A Go translation of the example code from Practical Object-Oriented Design in Ruby by Sandi Metz.

Use the Go Playground or go run to try these examples, eg:

chapter2> go run gear1.go

(intro)

• gear1.go defines a basic Gear with getters
• gear2.go introduces a new feature (and responsibility)
• gear3.go hide instance variables (behavior in one place)
• obscure.go depending on complicated data structures is bad
• revealing.go isolating the incoming array
• gear4.go extracting wheel as an internal structure
• gear5.go a real Wheel with dependency injection

• 1-dependencies.go Gear knows too much about Wheel (actually a step back from gear5.go)
• 2-duck-type.go We don't need a Wheel specifically, just an object that responds to Diameter()
• 3-isolate-new.go Isolate instance creation (if you can't inject the dependency for some reason)
• 4-isolate-messages.go Isolate external messages that could be vulnerable to change
• 5-map-init.go Remove argument order dependencies (probably not the best way to accomplish this)
• Skipped a factory method to work with an unwieldy constructor (gear-wrapper).
• 7-reverse-dependencies.go What if Wheel depends on Gear? (which is more stable?)

(It's all UML! :-)

(structural typing in Go)

• trip1.go A Trip that knows it needs the bicycles prepared.
• trip2.go Trip preparation becomes more complicated. It knows too much.
• trip3.go A Preparer interface, more abstract but easier to extend.

(which Go doesn't have)

• bikes1.go Starting with a road bike.
• bikes2.go We need mountain bikes too. Switching on the type.
• Skipped misapplying inheritance.
• bikes4.go Implicit delegation and type embedding instead of subclasses.
• bikes5.go Specializing the Spares method.
• bikes6.go Use a hook to push responsibilities into the embedded type.

The template method pattern would require a reference to the embedded type, after it is created. Seems like a pattern that shouldn't be attempted in Go.

• schedule1.go Scheduling as part of Bicycle, for later extraction.
• schedule2.go Extract and delegate to Schedulable.

• Skipping first transition, which still uses template methods and inheritance.
• parts2.go Bicycle composed of Parts, which is a slice of Part.
• parts3.go Rather than a PartsFactory, I use array-style composite literals.

Use go test to run these, eg:

chapter9> go test gear1/gear1_check_test.go

Your GOPATH matters for these, as we are importing a separate package for black box testing.

• gear1/gear1_test.go A basic example using Go's built in testing facilities.
• gear1/gear1_check_test.go The same code tested with gocheck and a custom matcher.