java

recent posts related to java, jdbc, spring, etc.

IT guy: I have successfully installed Java alvin November 3, 2019 - 11:47am

I don’t know where this originated, but I just saw it on a friend’s Facebook feed. Nothing like Java humor. :)

Notes on looking at MacOS application/process memory usage alvin July 20, 2019 - 4:14pm

Very briefly, I spent a little time today trying to understand how much memory/RAM a Java/Swing application was really using, and these are my notes from that excursion. Note that the app is bundled as a MacOS/Java app I named AlPad, but from some system perspectives it is only seen by the name `JavaAppLauncher`. Here are my notes, which will hopefully be useful to me in the future:

A Java method that returns a random boolean value based on a probability alvin June 30, 2019 - 8:14pm

If you ever need a Java method that returns a boolean value based on a given probability, I can confirm that this method works:

``````/**
* `probability` should be given as a percentage, such as
* 10.0 (10.0%) or 25.5 (25.5%). As an example, if `probability`
* is 60% (60.0), 100 calls to this function should return ~60
* `true` values.
* (Note that Math.random returns a value >= 0.0 and < 1.0.)
*/
static boolean getRandomBoolean(float probability) {
double randomValue = Math.random()*100;  //0.0 to 99.9
return randomValue <= probability;
}``````
How to sort data that’s in a Java HashMap (using a TreeMap) alvin June 29, 2019 - 12:32pm

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:

A Java JFreeChart x/y plot/chart/graph example alvin June 29, 2019 - 12:10pm

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:

Java: A Java list `tail` function (for ArrayList, LinkedList, etc.) alvin June 27, 2019 - 1:48pm

As a brief note today, I was working on a Java/Android application recently, and I needed a “tail” function when I was working on a Java list. What I mean by that is that Scala has a `tail` function that returns all elements of the list except for the head element, like this:

``````scala> val x = List(1,2,3,4)
x: List[Int] = List(1, 2, 3, 4)

scala> x.tail
res1: List[Int] = List(2, 3, 4)  //head element removed``````

and I wanted the same thing in Java.

Java: Examples of the Java list/array `subList` method (list subset) alvin June 27, 2019 - 12:26pm

As a little note today, if you ever need to extract a subset of a Java list or array, here are some examples of the Java `subList` method:

CAFEBABE? alvin April 16, 2019 - 7:14pm

And in the category of “Strangest Things I Never Knew About Java,” I give you ... CAFEBABE.

Best RxJava links: map, flatMap, concatMap, and more alvin March 28, 2019 - 2:28pm

As often happens, I have about 50 browser tabs open, and in an effort to close some of those down, these are some of the best links I found while working with RxJava a week or two ago: