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Golang basics…...for Java developersRobert Stern

AgendaOverviewBasic syntax and structureDatatypesFlow controlFunctions, interfacesJava vs. Go

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

OverviewBasic syntax and structureDatatypesFlow controlFunctions, interfacesConcurrencyJava vs. Go

History and featuresCreated in 2007 at Google, announced in 2009, v1.0 released in 2012) with strongdedication for server side programming.Compiled, statically typed, garbage collected language with multiplearchitecture/OS support (i386, amd64, arm/Windows, Linux, *BSD, etc..)(very) fast compilation AND execution time with low resource consumptionPowerful 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 genericsNot an OO languageNo constructorsNo operator/method overloadingNo circular package importsNo mandatory semicolonsGC pause is unpredictable

What is non-java-isch?Multiple return values + the blank variableGolang is a pass-by-value languageLightweight concurrency with goroutines and channelsNo SE/EE separation. All server stuff is built-in.No exceptions, use error insteadCompile time error about unused imports and variablesNo maven/ant like build system. Uses Makefile+vendoring+go get

Famous projects, companiesProjects: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// importsimport "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

PackagesLike in Java, golang also uses packages to organize code, but in a bit differentformat.Keyword: package <name>The app starts in the main package.the path to the package’s directory is the import pathThe package name is the last element of the import path:github.com/zooplus/golang-app/stringutil → stringutilAccessing 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 typesTo use one or more functions in a package we need to import it into our code. Wecan 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 neededRemote packages has complete import path pointing to the server like:github.com/emicklei/go-restful

Getting the dependenciesFor remote packages use go get to fetch into your workspace. We have twosyntaxesgo get in the repository folder downloads all remote packages based on theimportsgo get <remote path> downloads the specified package for later use

Variables, constantspackage mainimport "fmt"const greeting = "Hello"var name stringfunc 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 valuediscarded 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 mainimport ("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 solutionpackage mainimport ("fmt""time")func main() {name := "Zooplus"fmt.Printf("Hello %s, today is %s.", name, time.Now().Format("Monday"))}

Basic typesboolstringint int8 int16 int32 int64uint uint8 uint16 uint32 uint64 uintptrbyte // alias for uint8rune // alias for int32, represents a Unicode code pointfloat32 float64complex64 complex128 - “math/cmplx”:var number complex128 = cmplx.Sqrt(-5 + 12i)

Pointers - bit.ly/2a58jnIpackage mainimport "fmt"func main() {i, j := 42, 2701p := &i // point to ifmt.Println(*p) // read i through the pointer*p = 21 // set i through the pointerfmt.Println(i) // see the new value of ip = &j // point to j*p = *p / 37 // divide j through the pointerfmt.Println(j) // see the new value of j}

package mainvar (a intb int32c int16)func main() {a = 1b = ac = a}No autoboxing! You should convert your types by yourself.package mainimport "fmt"var (a intb int32c int16)func main() {a = 1b = int32(a)c = int16(a)fmt.Printf("a: %d, b: %d, c: %d", a, b, c)}Type conversionprog.go:11: cannot use a (type int) as type int32 in assignmentprog.go:12: cannot use a (type int) as type int16 in assignmenta: 1, b: 1, c: 1

Structtype ApacheLogRecord struct {ipstringtimestringmethod, uri, protocolstringstatus intresponseBytes intrefererstringuserAgentstring}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 mainimport "fmt"type Animal struct {legCount int}// define a behavior for Animalfunc (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/2a56VkSpackage mainimport "fmt"type Animal struct {legCount int}func (a Animal) numberOfLegs() 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 {AnimalMammal}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/2aKK7sipackage mainimport "fmt"type Animal struct {legCount int}func (a Animal) numberOfLegs() 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}}fmt.Println("A Dog has this many legs: ", d.numberOfLegs())d.setLegs(5)fmt.Println("A Dog has this many legs: ", d.numberOfLegs())d.setLegsPtr(6)fmt.Println("A Dog has this many legs: ", d.numberOfLegs())}

Arrays -bit.ly/2ahx5EAArrays has fix size and type with zero based indexing:var integers [20]intpackage mainimport "fmt"func main() {var a [2]string // std declarationa[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 declarationfmt.Println(fibonacci)}

Slices - bit.ly/2ahVarPSlice is a dynamic version of array (a sort of):var integers []int // decleration of a slice without sizepackage mainimport "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/2aai1Wxpackage mainimport "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 - boundsThere are default low and high bounds for slices. Given the following array:var a [10]intThe following slice declarations are equivalent:a[0:10]a[:10]a[0:]a[:]

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

Arrays, slices - for loop as foreachIterate through an array/slicefor 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, initializationMaps are not initialized by default, we should use make:package mainimport "fmt"var m map[string]stringfunc main() {m = make(map[string]string)m["Zooplus"] = "Gut"m["Bitiba"] = "Gut"m["Fressnapf"] = "Falsch"fmt.Println(m["Zooplus"])}

Maps - Mutating mapsm[key] = item // upsertitem = m[key] // retrievedelete (m, key) // guess…._, ok = m[key] // ok == true if key exists, map can store/return nilMaps are not thread safe!!!

