Posts in the “java” category

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 or to display in a user interface?

Solution: Java double formatting

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

Java date FAQ: How do I get the current date (now, 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, and others. This means that your printf knowledge is reusable, which is a good thing.

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.

Java HTTPS client FAQ: Can you share some source code for a Java HTTPS client application?

Sure, here's the source code for an example Java HTTPS client program I just used to download the contents of an HTTPS (SSL) URL. I actually found some of this in a newsgroup a while ago, but I can't find the source today to give them credit, so my apologies for that.

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 exceptions FAQ: How do I create a custom exception in Java?

Solution

The solution is to:

• Create a custom exception class in Java
• Throw the custom Java exception
• In other code, catch the custom exception, and
• Look at the output from our custom exception when we print a stack trace

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:

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 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:

Java array FAQ: Can you share some examples of how to create arrays in Java (Java object arrays)?

While I generally work with lists and maps in Java, I occasionally need to create object arrays in Java. Since I don't use arrays that often, I thought I'd share some examples here so I can have a handy Java array syntax reference.

A simple Java String array

I work with the String class a lot, and here's how to create a String array in Java:

Java String array FAQ: Can you share some Java array examples, specifically some String array examples, as well as the new for loop syntax that was introduced back in Java 5?

Solution

Sure. In this tutorial, I’ll show how to declare, populate, and iterate through Java string arrays, including the for loop syntax that was introduced with Java 5. Because creating a String array is just like creating and using any other Java object array, these examples also work as more generic object array examples.