Posts in the “java” category

Java date/time FAQ: How do I get the current date (i.e., now or today) in Java? Also how do I get the current time in Java?

Solution

With Java 8 and newer — i.e., Java 11, 14, 17, etc. — use any of the follow “now” methods on these Java classes to get the current data and time:

Summary: This page is a printf formatting cheat sheet or reference page. I originally created this printf cheat sheet for my own programming purposes, and then thought it might be helpful to share it here.

Many languages, same syntax

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, Python, Perl, PHP, Ruby, Scala, Kotlin, and others. This means that your printf knowledge is reusable, which is a good thing.

Java double FAQ: How do I format Java double and float output to two decimal places, such as when I want to format its output for printing — such as currency — or to display it in a user interface? (Also, how do I do the same thing in Kotlin or Scala?)

Solution

There are at least two ways to round a double or float value to two decimal places in Java:

Java math FAQ: How do I square a number in Java?

Solution

You can square a number in Java in at least two different ways:

1. Multiply the number by itself
2. Call the Math.pow function

Summary: This article provides definitions and descriptions of the Java stack and heap.

I just read a couple of emails about the concepts of a Java stack and heap, and thinking that their descriptions weren’t exactly right, I decided to do a little research. There’s no better source than the source, so directly from Oracle’s Java website, here are definitions for the Java stack and Java heap.

Java/Scala memory FAQ: How do I control the amount of memory my Java program uses (i.e., Java RAM usage)?

Java memory control: Short answer

The short answer is that you use these java command-line parameters to help control the memory (RAM) use of your application:

Java switch case statement FAQ: Can you provide an example of a Java switch/case statement syntax?

Here's a sample Java method that takes an int argument and attempts to turn that int into a month string, using a Java switch case statement to make that decision:

As a quick note to self, I used the following Ant build.xml file in 2014 to build my Wikipedia Reader client application on Mac OS X 10.9 with Java 7.x:

UPDATE: The solution below worked for Java 8, and here’s a solution that works with Java 14 and newer on macOS systems.

In this article I’ll show how to build a macOS application from a Java Jar file. I tested this with Java 1.8 on macOS 10.12.5 (Sierra) on June 29, 2017.

Three assumptions in this process are:

Java Mac application FAQ: Can you share a Java/Mac Ant build script that uses the Jarbundler task to make my Java application look like a native Mac OS X application?

NOTE: This solution is for Mac OS X systems running versions of Java prior to Java 7. If I remember right, it only works on those systems, and therefore only on Mac OS X systems 10.6 and earlier. I'm working on new tutorials for Java 7 and Mac OS X 10.7, 10.8, 10.9, and newer.

UPDATE: The approach below worked with Java 8, and here is a link to the new solution for macOS and Java 14 and newer.

I wrote earlier about how to use the javapackager command to create a macOS application bundle from a Java application, so I won’t repeat all of that information here. Instead, in this article I just want to show how to display an image that’s stored in the Contents/Resources/Java directory of a Mac/Java application bundle.

UPDATE: Please note that the approach shown below worked with Java 8, and here’s a new ‘jpackage’ tutorial for Java 14 and macOS.

I recently learned how to use the Java javapackager command to build a macOS application bundle — i.e., a regular macOS application — from a Java application. In this tutorial I’ll show how to create a Mac application bundle from a simple Java class, in this case a Java Swing class.

I ended up not using this code, but if you wanted to see one way to use JSoup’s OutputSettings (Document.OutputSettings) class to set some parameters before calling JSoup.clean, I hope this is helpful:

// tried some things to improve the html output
val settings: OutputSettings = new OutputSettings
settings.prettyPrint(true)  //`true` is default
settings.charset("UTF-8")
settings.outline(true)  //this is close to what i want, but too extreme
settings.indentAmount(4)
val cleanHtml: String = Jsoup.clean(html, "", wl, settings)

I can attest that this code works, it’s just not what I need at the moment.

Also, the code shown is written in Scala, but as you can see, it converts easily to Java.

Java jar file reading FAQ: Can you show me how a Java application can read a file from own of its own Jar files?

Here's an example of some Java code I'm using to read a file (a text file) from a Java Jar file. This is useful any time you pack files and other resources into Jar files to distribute your Java application.

Java - read Jar file example #1

The source code to read a file from a Java Jar file uses the getClass and getResourceAsStream methods:

Java socket FAQ: How do I set the timeout on a Java socket? That is, when I'm trying to read data from a Java socket, and I'm not getting any response from the server, how do I make sure my code doesn't hang up? (It needs to time out after several seconds.)

Java socket timeout

Answer: Just set the SO_TIMEOUT on your Java Socket, as shown in the following sample code:

Here's the source code for a simple JOptionPane showMessageDialog example, where I use a JTextArea inside of a JScrollPane to show a long text message in the showMessageDialog:

As a quick Java tip related to lists, 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.

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:

Java array FAQ: How do you create an array of Java int values (i.e., a Java “int array”)?

Answer: There are several ways to define an int array in Java; let’s take a look at a few examples.

1) Declare a Java int array with initial size; populate it later

If you know the desired size of your array, an you'll be adding elements to your array some time later in your code, you can define a Java int array using this syntax:

I’ve been working with the Java JOptionPane showMessageDialog a lot lately, so I thought I’d create a page here with a number of showMessageDialog examples, sort of a JOptionPane reference page.

I’ll walk you through some Java JOptionPane examples here, starting with a simple example and then increasing the level of difficulty as I go on.