Scala, Java, Unix, MacOS tutorials (page 32)

The Scala SBT build tool uses a file named build.sbt to define your build. The SBT syntax can be a little hard to understand without examples, so I try to collect good examples here when I see them.

Seeing good examples rather than scrolling through reams of documentation has helped me many times!

To that end, this is a build.sbt SBT build file example from the Kyo project:

I have no idea if I’ll keep using the script, but for the moment, this is a MacOS script (shell script) I named Turn-Off-Wifi-and-Run-Sleep.command:

time=2

networksetup -setairportpower en0 off
echo "Sleeping $time seconds ..."
sleep $time

osascript -e 'tell application "System Events" to sleep'

I saved this file to my desktop, made it executable with chmod, and then when I run it, it does what its name implies, turning off the Wi-Fi networking and then putting my Mac to sleep.

Note that when you run this script, it opens a MacOS Terminal window to show the result of the commands in the script.

I did this because the Mac had a weird error this morning this morning when I turned it on and it said it had run out of application memory while I was sleeping. ATM I don’t want the Mac working while I’m sleeping, so I wrote this script (though I don’t know how successful this approach will be).

As a brief note, here are some tests I just created with Scala 3 to test concepts like private, protected, def, val, and var with inheritance tests, i.e., where classes extend other classes or traits:

// FAILS
// package private_def:
    // trait StoreProduct:
    //     private def id: Int   // compiler:
    //                           // "abstract method id may not have `private` modifier"
    //
    // class Pizza extends StoreProduct:
    //     val id = 1

package def_val:
    trait StoreProduct:
        protected def id: Int

    class Pizza extends StoreProduct:
        val id = 1

package val_val:
    trait StoreProduct:
        protected val id: Int

    class Pizza extends StoreProduct:
        val id = 1

package var_var:
    trait StoreProduct:
        protected var id: Int

    class Pizza extends StoreProduct:
        var id = 1

// FAILS
// package val_var:
//     trait StoreProduct:
//         protected val id: Int
//
//     class Pizza extends StoreProduct:
//         var id = 1      // compiler:
//                         // "variable id of type Int needs to be a stable, immutable value"

// FAILS
// package var_val:
//     trait StoreProduct:
//         protected var id: Int
//
//     class Pizza extends StoreProduct:
//         val id = 1      // compiler:
//                         // "class Pizza needs to be abstract, since protected
//                         // var id_=(x$1: Int): Unit in trait StoreProduct in
//                         // package var_val is not defined"

Hopefully you can see from the comments what works and what doesn’t work when you’re using inheritance in Scala.

As a brief note, the Scala example at this URL (www.geeksforgeeks.org/controlling-method-scope-in-scala/) is not 100% correct, and this code Scala 3 code shows the issue and solution:

package a:
    class ClassA:
        protected var aProtected = 1
        var a = 1

package b:
    import a.ClassA

    class ClassB extends ClassA:
        a = 2
        aProtected = 2                   // ClassB *can* access this field.
        def getAProtected = aProtected   // make that field available to others
                                         //   via this public method.

    object Main:
        def main(args: Array[String]): Unit =
            val b = ClassB()
            b.a
            b.getAProtected   // compiles/works
            // b.aProtected   // does not compile

They correctly note that the variable I call aProtected can’t be accessed from the main method in the second package, but aProtected can be accessed by ClassB, which extends ClassA, though ClassB is in package b and ClassA is in package a.

So if you wanted to see an example of how to access a protected field in Scala code, where your class extends a class in a different package, and that original class has a protected field, I hope this example is helpful.