Go Programming for Java Developers

Notes for my Go programming workshop.

1 About Me

Fabian Stäber, ConSol Software GmbH
Java Developer, Gopher
Some Go projects on GitHub, like github.com/fstab/grok_exporter

2 Resources

Online tutorial: https://tour.golang.org
Book: Alan A. A. Donovan, Brian W. Kernighan: The Go Programming Language, Addison-Wesley, 2015
Google: Google keyword for the Go programming language is 'golang'
This text: https://github.com/fstab/go-programming-for-java-developers
Slides: https://goo.gl/P7XAXn (docs.google.com)

3 About Go

Created at Google in 2007 by Robert Griesemer, Rob Pike, and Ken Thompson
Main design goal: Simplicity (not many features)
Main purpose: System tools programming

4 Go for Java Developers

Go is very similar to Java

C-like Syntax
Strongly typed
Packages, Imports
...

Java developers will learn Go very quickly.

5 Installation

Java:

Download and extract JDK.
Set environment variable JAVA_HOME to JDK directory.
Include $JAVA_HOME/bin in PATH.

Go:

Download and extract Go.
Set environment variable GOROOT to Go directory (default /usr/local/go or C:\Go).
Include $GOROOT/bin in PATH.

Optional (not needed in this workshop): Set GOPATH to workspace, like $HOME/go.

6 Exercise

Install go
Run the following commands

go
go help
go help run
go help fmt

7 Go Tool

The go command is more like mvn than like javac.

go get ...               <-- download from github, bitbucket, etc.
go test ...              <-- like 'mvn test'
go install ...           <-- install binary to $GOPATH/bin

8 Go Directory Structure

Like mvn, the go command assumes a defined directory structure:

$GOPATH/bin                     <- compiled executables
$GOPATH/pkg                     <- compiled libraries
$GOPATH/src/github.com/...      <- source code
$GOPATH/src/bitbucket.org/...   <- source code
$GOPATH/src/...                 <- source code

In real-world go development, you run go install to compile the source code and create an executable in $GOPATH/bin.

However, in this workshop we use go run, which is a shortcut to quickly run a single go file. The go run command does not require the directory structure above.

9 Hello, World!

package main

import (
    "fmt"
)

func main() {
    fmt.Printf("Hello, World!\n")
}

10 Exercise

Create a file hello.go containing the hello world source code.
Format and run the program:

go fmt hello.go
go run hello.go

Read the documentation of the fmt package and the Printf call

go doc fmt
go doc fmt.Printf

11 Hello, <name>!

os.Args is an []string (like String[] in Java)
len(os.Args) returns the number of elements (like arr.length in Java)

package main

import (
    "fmt"
    "os"
)

func main() {
    if len(os.Args) != 2 {
        fmt.Printf("Usage: ./hello <name>\n")
        return
    }
    fmt.Printf("Hello, %v\n", os.Args[1])
}

12 os.Args[0]

os.Args[0] is the name of the executable
Example: ./hello Fabian:
- os.Args[0]: ./hello
- os.Args[1]: Fabian
With go run hello.go Fabian, the program will be compiled to an executable in a temporary directory, this temporary executable is executed. -> os.Args[0] is the path to the temporary executable created by go run.

13 Loops

Use of for like Java's for

for i := 0; i < 10; i++ {
    fmt.Printf("%v\n", i)
}

Use of for like Java's while

i := 0
for i < 10 {
    fmt.Printf("%v\n", i)
    i++
}

Use of for like Java's while(true)

i := 0
for {
    if i >= 10 {
        break
    }
    fmt.Printf("%v\n", i)
    i++
}

14 Exercise

Write a program that says hello to multiple people:

> go run hello.go Fabian Thomas Christian
Hello, Fabian!
Hello, Thomas!
Hello, Christian!

