java

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

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.)

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.