Golang basics for Java developers - Part 1

Golang basics…
...for Java developers
Robert Stern

Agenda
Overview
Basic syntax and structure
Datatypes
Flow control
Functions, interfaces
Java vs. Go

All you need is…
https://play.golang.org/

Overview
Basic syntax and structure
Datatypes
Flow control
Functions, interfaces
Concurrency
Java vs. Go

History and features
Created in 2007 at Google, announced in 2009, v1.0 released in 2012) with strong
dedication for server side programming.
Compiled, statically typed, garbage collected language with multiple
architecture/OS support (i386, amd64, arm/Windows, Linux, *BSD, etc..)
(very) fast compilation AND execution time with low resource consumption
Powerful cli (go get, go build, go test, go install…)
Current stable version is 1.6.3 and still being actively developed

What is missing or disturbing?
No generics
Not an OO language
No constructors
No operator/method overloading
No circular package imports
No mandatory semicolons
GC pause is unpredictable

What is non-java-isch?
Multiple return values + the blank variable
Golang is a pass-by-value language
Lightweight concurrency with goroutines and channels
No SE/EE separation. All server stuff is built-in.
No exceptions, use error instead
Compile time error about unused imports and variables
No maven/ant like build system. Uses Makefile+vendoring+go get

Famous projects, companies
Projects:
App Engine with Go, AWS with Go SDK, Docker, Kubernetes, Grafana, Terraform,
Gogs (Go Git Service), etcd, drone.io, Syncthing, CockroachDB, rkt, nsq, consul,
boltDB, weave, swarm, vulcand, gryffin, fleet, minio…
Companies:
Google (of course…), Netflix, Dropbox, CloudFlare, SoundCloud, BBC, Uber,
Couchbase, MongoDB...

A bit bigger picture about the trends

Hello world in golang:
package main
// imports
import "fmt"
/*
This is the main function
*/
func main() {
fmt.Println("Hello, World!")
}
Try it in the Go playground!
Hello world

Go project structure
❏ Workspace ($GOPATH or %GOPATH%)
❏ src/pkg/bin roots
❏ Repository (github.com, etc…)
❏ Package
❏ Go source files

Packages
Like in Java, golang also uses packages to organize code, but in a bit different
format.
Keyword: package <name>
The app starts in the main package.
the path to the package’s directory is the import path
The package name is the last element of the import path:
github.com/zooplus/golang-app/stringutil → stringutil
Accessing stringutil functions/elements: stringutil.<name>

import (
"reflect"
"strings"
"github.com/emicklei/go-restful"
)
import "reflect"
import "strings"
import "github.com/emicklei/go-restful"
Importing packages

Package types
To use one or more functions in a package we need to import it into our code. We
can import system, local and remote packages.
System packages are the golang out of box packages like http, testing, etc..
Local package resides in your repository, no full path needed
Remote packages has complete import path pointing to the server like:
github.com/emicklei/go-restful

Getting the dependencies
For remote packages use go get to fetch into your workspace. We have two
syntaxes
go get in the repository folder downloads all remote packages based on the
imports
go get <remote path> downloads the specified package for later use

Variables, constants
package main
import "fmt"
const greeting = "Hello"
var name string
func main() {
name = "Zooplus"
space := " "
fmt.Println(greeting + space + name)
}

if _, err := os.Stat(path); os.IsNotExist(err) {
fmt.Printf("%s does not existn", path)
}
The blank identifier
“The blank identifier can be assigned or declared with any value of any type, with the value
discarded harmlessly”
(https://golang.org/doc/effective_go.html#blank)

Exported names
“In Go, a name is exported if it begins with a capital letter.
For example, Pizza is an exported name, as is Pi, which is exported from the math package.”
package main
import (
"fmt"
"math"
)
func main() {
fmt.Println(math.pi)
}

Practice!
Refactor the greeting app to use fmt.Printf(format, args...)** and display the following message:
Hello Zooplus, today is Friday.
The name of the day as string can be printed with:
import "time"
...
time.Now().Format("Monday")
**(Use %s for string)

A solution
package main
import (
"fmt"
"time"
)
func main() {
name := "Zooplus"
fmt.Printf("Hello %s, today is %s.", name, time.Now().Format("Monday"))
}

Basic types
bool
string
int int8 int16 int32 int64
uint uint8 uint16 uint32 uint64 uintptr
byte // alias for uint8
rune // alias for int32, represents a Unicode code point
float32 float64
complex64 complex128 - “math/cmplx”:
var number complex128 = cmplx.Sqrt(-5 + 12i)