15 Variables

Variable declaration

var a int
a = 3

Variable declaration with initializer

var a = 3

Short variable declaration

a := 3

Declaring multiple variables

var (
    a, b int
    c string
)

var a, b, c = 3, 4, "hello"

Constants (like final in Java)

const a = 3

16 Functions

func add(a int, b int) int {
    return a + b
}

func add(a, b int) int {
    return a + b
}

func addAndMult(a, b int) (int, int) {
    return a + b, a * b
}

func main() {
    sum, product := addAndMult(3, 7)
    fmt.Printf("sum = %v, product = %v\n", sum, product)
}

17 Function Example

package main

import "fmt"

func swap(x, y string) (string, string) {
    return y, x
}

func main() {
    a, b := swap("world", "hello")
    fmt.Printf("%v, %v\n", a, b)
}

18 Error Handling with Multiple Return Values

package main

import (
    "fmt"
    "strconv"
    "os"
)

func main() {
    // We assume that len(os.Args) >= 2
    n, err := strconv.Atoi(os.Args[1])
    if err != nil {
        fmt.Printf("Error: %v is not a valid number.\n", os.Args[1])
        return
    }
    fmt.Printf("Your number is %v\n", n)
}

Go's nil is like Java's null.

19 Ignoring Return Values

In go it's a syntax error if a variable is declared but not used:

func main() {
    sum, product := addAndMult(3, 7)
    fmt.Printf("sum = %v\n", sum)
}

Syntax error, because variable product is declared but not used.

To explicitly ignore a return value, use _:

func main() {
    sum, _ := addAndMult(3, 7)
    fmt.Printf("sum = %v\n", sum)
}

20 Exercise

Write a program that prints the first n Fibonacci numbers.

> go run fibonacci.go 10
1 1 2 3 5 8 13 21 34 55

21 Bonus Exercise

From http://tour.golang.org

As a simple way to play with functions and loops, implement the square root function using Newton's method.

In this case, Newton's method is to approximate Sqrt(x) by picking a starting point z and then repeating:

To begin with, just repeat that calculation 10 times and see how close you get to the answer for various values (1, 2, 3, ...).

Next, change the loop condition to stop once the value has stopped changing (or only changes by a very small delta). See if that's more or fewer iterations. How close are you to the math.Sqrt?

Hint: to declare and initialize a floating point value, give it floating point syntax or use a conversion:

z := float64(1)

z := 1.0

22 Deferred Methods

package main

import (
    "bufio"
    "fmt"
    "os"
)

const path = "./hello.txt"

func main() {
    file, err := os.Open(path)
    if err != nil {
        fmt.Printf("failed to open file: %v\n", err)
        os.Exit(-1)
    }
    defer file.Close()
    reader := bufio.NewReader(file)
    for {
        line, err := reader.ReadString('\n')
        if err != nil {
            break
        }
        fmt.Printf("%v", line)
    }
}

23 Exercise

Modify the hello world program and add some deferred method calls, like

defer fmt.Printf("defer 1\n")
defer fmt.Printf("defer 2\n")
defer fmt.Printf("defer 3\n")

Figure out the order in which these calls are executed.

24 Pointers

package main

import "fmt"

func double(i int) {
    i = i * 2
}

func main() {
    i := 2
    double(i)
    fmt.Printf("i = %v\n", i)
}

Result: i = 2

package main

import "fmt"

func double(i *int) {
    *i = *i * 2
}

func main() {
    i := 2
    double(&i)
    fmt.Printf("i = %v\n", i)
}

Result: i = 4

25 Structs

package main

import "fmt"

type rectangle struct {
    height float64
    width  float64
}

func main() {
    square := &rectangle{
        height: 4.0,
        width: 4.0,
    }
    fmt.Printf("square has area of %v\n", square.height * square.width)
}

A struct is like a final class in Java.
Variables starting with an upper case letter are public, variables starting with a lower case letter are package private.

26 Methods

package main

import "fmt"

type rectangle struct {
    height float64
    width  float64
}

func (r *rectangle) area() float64 {
    return r.width * r.height
}

func main() {
    square := &rectangle{
        height: 4.0,
        width: 4.0,
    }
    fmt.Printf("square has area of %v\n", square.area())
}

27 Exercise

Define struct circle with variable radius that also has a method area().

Hint: go doc math.Pi

28 String Concatenation

Java: Implicit conversion from float64 to String

return "the area is " + s.area();

Go: Explicit string formatting

return fmt.Sprintf("area is %v", s.area())

29 Interfaces

type shape interface {
    area() float64
}

func areaInfo(s shape) string {
    return fmt.Sprintf("area is %v", s.area())
}

func main() {
    c := &circle{
        radius: 2.0,
    }
    r := &rectangle{
        width: 4.0,
        height: 4.0,
    }
    fmt.Printf("circle:    %v\n", areaInfo(c))
    fmt.Printf("rectangle: %v\n", areaInfo(r))
}

30 Pre-Defined Interfaces

Go has some pre-defined interfaces. The most important ones are:

type Stringer interface {
    String() string
}

Like Java's toString()

type error interface {
    Error() string
}

Remember file, err := os.Open(path)? The err is of type error.

