Posts in the “java” category

A `printf` format reference page (cheat sheet) (C, Java, Scala, etc.)

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: How to square a number


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


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

Java Jar file: How to read a file from a Jar file

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:

A Java email address validation class

I thought I'd share the source code for my Java email address validator class. I'm not sure if there's a big need for it ... I wrote it a long time ago, and I think I created it because Java's javax.mail.internet.InternetAddress class wasn't validating email addresses as deeply as I wanted it to. For instance, I think it would allow the string "fred" to be a valid email address, but on the internet you really want to see something like "". So I think that's where this class comes from.

Java ‘int’ array examples (declaring, initializing, populating)

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:

JDBC Timestamp - How to select a Java Timestamp field from a database timestamp column

Here's a JDBC Timestamp example that shows how to read a Java Timestamp field from a database timestamp column (a MySQL Timestamp field) in a SQL SELECT statement. I pulled this source code out of a real-world Java application, so I'll break it into small steps, and hopefully it will make sense.

The MySQL timestamp field definition

My Java application uses a MySQL database, including one table named commands that has a MySQL Timestamp field I need to read. The SQL definition for this MySQL table is shown below:

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Java Timestamp example: How to create a “current timestamp” (i.e., now)

Java date/time FAQ: When working with the Timestamp class, how do I create a “Java current timestamp”? For instance, how do I create a JDBC Timestamp object to represent the “current time” (“now”)?


You can create a “current time” JDBC Timestamp in just a few lines of code, using the Java Calendar class and a java.util.Date instance, as shown in this example code:

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Java: How to create and throw a custom exception


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

As a solution, here’s a quick example that shows how to create and throw a custom exception class in Java. In this tutorial I'll demonstrate how to:

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

The Java ‘keytool’ command, keystore files, and certificates


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.

Java Iterator Design Pattern examples

Summary: The Iterator Pattern is demonstrated using Java source code examples.

The Iterator Design Pattern is one of the most simple and frequently used design patterns. The Iterator Pattern lets you sequentially move through a collection of objects using a standard interface, and without having to know the internal representation of that collection.

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Chain of Responsibility Pattern in Java

The Chain of Responsibility Pattern is a design pattern whose intent is to avoid coupling the sender of a request to its receivers by giving more than one object a chance to handle a request. The Chain of Responsibility works like this:

Java BufferedReader examples

Java file FAQ: Can you share some examples of the Java BufferedReader class?

When it comes to reading character input streams, the Java BufferedReader class is extremely important, and I'll demonstrate this in several different source code examples.

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