Posts in the “java” category

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:

Java Robot class FAQ: Can you show me an example of how to use the Java Robot class?

Answer: Um, yeah, sure ... I say that a little jokingly. Okay, what really happened is that while developing this Java Robot example code on my Mac, I had to reboot it about 10 times. When you use the Java Robot class, you're poking your head out into the native operating system, and if you mess up with your GUI events -- at least on a Mac OS X system -- a lot of bad things can happen.

Java String FAQ: How do I merge/combine two Java String fields?

Solution

NOTE: If performance is an important consideration, use the StringBuffer approach I show below.

You can merge/concatenate/combine two Java String fields using the + operator, as shown in this example code:

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 RAM use of your application:

Summary: This tutorial demonstrates how to use the Java SimpleDateFormat class to convert a Java Date to a formatted String.

It's been so long since I create a two-dimensional Java array in I almost couldn't remember how to do it. Fortunately I did, and I thought I'd include my sample code here in case it will help anyone else.

So, in this quick tutorial, I'll show you how to create a two-dimensional array (2D array) of Java String objects, and then I'll show you how to access each element in the array.

How to create a two-dimensional Java array

First, here's how I create the two-dimensional arrays of strings with this code:

While working with various "Java instanceof" tests recently, my curiosity was piqued, and I thought I'd take a look at how the instanceof operator works when testing against a Java array.

Scala FAQ: How do I use the Scala varargs syntax (or, What is the Scala varargs syntax)?

You can define a Scala function that accepts a varargs parameter like this:

def printAll(strings: String*) {
    strings.map(println)
}

The only magic in using the varargs syntax is adding the * symbol after the String declaration, as highlighted in this line of code in the function declaration:

Java file writing FAQ: How do I append text to the end of a text file in Java?

The short answer is that you should create a FileWriter instance with the append flag set to true, like this:

BufferedWriter bw = new BufferedWriter(new FileWriter("checkbook.dat", true));

The rest of this article explains this.

Summary: This is a Java/MySQL SQL SELECT example, demonstrating how to issue a SQL SELECT command from your Java source code, using a MySQL database.

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 enumerations 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.

Java String formatting FAQ: How can I format Java String output?

Java String formatting solution

One way to format Java string output is with the format method of the String class, which works like a “Java sprintf” method. Here are some examples:

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

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 "fred@foo.bar". So I think that's where this class comes from.

Summary: This article shares the source code for a simple Java Socket client class. This article demonstrates both (a) how to write to a Java socket, and (b) how to read from a Java socket.

Summary: This is a Java SimpleDateFormat (date formatting) example, showing how to convert a String to a Date.

In an earlier example I showed how to use the Java SimpleDateFormat class to convert from a Date to a String, but you can also use the SimpleDateFormat class to convert in the opposite direction, from a given Java String to a Date object.

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:

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”)?

Solution

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:

Spring JDBC/Dao FAQ: Can you share some Spring JDBC examples, specifically SELECT query examples using Spring Dao objects?