31 Exercise

Implement a String() method for rectangle and circle.

The following code:

func main() {
    c := &circle{
           radius: 2.0,
    }
    r := &rectangle{
           width: 4.0,
           height: 4.0,
    }
    fmt.Printf("Shape 1: %v\n", c)
    fmt.Printf("Shape 2: %v\n", r)
}

Should produce the following output:

Shape 1: circle with radius 2 and area 12.566370614359172
Shape 2: rectangle with width 4, height 4, and area 16

32 Functional Programming

Example 1: Define an anonymous function and assign it to a variable

package main

import "fmt"

func main() {
    f := func(n int) int {
        return 2 * n
    }
    fmt.Printf("f(21) = %v\n", f(21))
}

Example 2: Pass a function as parameter to another function

package main

import "fmt"

func applyFunction(n int, f func(int) int) {
    fmt.Printf("f(%v) = %v\n", n, f(n))
}

func main() {
    f := func(n int) int {
        return 2 * n
    }
    applyFunction(21, f)
}

33 Arrays & Slices

Java Arrays:

int size = 10;
int[] arr = new int[size];

Go Slices:

size := 10
slice := make([]int, size)

Iterate over a slice:

for i, val := range slice {
    fmt.Printf("slice[%v] = %v\n", i, val)
}

34 Arrays & Slices Example

package main

import "fmt"

func createSlice(size int) []int {
    slice := make([]int, size)
    for i := 0; i < size; i++ {
        slice[i] = i + 1
    }
    return slice
}

func applyFunction(slice []int, f func(int) int) {
    for i, n := range slice {
        slice[i] = f(n)
    }
}

func main() {
    f := func(n int) int {
        return 2 * n
    }
    slice := createSlice(10)
    fmt.Printf("%v\n", slice)
    applyFunction(slice, f)
    fmt.Printf("%v\n", slice)
}

Result:

[1 2 3 4 5 6 7 8 9 10]
[2 4 6 8 10 12 14 16 18 20]

35 Slicing

Example 1:

slice := createSlice(10)
fmt.Printf("%v\n", slice)
part := slice[3:7]
applyFunction(part, f)
fmt.Printf("%v\n", slice)

Result:

[1 2 3 4 5 6 7 8 9 10]
[1 2 3 8 10 12 14 8 9 10]

Example 2:

slice := createSlice(10)
fmt.Printf("%v\n", slice)
firstHalf := slice[:len(slice)/2]
applyFunction(firstHalf, f)
fmt.Printf("%v\n", slice)
secondHalf := slice[len(slice)/2:]
applyFunction(secondHalf, f)
fmt.Printf("%v\n", slice)

Result:

[1 2 3 4 5 6 7 8 9 10]
[2 4 6 8 10 6 7 8 9 10]
[2 4 6 8 10 12 14 16 18 20]

36 Exercise

Write a function combine() such that

func main() {
    add := func(a, b int) int {
            return a + b
    }
    mult := func(a, b int) int {
            return a * b
    }
    slice := createSlice(10)
    fmt.Printf("add(1..10)=%v\n", combine(slice, add))
    fmt.Printf("mult(1..10)=%v\n", combine(slice, mult))
}

Prints the following output

add(1..10)=55
mult(1..10)=3628800

Bonus task: Implement two versions of combine(): one with a for loop, and one recursive version without using for.

37 Multithreading

package main

import (
    "fmt"
    "time"
)

func say(s string) {
    for i := 0; i < 5; i++ {
        time.Sleep(100 * time.Millisecond)
        fmt.Printf("%v\n", s)
    }
}

func main() {
    go say("world")
    say("hello")
}

38 Channels

package main

import (
    "time"
    "fmt"
)

func produceNumber(c chan int) {
    time.Sleep(10 * time.Second)
    c <- 3
}

func main() {
    c := make(chan int)
    go produceNumber(c)
    n := <-c
    fmt.Printf("n = %v\n", n)
}

39 Loop over Data Read from Channel

package main

import (
    "time"
    "fmt"
)

func produceNumber(c chan int) {
    for i := 0; i < 10; i++ {
        time.Sleep(1 * time.Second)
        c <- i + 1
    }
    close(c)
}

func main() {
    c := make(chan int)
    go produceNumber(c)
    for n := range c {
        fmt.Printf("n = %v\n", n)
    }
}

40 Exercise

Re-write the channel examples with two go-routines:

One producer routine producing numbers
One consumer routine printing numbers to stdout

Terminology: A function running as a background thread is a go-routine.