Pointers - bit.ly/2a58jnI
package main
import "fmt"
func main() {
i, j := 42, 2701
p := &i // point to i
fmt.Println(*p) // read i through the pointer
*p = 21 // set i through the pointer
fmt.Println(i) // see the new value of i
p = &j // point to j
*p = *p / 37 // divide j through the pointer
fmt.Println(j) // see the new value of j
}

package main
var (
a int
b int32
c int16
)
func main() {
a = 1
b = a
c = a
}
No autoboxing! You should convert your types by yourself.
package main
import "fmt"
var (
a int
b int32
c int16
)
func main() {
a = 1
b = int32(a)
c = int16(a)
fmt.Printf("a: %d, b: %d, c: %d", a, b, c)
}
Type conversion
prog.go:11: cannot use a (type int) as type int32 in assignment
prog.go:12: cannot use a (type int) as type int16 in assignmenta: 1, b: 1, c: 1

Struct
type ApacheLogRecord struct {
ip
string
time
string
method, uri, protocol
string
status int
responseBytes int
referer
string
userAgent
string
}
1. result = &ApacheLogRecord{
ip: nvl(req.RemoteAddr),
time: timeFormatted,
method: nvl(req.Method),
uri: nvl(req.URL.Path),
protocol: nvl(req.Proto),
status: resp.StatusCode(),
responseBytes:
resp.ContentLength(),
referer: nvl(req.Referer()),
userAgent:
nvl(req.UserAgent()),
}
2.) result = new(ApacheLogRecord)

package main
import "fmt"
type Animal struct {
legCount int
}
// define a behavior for Animal
func (a Animal) numberOfLegs() int {
return a.legCount
}
type Dog struct {
Animal //anonymous field Animal
}
func main() {
d := Dog{Animal{4}}
//no method defined for Dog, but we have the same behavior as Animal.
fmt.Println("A Dog has this many legs: ", d.numberOfLegs())
}
Embedding, inheritance -bit.ly/2axY1yu

Receiver methods -bit.ly/2a56VkS
package main
import "fmt"
legCount int
}
return a.legCount
}
func (a Animal) setLegs(c int) {
a.legCount = c
}
type Mammal struct {
weight int
}
func (m Mammal) getWeight() int {
return m.weight
}
func (m Mammal) setWeight(w int) {
m.weight = w
}
type Dog struct {
Animal
Mammal
}
func main() {
d := Dog{Animal{4}, Mammal{10}}
fmt.Printf("A Dog has this many legs: %d oh... and weight: %d", d.numberOfLegs(), d.getWeight())
}

Pointer receivers -bit.ly/2aKK7si
package main
import "fmt"
legCount int
}
return a.legCount
}
func (a Animal) setLegs(c int) {
a.legCount = c
}
func (a *Animal) setLegsPtr(c int) {
a.legCount = c
}
type Dog struct {
Animal //anonymous field Animal
}
func main() {
d := Dog{Animal{4}}
d.setLegs(5)
d.setLegsPtr(6)
}

Arrays -bit.ly/2ahx5EA
Arrays has fix size and type with zero based indexing:
var integers [20]int
package main
import "fmt"
func main() {
var a [2]string // std declaration
a[0] = "Hello"
a[1] = "Zooplus"
fmt.Println(a[0], a[1])
fmt.Println(a)
fibonacci := [6]int{1, 1, 2, 3, 5, 7} // on-the-fly declaration
fmt.Println(fibonacci)
}

Slices - bit.ly/2ahVarP
Slice is a dynamic version of array (a sort of):
var integers []int // decleration of a slice without size
package main
import "fmt"
func main() {
fibonacci := [6]int{1, 1, 2, 3, 5, 7}
var p []int = fibonacci[1:4]
fmt.Println(p)
}

Slices are not storing data - bit.ly/2aai1Wx
package main
import "fmt"
func main() {
names := [4]string{
"John",
"Paul",
"George",
"Ringo",
}
fmt.Println(names)
a := names[0:2]
b := names[1:3]
fmt.Println(a, b)
b[0] = "XXX"
fmt.Println(a, b)
fmt.Println(names)
}

Slices - bounds
There are default low and high bounds for slices. Given the following array:
var a [10]int
The following slice declarations are equivalent:
a[0:10]
a[:10]
a[0:]
a[:]

Creating slice:
a := make([]int, 5) // len(a)=5
Creating slice with capacity:
b := make([]int, 0, 5) // len(b)=0, cap(b)=5
Appending slice:
b = append(b, 10)
Further reading: https://blog.golang.org/go-slices-usage-and-internals
Slices - create, append

Arrays, slices - for loop as foreach
Iterate through an array/slice
for index, value := range arr {
fmt.Println("Index:", index, "Value:", value)
}
Get the index only:
for index := range arr {
fmt.Println("Index:", index)
}
Get the values:
for _, value := range arr {
fmt.Println("Value:", value)
}

Maps - declaration, initialization
Maps are not initialized by default, we should use make:
package main
import "fmt"
var m map[string]string
func main() {
m = make(map[string]string)
m["Zooplus"] = "Gut"
m["Bitiba"] = "Gut"
m["Fressnapf"] = "Falsch"
fmt.Println(m["Zooplus"])
}

Maps - Mutating maps
m[key] = item // upsert
item = m[key] // retrieve
delete (m, key) // guess….
_, ok = m[key] // ok == true if key exists, map can store/return nil
Maps are not thread safe!!!

