Java enum FAQ: Can you share a Java enum switch example, i.e., how to use an enum with a Java switch statement?
In my earlier Java enum examples tutorial, I demonstrated how to declare a simple Java enum, and then how to use a Java enum with a variety of Java constructs, including a Java switch statement, a for loop, and an if/then statement.
Java exception FAQ: What is a Java NumberFormatException?
Answer: A Java NumberFormatException usually occurs when you try to do something like convert a String to a numeric value, like an int, float, double, long, etc.
The best way to show a NumberFormatException is by example, so here’s an example where I intentionally write bad Java code to throw a NumberFormatException:
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Java exceptions FAQ: How do I create a custom exception in Java?
Here's a quick example of how to create and throw a custom exception class in Java. In this tutorial I'll demonstrate how to:
Java file FAQ: How can I test to see if a file or directory exists in Java (or Scala)?
Solution: Use the Java File.exists method. Here’s an example that shows the basic technique:
File tmpDir = new File("/var/tmp"); // create a File object
boolean exists = tmpDir.exists(); // call its 'exists' method
The exists method of the Java File class returns true if the file or directory exists, and false otherwise.
Here's a quick demonstration of how to create a Tomcat connection pool (database connection pool) using the Tomcat DBCP library.
I'm not going to go into a detailed explanation here of how Tomcat DBCP works, other than to say that it works for me, and I've tried to include everything here that you'll need to implement your own Tomcat DBCP database connection pool in your web applications.
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Summary: This page is a printf formatting cheat sheet. I originally created this cheat sheet for my own purposes, and then thought I would share it here.
A great thing about the printf formatting syntax is that the format specifiers you can use are very similar — if not identical — between different languages, including C, C++, Java, Perl, PHP, Ruby, Scala, and others. This means that your printf knowledge is reusable, which is a good thing.
After writing several recent Model/View/Controller (MVC) pattern articles (A Model View Controller diagram, Model View Controller definitions), I thought it might help to share a real-world implementation of an MVC design. To that end, I’d like to share some information from a Java/Swing GUI application I wrote several years ago to help me in my work with Function Point Analysis (FPA).
Java File I/O FAQ: How do I create a Java temporary file? Also, after I create a temporary file, when is it deleted?
I just ran into this question on the Mac/Java mailing list, so I thought I'd write a quick "Java temporary file" test to explore this. Here are the results.
Java JMX FAQ: Can you share a simple Java JMX example?
Here is some sample Java source code for some JMX tests that I created recently. I got a large percentage of this code from Sun's JMX MBean tutorial when I first started working with JMX, and I think it's some decent "Hello World" starter code. The main thing I've done here is (a) clarify what they've written and (b) added a shell script to start the JMX application. (I'm also about to publish some other JMX source code that I know I wrote myself.)
Java array FAQ: Can you share an example of how to sort a Java String array?
Sure. The following source code shows how to sort an array of strings. In short, you can easily sort a string array with the Arrays class sort method, shown in bold below:
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Java keytool/keystore FAQ: Can you share some Java keytool and keystore command examples?
Sure. As a little bit of background, in creating my "Hyde (Hide Your Mac Desktop)" software application, I decided to venture into the world of commercial software, selling my app for a whopping 99 cents. While that price is trivial, creating the “software licensing” code for this application was anything but trivial.
I finally decided to use a Java licensing tool named TrueLicense to assist with the software licensing, and TrueLicense quickly led me down the path of learning about the Java keytool and keystore path. So that’s what this article is about: How to use the Java keytool command to work with private and public keys, and work with intermediate certificate files.
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Java enum FAQ: Can you share some Java enum examples, such as how to declare a Java enum, and how to use a Java enum in a for loop, if/then statement, and Java switch statement?
Sure. As described in the Sun/Oracle Java documentation, “you should use enum types any time you need to represent a fixed set of constants.” Let's take a look at some enum examples to see how this works.
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Back when I was interviewing for computer programming positions in Boulder and Louisville, Colorado, I found that many interviewers ask questions about Java serialization. After being asked about serialization for the third time, I remembered an old Java deep clone hack that takes advantage of serialization.
Java Fonts FAQ: How do I create a list of all the fonts available on the current platform?
Answer: To list all the fonts available to you in a Java application (a Java Swing application), I use the GraphicsEnvironment.getLocalGraphicsEnvironment().getAvailableFontFamilyNames() method of the GraphicsEnvironment class, which technically returns an array of all the font family names it finds on the local system.
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Java memory FAQ: How do I control the amount of memory my Java program uses (i.e., Java RAM usage)?
The short answer is that you use these java command-line parameters to help control the RAM use of application:
Here’s the source code for a Java “memory eating” program I wrote. Its purpose is to consume all of the memory (RAM) on a PC by allocating 1 MB byte arrays until it runs out of RAM:
import java.util.Vector;
public class MemoryEater
{
public static void main(String[] args)
{
Vector v = new Vector();
while (true)
{
byte b[] = new byte[1048576];
v.add(b);
Runtime rt = Runtime.getRuntime();
System.out.println( "free memory: " + rt.freeMemory() );
}
}
}
It’s also important to run this Java memory eating program with the -Xmx option to make sure your JVM gets all the memory you want it to have:
$ java -Xmx1024M MemoryEater
The example shown above will try to consume up to 1,024 MB RAM (1 GB). As I write about in this How to control Java heap size (memory) allocation (xmx, xms) post, you can also specify the Xmx parameter in gigabytes, such as this setting for one gigabyte:
-Xmx1g or -Xmx1G
I wrote a program similar to this when I started getting the Windows blue screen of death several times. I figured there was some type of hardware failure going on, so I tried to consume all the RAM in the system and then read it back in to see if the hardware failure was related to the RAM.
I was just reminded that if you need to print every element in a Java List, you can use the forEach method on the List:
// [1] create a List of strings. java.util.List<String> listOfStrings = CollectionConverters.asJava(xs); // [2] print the List of strings using forEach and System.out.println. // note that there is no need for a 'for' loop. listOfStrings.forEach(System.out::println);
I can confirm that as of August, 2021, this solution works just fine. So if you ever need to print every element in a Java List — without using a for loop — I hope this example is helpful.