41 Waiting for termination

package main

import (
    "time"
    "fmt"
)

func produceNumbers(c chan int) {
    for i := 0; i < 10; i++ {
        time.Sleep(1 * time.Second)
        c <- 3
    }
    close(c)
}
func consumeNumbers(c chan int, done chan interface{}) {
    for n := range c {
        fmt.Printf("n = %v\n", n)
    }
    close(done)
}
func main() {
    c := make(chan int)
    done := make(chan interface{})
    go produceNumbers(c)
    go consumeNumbers(c, done)
    <-done
}

42 Polling on multiple channels with select

chan c int
chan done interface{}

select {
    case n <- c:
        // do something with n
    case <- done:
        // shut down
}

43 Exercise

Implement a deadlock (go-routine A is reading from channel of go-routine B and vice versa) and see what happens.

44 Buffered Channels

Write to channel blocks
Channels may have capacity
Reading from a closed channel returns nil
Writing to a closed channel panics

-> Always the producer should close a channel.

45 HTTP Server

package main

import (
    "fmt"
    "net/http"
    "os"
)

func handler(w http.ResponseWriter, r *http.Request) {
    fmt.Fprintf(w, "URL.Path = %q\n", r.URL.Path)
}

func main() {
    http.HandleFunc("/", handler)
    err := http.ListenAndServe("localhost:8000", nil)
    if err != nil {
        fmt.Fprintf(os.Stderr, "Failed to open Web server on localhost:8080: %v\n", err)
    }
}

46 Exercise

Include a unique number in each response:

First response gets number 1
Second response gets number 2
etc.

Avoid race conditions. Don't communicate through shared memory, use channels!!!

47 Mutexes

From The Go Programming Language, Chapter 1.7:

package main

import (
    "fmt"
    "log"
    "net/http"
    "sync"
)

var mu sync.Mutex
var count int

func main() {
    http.HandleFunc("/", handler)
    http.HandleFunc("/count", counter)
    log.Fatal(http.ListenAndServe("localhost:8000", nil))
}

// handler echoes the Path component of the requested URL.
func handler(w http.ResponseWriter, r *http.Request) {
    mu.Lock()
    count++
    mu.Unlock()
    fmt.Fprintf(w, "URL.Path = %q\n", r.URL.Path)
}

// counter echoes the number of calls so far.
func counter(w http.ResponseWriter, r *http.Request) {
    mu.Lock()
    fmt.Fprintf(w, "Count %d\n", count)
    mu.Unlock()
}

48 Summary

Topics covered in this workshop:

control flow: if/else, for
functions, error handling, deferred methods
pointers, structs, methods, interfaces
arrays & slices
multithreading

Solution to Exercise 14

package main

import (
    "fmt"
    "os"
)

func main() {
    for i := 1; i<len(os.Args); i++ {
        fmt.Printf("Hello, %v\n", os.Args[i])
    }
}

Solution to Exercise 20

package main

import (
    "fmt"
    "os"
    "strconv"
)

func main() {
    if len(os.Args) != 2 {
        fmt.Print("Usage: fibonacci <n>\n")
        return
    }
    n, err := strconv.Atoi(os.Args[1])
    if err != nil {
        fmt.Print("Invalid number.\n")
        return
    }
    fmt.Print("Result:")
    var prev, cur = 0, 1
    for i := 0; i < n; i++ {
        fmt.Printf(" %v", cur)
        prev, cur = cur, prev + cur
    }
    fmt.Print("\n")
}

Solution to Exercise 23

package main

import (
    "fmt"
)

func main() {
    defer fmt.Printf("defer 1\n")
    defer fmt.Printf("defer 2\n")
    fmt.Printf("Hello, World!\n")
    defer fmt.Printf("defer 3\n")
}