Maps - for loop as foreach
Iterate through a map:
for key, value := range m {
fmt.Println("Key:", key, "Value:", value)
}
Get the keys:
for key := range m {
fmt.Println("Key:", key)
}
Get the values:
for _, value := range m {
fmt.Println("Value:", value)
}

The one and only: for loop
// normal loop
sum := 0
// init-condition-post
for i := 0; i < 10; i++ {
sum += i
}
fmt.Println(sum)
// partial loop
sum := 0
// init and post are optional
for ; sum < 10; {
sum += sum
}
// “while” loop
sum := 1
// init and post are optional
for sum < 1000 {
sum += sum
}
fmt.Println(sum)
// infinite loop
for {
}

func GetProject(req *restful.Request, resp *restful.Response) {
resp.Header().Set("Pragma", "no-cache")
//
pId := req.PathParameter("projectId")
if project, ok := projects.GetProjectByIdOrName(pId); ok
{
ReturnJSON(resp, http.StatusOK, project)
}
ReturnMessage(resp, http.StatusNotFound, "Project not
found!")
}
If-then-else
existingPipeline, ok := nameSpace.Pipelines[newPipeline.Name]
if ok {
log.Debug("Entry exists!")
} else {
nameSpace.Pipelines[newPipeline.Name] = existingPipeline
log.Debug("New entry stored")
}
?

Switch
Switch in Golang is a bit different from Java:
No automatic fallthrough, no need to put break at the end in every cases
You can delegate expression evaluation into the case block
fallthrough keyword for java-ish behavior
Multiple conditions in one case (no empty cases like in java)
Not limited to string and int
Type switch can be used for interface{} to find out the real type (instanceof)

Switch - no fallthrough by default
switch c {
case '&':
esc = "&"
case ''':
esc = "'"
case '<':
esc = "<"
case '>':
esc = ">"
case '"':
esc = """
default:
panic("unrecognized escape character")
}

Switch - strings are welcome
switch syscall.OS {
case "windows":
...
case "plan9":
...
default:
...
}

Switch - (multiple)expression(s) in cases
switch {
case '0' <= c && c <= '9':
return c - '0'
case 'a' <= c && c <= 'f':
return c - 'a' + 10
case 'A' <= c && c <= 'F':
return c - 'A' + 10
}
switch chars[code].category {
case "de", "uk", "fr", "sp", "nl":
return true
}

Switch - Type switch
switch v.(type) {
case int:
return "int"
case string:
return "string"
default:
return "unknown"
}

Switch - you can break it
switch argv[0] {
case "echo":
fmt.Print(argv[1:]...)
case "cat":
if len(argv) <= 1 {
fmt.Println("Usage: cat <filename>")
break
}
PrintFile(argv[1])
default:
fmt.Println("Unknown command; try 'echo' or 'cat'")
}

Switch - fallthrough
switch len(src) {
default:
v |= uint32(src[3])
fallthrough
case 3:
v |= uint32(src[2]) << 8
fallthrough
case 2:
v |= uint32(src[1]) << 16
fallthrough
case 1:
v |= uint32(src[0]) << 24
}

Errors instead of exceptions
Golang has no exceptions
and of course there are no checked exceptions :)))))))
No try-catch-finally.
Error is just an interface
Functions are often returning a value and an error together.
Checking this error for nil is the exception handling in Golang:
// validate
pipeline, err := validateYaml(buf.Bytes())
if err != nil {
errorMessage := fmt.Sprintf("Error parsing pipeline config: %s", err)
log.Error(errorMessage)
http.Error(w, errorMessage, http.StatusBadRequest)
} else {...}
NO CHECKED EXCEPTIONS!!!

