Short source code examples

As a brief note to self, I was experimenting with passing methods as parameters to Scala functions — in particular trying to overload those method signatures to see what would work and what would not work — and I came up with these simple function signature overloading tests:

As a brief note to self, you compile a TypeScript source code file with the tsc command:

tsc Foo.ts

Depending on what you’re doing, you can also run the resulting JavaScript file from the command line with this node command:

tsc Foo.ts && node Foo.js

That command can be read as, “Compile the TypeScript file and then if that compiles successfully, run the resulting JavaScript file.” Note the ts and js extensions on those files.

As a brief note today, here’s a Scala method that writes the strings in a list — more accurately, a Seq[String] — to a file:

def writeFile(filename: String, lines: Seq[String]): Unit = {
    val file = new File(filename)
    val bw = new BufferedWriter(new FileWriter(file))
    for (line <- lines) {
        bw.write(line)
    }
    bw.close()
}

Here’s a brief example of how to sort data that’s already in a Java HashMap. First, some HashMap data:

import java.util.*;

Map<Integer, Float> playMap = new HashMap<Integer, Float>() {{
    put(10,  1f);
    put(12,  2f);
    put(15,  4f);
    put(18,  6f);
    put(20,  6f);
    put(25,  7f);
    put(30,  6f);
    put(35,  5f);
    put(40,  5f);
}};

A HashMap won’t keep that data in sorted order, so if you want your data sorted, one way to sort it is to add the elements to a Java TreeMap:

In an effort to share some source code (but without taking the time to explain it), here’s some Java source code that I just used to create a JFreeChart chart/graph of some data that I use in my Android football game:

If you need to copy a directory on Unix/Linux and want to preserve the date/time information while copying the directory and files, use the -p option to save the date/time information, and the -r option to copy the directory properly. For instance, I just used this cp command to copy a directory named OldDir to a new directory named NewDir, while retaining all of the date/time file information:

I was just going through some old notes and found this Scala DSL example from 2010:

As a brief note today, here’s an example of stackable modifications in Scala.

Lately I was curious about what super means when you mix Scala traits into a class or object. A simplified answer is that super refers to the last trait that’s mixed in, though I should be careful and note that this is an oversimplification.

This can be demonstrated in an example that uses both inheritance and mixins with traits. Given this combination of traits and classes:

If you ever need to left-justify a multiline Scala string, the solution is to use the stripMargin method, as shown in this example:

def getWordFormatted(s: String): String = {
    s"""
      |
      |Word of the Day
      |---------------
      |Word: $s
    """.stripMargin
}

The | character has a special meaning when used in conjunction with stripMargin, so that method results in a string like this being output:

As a note to self, the Scala 3 “Dotty” procedure syntax is going to look like this:

def f() = { ... }
def f(): Unit = { ... }

This Scala 2 procedure syntax is being dropped:

def f() { ... }

You can find the official Dotty procedure syntax page at this link.

As for me, I’m adding this page right now because I noticed today that this procedure syntax (without the parentheses) also works with Dotty:

I can never remember how to create a Scala 3 (Dotty) project with SBT (in early 2019), so:

# create a new Dotty project
sbt new lampepfl/dotty.g8

# create a dotty project that cross compiles with scala 2
sbt new lampepfl/dotty-cross.g8

# start a dotty reply from within your sbt project
$ sbt

> console
scala> _

Here’s what an example interaction looks like — include SBT renaming my directory name to all lowercase, which brings me no joy:

As a brief note to self, when you need to debug a chain of RxJava Observable method calls, you can use the doOnNext method to log the current values or print them to STDOUT or STDERR with println. Here’s an example from RxJava For Android Developers, where the debug output is logged with the Android Log.d method:

As a quick note today, if you need to configure an Android project to use Java 8, I found that adding this setting to the app-level build.gradle file worked:

compileOptions {
    sourceCompatibility = '1.8'
    targetCompatibility = '1.8'
}

You can also use this syntax:

compileOptions {
    sourceCompatibility JavaVersion.VERSION_1_8
    targetCompatibility JavaVersion.VERSION_1_8
}

That setting goes inside the android setting in the build.gradle file, as shown here:

If for some reason you ever need a list of people’s given names for testing your Java/Scala/Kotlin/JVM code, here’s a Java class with a sorted, static list of over 5,000 male and female given (first) names:

As a brief note to self, this is how I compiled/built an Android application (APK) from the MacOS command line and then ran it in an emulator. I include both my application- and system-specific notes, as well as the more generic commands I found at this Android.com URL:

When I tried to open an Android project with Android 3 that I probably created with Android Studio 2 and haven’t worked on in about two years, I got this error message:

As a brief note, here’s an example that shows how to create an RxJava 2 Observable from a Java List:

When you’re working with multi-threaded programming in Java — such as when working with Thread, Runnable, SwingUtilities.invokeLater, Akka, futures, or Observable in RxJava — you may need to get the name of the current thread as a way to debug your code. Here’s how you get the name of the current thread in Java:

I have no idea what I was thinking, but today I learned (or was reminded) that if you want the first element from a Scala Set you should use its head method, or headOption. For some reason I kept thinking that take should do the job, but you can see the results in the Scala REPL, where take(1) returns a Set:

As a bit of warning, this is some old Java code, but if you want to create your own Java file utilities (utility methods), this code might help you get started: