Make a mini calendar app for Android, celebrating the New Year

Celebrate the New Year and make a small app! As you can see from the calendar, there are several months with the same day of the week. So, let's make an application that groups months by the day of the week on the 1st day as a key.

[ For Japanese version, please click here.]

grouping months by the day of the week on the 1st day

Although this mini app is not so practical, I will make it with the following five goals:

[1] Make the application logic in Java for Android.
[2] Use lambda expressions, streams, and the new date classes provided by Java 8.
[3] Create a GUI with App Inventor and call Java from there.
[4] Create as a general-purpose framework applicable to other applications.
[5] Customize the ActivityStarter to create a new block.

First, I will talk about [1] and [2]. With the recent Android, we can use Java 8 features. So, I'm going to write this app as concisely as possible using lambda expressions and streams. The logic part is as follows. The essential part is only one assignment statement to a Map object named "mp". The element of this object is a pair of "day of the week" and "a list of months whose first day matches that day of the week".

Next is [3] and [4]. Java programs like the above can be called using App Inventor 's Activity Starter function (block). I will use it, but in order to make it a general-purpose framework, I prepared a template for both the App Inventor side and the Java side. On the Java side, I prepared an input processing part and an output processing part beforehand so that users can complete it by rewriting only the application logic. Also, the App Inventor side has been designed to be applicable to many other applications as shown below.　

A generic App Inventor application that calls Java programs

With regard to the last item [5], I wanted to slightly change the input data and parameters passing to Activity Starter, as a result of considering the versatility as described above. So, I customized ActivityStarter and created new blocks shown below, using AppyBuilder Code Editor. I / O with the Java side was designed to do via a text file. Regarding the input, I provide two ways, passing the path of the input file and passing the text data directly. The boolean parameter "isFile" in the Setup block blow makes this selection possible.

Newly created App Inventor block (customized version of Activity Starter)

The entire App Inventor program created using this new block is as follows.

App Inventor program with newly created block

[Note] About Android and Java 8
Under Android 5.0 (API level 22), it is Java 7 compliant and Java 8 cannot be used. Android 6.0 (API level 23) can use part of lambda expressions etc., but seems to require Android 8.0 (API level 26) or higher in order to use full functions including streams.