Maps - for loop as foreachIterate 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 loopsum := 0// init-condition-postfor i := 0; i < 10; i++ {sum += i}fmt.Println(sum)// partial loopsum := 0// init and post are optionalfor ; sum < 10; {sum += sum}// “while” loopsum := 1// init and post are optionalfor sum < 1000 {sum += sum}fmt.Println(sum)// infinite loopfor {}

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 notfound!")}If-then-elseexistingPipeline, ok := nameSpace.Pipelines[newPipeline.Name]if ok {log.Debug("Entry exists!")} else {nameSpace.Pipelines[newPipeline.Name] = existingPipelinelog.Debug("New entry stored")}?

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

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

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

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

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

Switch - you can break itswitch 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 - fallthroughswitch len(src) {default:v |= uint32(src[3])fallthroughcase 3:v |= uint32(src[2]) << 8fallthroughcase 2:v |= uint32(src[1]) << 16fallthroughcase 1:v |= uint32(src[0]) << 24}

Errors instead of exceptionsGolang has no exceptionsand of course there are no checked exceptions :)))))))No try-catch-finally.Error is just an interfaceFunctions are often returning a value and an error together.Checking this error for nil is the exception handling in Golang:// validatepipeline, 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!!!

Deferfunc 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/2a5m0mJfunc 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/2a5mg4Wpackage mainimport "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/2aymMe6package mainimport "fmt"func split(sum int) (x, y int) {x = sum * 4 / 9y = sum - xreturn}func main() {fmt.Println(split(17))}

InterfacesGolang has no “implements” keyword. If you implement all methods defined by theinterface, then you are implemented the interface implicitly.package mainimport "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/2awwq3ZLike Object in Java, Golang has the interface{} as type that compatible with everyvalue (even with primitives).So it can hold any value. You need type assertion to find out what is it:package mainimport "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) // panicfmt.Println(f)}

Reflection“In computer science, reflection is the ability of a computer program to examineand 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 runtimelike in Java and also you can change the values of the fields.What you’ll miss is some bytecode manipulation and proxy generation. Mockitolike framework is not possible in Golang.See more: http://blog.ralch.com/tutorial/golang-reflection/

Similarities with JavaGoroutine = ThreadLocks (R, RW)Atomic package thread safe variable handlingWaitgroup = Join

Differences from JavaGoroutine is lightweight, but not reusableNo Threadlocal or Executorservice in GolangGolang has no thread safe map implementation by default(https://github.com/streamrail/concurrent-map)Golang is a pass-by-value languageGolang uses channels to communicate between threadsGolang unsafe package is more limitedTo summarize: Java originally built-on the shared memory concept, but theGolang is focusing on the communication with channels

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

Looping on a channelpackage mainimport ("fmt")func fibonacci(n int, c chan int) {x, y := 0, 1for i := 0; i < n; i++ {c <- xx, 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 pointsC like (braces)Garbage collectorMemory safety (nil/null references, runtime bounds checks)Statically typedVariables initialized by default(zero/nil/false)Methods, Interfaces

Go and Java differencesNative code without VM + statically linked binariesControl over memory layoutFunction values and lexical closuresBuilt-in strings (UTF-8)Built-in generic maps and arrays/slicesBuilt-in concurrency

Go missing parts (by intention…)No inheritanceNo constructorsNo classesNo implementsNo finalNo exceptions

A restful API in GolangBuilt with MakefileDockerizedDocumented with SwaggerTested with Cucumber

Next stepsGo 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 frameworksGorilla: http://www.gorillatoolkit.org/ OR https://github.com/emicklei/go-restful (withSwagger)BDD: https://github.com/DATA-DOG/godogConfiguration: https://github.com/spf13/viperLogging: https://github.com/Sirupsen/logrusCLI writing: https://github.com/spf13/cobraMicroservices: https://gokit.io/

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