Defer
func CopyFile(dstName, srcName string) (written int64, err error) {
src, err := os.Open(srcName)
if err != nil {
return
}
defer src.Close()
dst, err := os.Create(dstName)
if err != nil {
return
}
defer dst.Close()
defer fmt.Println(“All closed”)
return io.Copy(dst, src)
}

Panic and Recover -bit.ly/2a5m0mJ
func main() {
f()
fmt.Println("Returned normally from f.")
}
func f() {
defer func() {
if r := recover(); r != nil {
fmt.Println("Recovered in f", r)
}
}()
fmt.Println("Calling g.")
g(0)
fmt.Println("Returned normally from g.")
}
func g(i int) {
if i > 3 {
fmt.Println("Panicking!")
panic(fmt.Sprintf("%v", i))
}
defer fmt.Println("Defer in g", i)
fmt.Println("Printing in g", i)
g(i + 1)
}

Functions -bit.ly/2a5mg4W
package main
import "fmt"
func swap(x, y string) (string, string) { // also can use (x string, y string)
return y, x
}
func main() {
a, b := swap("hello", "zooplus")
fmt.Println(a, b)
}

Functions -bit.ly/2aymMe6
package main
import "fmt"
func split(sum int) (x, y int) {
x = sum * 4 / 9
y = sum - x
return
}
func main() {
fmt.Println(split(17))
}

Interfaces
Golang has no “implements” keyword. If you implement all methods defined by the
interface, then you are implemented the interface implicitly.
package main
import "fmt"
type I interface {
M()
}
type T struct {
S string
}
// This method means type T implements the interface I,
// but we don't need to explicitly declare that it does so.
func (t T) M() {
fmt.Println(t.S)
}
func main() {
var i I = T{"hello"}
i.M()
}

The empty interface - bit.ly/2awwq3Z
Like Object in Java, Golang has the interface{} as type that compatible with every
value (even with primitives).
So it can hold any value. You need type assertion to find out what is it:
package main
import "fmt"
func main() {
var i interface{} = "hello"
s := i.(string)
fmt.Println(s)
s, ok := i.(string)
fmt.Println(s, ok)
f, ok := i.(float64)
fmt.Println(f, ok)
f = i.(float64) // panic
fmt.Println(f)
}

Reflection
“In computer science, reflection is the ability of a computer program to examine
and modify its own structure and behavior (specifically the values, meta-data,
properties and functions) at runtime.” /Wikipedia/
Golang has a reflection package to inspect structs, interfaces or methods runtime
like in Java and also you can change the values of the fields.
What you’ll miss is some bytecode manipulation and proxy generation. Mockito
like framework is not possible in Golang.
See more: http://blog.ralch.com/tutorial/golang-reflection/

Similarities with Java
Goroutine = Thread
Locks (R, RW)
Atomic package thread safe variable handling
Waitgroup = Join

Differences from Java
Goroutine is lightweight, but not reusable
No Threadlocal or Executorservice in Golang
Golang has no thread safe map implementation by default
(https://github.com/streamrail/concurrent-map)
Golang is a pass-by-value language
Golang uses channels to communicate between threads
Golang unsafe package is more limited
To summarize: Java originally built-on the shared memory concept, but the
Golang is focusing on the communication with channels

Channels
ch := make(chan int) // single item channel
v:= 5
ch <- v // Send v to channel ch.
v := <-ch // Receive from ch, and
// assign value to v.
ch := make(chan int,100) // buffered channel
v:= 5
ch <- v // Send v to channel ch.
v := <-ch // Receive from ch, and
// assign value to v.

Looping on a channel
package main
import (
"fmt"
)
func fibonacci(n int, c chan int) {
x, y := 0, 1
for i := 0; i < n; i++ {
c <- x
x, y = y, x+y
}
close(c)
}
func main() {
c := make(chan int, 10)
go fibonacci(cap(c), c)
for i := range c {
fmt.Println(i)
}
}

Go and Java common points
C like (braces)
Garbage collector
Memory safety (nil/null references, runtime bounds checks)
Statically typed
Variables initialized by default(zero/nil/false)
Methods, Interfaces

Go and Java differences
Native code without VM + statically linked binaries
Control over memory layout
Function values and lexical closures
Built-in strings (UTF-8)
Built-in generic maps and arrays/slices
Built-in concurrency

Go missing parts (by intention…)
No inheritance
No constructors
No classes
No implements
No final
No exceptions

A restful API in Golang
Built with Makefile
Dockerized
Documented with Swagger
Tested with Cucumber

Next steps
Go by example (https://gobyexample.com)
Tutorial (https://tour.golang.org/welcome/1)
Effective go (https://golang.org/doc/effective_go.html)
Awesome Go: https://go.libhunt.com/

Recommended frameworks
Gorilla: http://www.gorillatoolkit.org/ OR https://github.com/emicklei/go-restful (with
Swagger)
BDD: https://github.com/DATA-DOG/godog
Configuration: https://github.com/spf13/viper
Logging: https://github.com/Sirupsen/logrus
CLI writing: https://github.com/spf13/cobra
Microservices: https://gokit.io/

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