Solution to Exercise 27

package main

import (
    "fmt"
    "math"
)

type rectangle struct {
    height float64
    width  float64
}

func (r *rectangle) area() float64 {
    return r.width * r.height
}

type circle struct {
    radius float64
}

func (c *circle) area() float64 {
    return math.Pi * c.radius * c.radius
}

func main() {
    r := &rectangle{
        height: 4.0,
        width: 4.0,
    }
    c := &circle{
        radius: 2.0,
    }
    fmt.Printf("square has area of %v\n", r.area())
    fmt.Printf("circle has area of %v\n", c.area())
}

Solution to Exercise 31

package main

import (
    "fmt"
    "math"
)

type rectangle struct {
    height float64
    width float64
}

func (r *rectangle) area() float64 {
    return r.width * r.height
}

type circle struct {
    radius float64
}

func (c *circle) area() float64 {
    return math.Pi * c.radius * c.radius
}

func (r *rectangle) String() string {
    return fmt.Sprintf("rectangle with width %v, height %v, and area %v", r.width, r.height, r.area())
}

func (c *circle) String() string {
    return fmt.Sprintf("circle with radius %v and area %v", c.radius, c.area())
}

func main() {

    c := &circle{
        radius: 2.0,
    }

    r := &rectangle{
        width: 4.0,
        height: 4.0,
    }

    fmt.Printf("Shape 1: %v\n", c)
    fmt.Printf("Shape 2: %v\n", r)
}

Solution to Exercise 36

package main

import "fmt"

// for loop
func combine1(slice []int, f func(int, int) int) int {
    result := slice[0]
    for i := 1; i < len(slice); i++ {
        result = f(result, slice[i])
    }
    return result
}

// recursive
func combine2(slice []int, f func(int, int) int) int {
    if len(slice) == 1 {
        return slice[0]
    }
    if len(slice) == 2 {
        return f(slice[0], slice[1])
    }
    left := combine2(slice[:len(slice)/2], f)
    right := combine2(slice[len(slice)/2:], f)
    return f(left, right)
}

func createSlice(size int) []int {
    slice := make([]int, size)
    for i:=0; i<size; i++ {
        slice[i] = i+1
    }
    return slice
}

func main() {
    add := func(a, b int) int {
        return a + b
    }
    mult := func(a, b int) int {
        return a * b
    }
    slice := createSlice(10)
    fmt.Printf("for loop:\n")
    fmt.Printf("add(1..10)=%v\n", combine1(slice, add))
    fmt.Printf("mult(1..10)=%v\n", combine1(slice, mult))

    fmt.Printf("\nrecursive:\n")
    fmt.Printf("add(1..10)=%v\n", combine2(slice, add))
    fmt.Printf("mult(1..10)=%v\n", combine2(slice, mult))
}

Solution to Exercise 40

package main

import (
    "time"
    "fmt"
)

func produceNumbers(c chan int) {
    for i := 0; i < 10; i++ {
        time.Sleep(1 * time.Second)
        c <- i
    }
    close(c)
}

func consumeNumbers(c chan int) {
    for n := range c {
        fmt.Printf("n = %v\n", n)
    }
}

func main() {
    c := make(chan int)
    go produceNumbers(c)
    go consumeNumbers(c)
    time.Sleep(12 * time.Second)
}

Solution to Exercise 43

package main

func main() {
    c1, c2 := make(chan int), make(chan int)
    done := make(chan interface{})
    go func() {
        <-c1     // read from c1
        c2 <- 42 // write to c2
        close(done)
    }()
    go func() {
        <-c2     // read from c2
        c1 <- 42 // write to c1
        <-done
    }()
    <-done
}

Solution to Exercise 46

package main

import (
    "fmt"
    "net/http"
    "os"
)

var c = make(chan int)

func producer() {
    i := 0
    for {
        i++
        c <- i
    }
}

func handler(w http.ResponseWriter, r *http.Request) {
    fmt.Fprintf(w, "Request number is %v. Note that you browser might send two requests per 'reload'.\n", <- c)
}

func main() {
    go producer()
    http.HandleFunc("/", handler)
    err := http.ListenAndServe("localhost:8000", nil)
    if err != nil {
        fmt.Fprintf(os.Stderr, "Failed to open Web server on localhost:8080: %v\n", err)
    }
}

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