Posts in the “java” category

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 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.

This morning when I saw some Java JFrame code on a mailing list, it made me think that I needed to put a simple JFrame example out here, something that would show how to properly construct and display a JFrame without getting into a discussion of anything else. Here are two examples that show the correct technique.

1) A simple Java JFrame example

To that end, here is the source code for a simple "JFrame example" demo class. This example shows how to construct a JFrame, and make sure it's properly displayed using the SwingUtilities invokeLater method:

As a quick note, here are some examples of the Java 8 lambda Thread and Runnable syntax. As a little bonus I also show the Java lambda syntax in other situations, such as with an ActionListener, and several “handler” examples, including when a lambda has multiple parameters.

The Java Thread/Runnable lambda syntax

First, here’s the lambda syntax for a Runnable that was introduced with Java 8, and now works with Java 11, Java 14, Java 17, etc., where I create a Runnable and pass it to a Thread:

Runnable runnable = () -> { 
    // your code here ...
};
Thread t = new Thread(runnable);
t.start();

And here’s the Java Thread lambda syntax (without a Runnable):

Thread t = new Thread(() -> {
    // your code here ...
});

You can also use this lambda approach to create a Java Thread, without creating a reference (variable) to the thread:

new Thread(() -> // your code here).start();

Note: There’s an interesting approach documented here:

def run2() = {println("hi2")}
new Thread(() => run2).start

The older Thread and Runnable syntax

If you can’t use Java 8+ lambdas — or don’t want to — here’s the pre-lambda thread syntax using a Runnable:

// pre java 8 lambdas
Thread t = new Thread(new Runnable() {
    public void run() {
        // your code here ...
    }
});

t.start();

Here’s the old Thread syntax, using the anonymous class approach:

Thread thread = new Thread() {
    public void run() {
        // your code here
    }
}

thread.start();

You can also create a class to extend a Thread and then run it, like this:

public class MyThread extends Thread {
    public void run() {
        // your code here
    }
}

MyThread myThread = new MyThread();
myTread.start();

ActionListener actionListener = new ActionListener() {
    public void actionPerformed(ActionEvent actionEvent) {
        handleMakeTheImageLargerAction();
}};

ActionListener actionListener = actionEvent -> handleMakeTheImageLargerAction();

desktop.setAboutHandler(e ->
    JOptionPane.showMessageDialog(null, "About dialog")
);
desktop.setPreferencesHandler(e ->
    JOptionPane.showMessageDialog(null, "Preferences dialog")
);
desktop.setQuitHandler((e,r) -> {
        JOptionPane.showMessageDialog(null, "Quit dialog");
        System.exit(0);
    }
);

JFrame title FAQ: How do I set the title of a Java JFrame?

Solution

There are two ways to set the JFrame title. First, you can set the JFrame title when you construct your JFrame, like this:

Java FAQ: How do I initialize/populate a static List (ArrayList, LinkedList) in Java?

Java 9

When you’re using Java 9 and newer — Java 11, 14, 17, etc. — you can use this concise syntax to populate a static List:

Java String array FAQ: Can you share an example of how to determine the largest String in a Java String array?

Sure, in this tutorial I'll share the source code for a complete Java class with a method that demonstrates how to find the longest String in a Java string array.

Finding the longest string in a Java string array

Here's the source code that shows how to find the longest string in a Java String array: