/h3>\n\n\n\n
Wrong:<\/p>\n\n\n\n
func (x int) Double() int<\/code><\/pre>\n\n\n\n
Correct:<\/p>\n\n\n\n
type MyInt int\n\nfunc (x MyInt) Double() MyInt<\/code><\/pre>\n\n\n\n
\n\n\n\n
Mistake 3: Expecting value receiver changes to persist<\/h3>\n\n\n\n
This:<\/p>\n\n\n\n
func (u User) ChangeName()<\/code><\/pre>\n\n\n\n
works on a copy.<\/p>\n\n\n\n
For modification, usually use:<\/p>\n\n\n\n
func (u *User) ChangeName()<\/code><\/pre>\n\n\n\n
\n\n\n\n
Mistake 4: Thinking receiver is a normal parameter<\/h3>\n\n\n\n
This:<\/p>\n\n\n\n
func (u User) Greet(name string)<\/code><\/pre>\n\n\n\n
contains:<\/p>\n\n\n\n
u      \u2192 receiver\nname   \u2192 parameter<\/code><\/pre>\n\n\n\n
They serve different roles.<\/p>\n\n\n\n
\n\n\n\n
Mistake 5: Thinking method must be in same file<\/h3>\n\n\n\n
No.<\/p>\n\n\n\n
It must belong to the appropriate package, not necessarily the same source file.<\/p>\n\n\n\n
\n\n\n\n
49. Quick Decision Guide<\/h1>\n\n\n\n
Ask these questions:<\/p>\n\n\n\n
Question<\/th>Choice<\/th><\/tr><\/thead>
Does behavior belong to a type?<\/td>Method<\/td><\/tr>
Is it a general operation?<\/td>Function<\/td><\/tr>
Need to change original value?<\/td>Pointer receiver<\/td><\/tr>
Only reading small value?<\/td>Value receiver often fine<\/td><\/tr>
Want type to satisfy interface?<\/td>Implement required methods<\/td><\/tr>
Want method on int<\/code>\/string<\/code>?<\/td>First define your own type<\/td><\/tr>
Type belongs to another package?<\/td>Cannot add methods directly<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n
\n\n\n\n
50. Final Mental Model<\/h1>\n\n\n\n
Remember this:<\/p>\n\n\n\n
TYPE\n \u2502\n \u2502 define data\/value\n \u2502\n \u25bc\nMETHOD\n \u2502\n \u2502 adds behavior\n \u2502\n \u25bc\nINTERFACE\n \u2502\n \u2502 describes required behavior\n \u2502\n \u25bc\nPOLYMORPHISM<\/code><\/pre>\n\n\n\n
And the core syntax is simply:<\/p>\n\n\n\n
func (receiver ReceiverType) MethodName() {\n}<\/code><\/pre>\n\n\n\n
Example:<\/p>\n\n\n\n
func (u User) Greet() {\n}<\/code><\/pre>\n\n\n\n
Or with a pointer:<\/p>\n\n\n\n
func (u *User) ChangeName() {\n}<\/code><\/pre>\n\n\n\n
The most important lesson is:<\/p>\n\n\n\n
\n
A Go method is a function attached to a locally defined type through a receiver. The receiver is not limited to structs. Your own types based on integers, strings, floats, slices, maps, arrays, functions, and channels can also have methods.<\/strong><\/p>\n<\/blockquote>\n\n\n\n
For everyday Go, however, you will see methods most frequently on:<\/p>\n\n\n\n
type Something struct {\n\t...\n}<\/code><\/pre>\n\n\n\n
because structs are how Go commonly groups related data, and methods naturally provide the behavior associated with that data.<\/p>\n","protected":false},"excerpt":{"rendered":"
Go methods become much easier once you understand one idea: A method is simply a function attached to a type. And the important rule is: A receiver… <\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[],"class_list":["post-1101","post","type-post","status-publish","format-standard","hentry","category-uncategorized"],"_links":{"self":[{"href":"https:\/\/www.devopsschool.com\/tutorials\/wp-json\/wp\/v2\/posts\/1101","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.devopsschool.com\/tutorials\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.devopsschool.com\/tutorials\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.devopsschool.com\/tutorials\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.devopsschool.com\/tutorials\/wp-json\/wp\/v2\/comments?post=1101"}],"version-history":[{"count":1,"href":"https:\/\/www.devopsschool.com\/tutorials\/wp-json\/wp\/v2\/posts\/1101\/revisions"}],"predecessor-version":[{"id":1102,"href":"https:\/\/www.devopsschool.com\/tutorials\/wp-json\/wp\/v2\/posts\/1101\/revisions\/1102"}],"wp:attachment":[{"href":"https:\/\/www.devopsschool.com\/tutorials\/wp-json\/wp\/v2\/media?parent=1101"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.devopsschool.com\/tutorials\/wp-json\/wp\/v2\/categories?post=1101"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.devopsschool.com\/tutorials\/wp-json\/wp\/v2\/tags?post=1101"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}