Posts in the “java” category

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

Solution: There are several different ways to set the size of a Java JFrame, but I generally use the setPreferredSize method of the JFrame class in combination with the Java Dimension class, like this:

jframe.setPreferredSize(new Dimension(400, 300));

A JFrame size example

Here's the source code for a complete "JFrame size" example, showing how to use this setPreferredSize method call in an actual Java program.

Java File I/O FAQ: How do I open a file and read text from it using Java? (Also written as, “How do I use the Java BufferedReader and FileReader classes to read from a text file?”)

Solution: Here’s a method taken from a Java class I wrote that shows how to open and read a file using the Java FileReader class.

In my previous tutorial on using the Java String format method ("Java sprintf") I showed how to use the format method of the Java String class to format strings and text output. That method works fine in many situations where you use the sprintf function in other languages, such as when you need to either concatenate strings, or print formatted output using something like Log4J.

Java file-reading FAQ: Can you share some off-the-shelf Java methods I can use to open and read files in Java?

Sure. Taken directly from my Java file utilities article (Java file utilities to open, read, write, and copy files), here is the source code for two Java methods that let you read a text file.

Java enum FAQ: Can you share a Java enum switch example, i.e., how to use an enum with a Java switch statement?

In my earlier Java enum examples tutorial, I demonstrated how to declare a simple Java enum, and then how to use a Java enum with a variety of Java constructs, including a Java switch statement, a for loop, and an if/then statement.

Here's a quick demonstration of how to create a Tomcat connection pool (database connection pool) using the Tomcat DBCP library.

I'm not going to go into a detailed explanation here of how Tomcat DBCP works, other than to say that it works for me, and I've tried to include everything here that you'll need to implement your own Tomcat DBCP database connection pool in your web applications.

Java regex Problem: In a Java program, you want to determine whether a String contains a case-insensitive pattern. The pattern can be a simple String, or a more complicated regular expression (regex).

Java File I/O FAQ: How do I create a Java temporary file? Also, after I create a temporary file, when is it deleted?

I just ran into this question on the Mac/Java mailing list, so I thought I'd write a quick "Java temporary file" test to explore this. Here are the results.

Java JMX FAQ: Can you share a simple Java JMX example?

Here is some sample Java source code for some JMX tests that I created recently. I got a large percentage of this code from Sun's JMX MBean tutorial when I first started working with JMX, and I think it's some decent "Hello World" starter code. The main thing I've done here is (a) clarify what they've written and (b) added a shell script to start the JMX application. (I'm also about to publish some other JMX source code that I know I wrote myself.)

Java Fonts FAQ: How do I create a list of all the fonts available on the current platform?

Answer: To list all the fonts available to you in a Java application (a Java Swing application), I use the GraphicsEnvironment.getLocalGraphicsEnvironment().getAvailableFontFamilyNames() method of the GraphicsEnvironment class, which technically returns an array of all the font family names it finds on the local system.

Here’s the source code for a Java “memory eating” program I wrote. Its purpose is to consume all of the memory (RAM) on a PC by allocating 1 MB byte arrays until it runs out of RAM:

import java.util.Vector;

public class MemoryEater
{
  public static void main(String[] args)
  {
    Vector v = new Vector();
    while (true)
    {
      byte b[] = new byte[1048576];
      v.add(b);
      Runtime rt = Runtime.getRuntime();
      System.out.println( "free memory: " + rt.freeMemory() );
    }
  }
}

It’s also important to run this Java memory eating program with the -Xmx option to make sure your JVM gets all the memory you want it to have:

$ java -Xmx1024M MemoryEater

The example shown above will try to consume up to 1,024 MB RAM (1 GB). As I write about in this How to control Java heap size (memory) allocation (xmx, xms) post, you can also specify the Xmx parameter in gigabytes, such as this setting for one gigabyte:

-Xmx1g or -Xmx1G

I wrote a program similar to this when I started getting the Windows blue screen of death several times. I figured there was some type of hardware failure going on, so I tried to consume all the RAM in the system and then read it back in to see if the hardware failure was related to the RAM.

Way back in the late 1990s, I wrote a Java CRUD generator, which was based on the work of someone else. It was a static code generator, but like future dynamic frameworks like Ruby on Rails, CakePHP, and others, it scanned the database and generated source code from the database table definitions.

Java FAQ: How do I populate (initialize) a static HashMap in Java?

In Java 9 and newer you can create and populate a Map like this:

As I started to mention in another blog post, your Java code will be more flexible when you learn to return more-general object references. In most cases other developers only need to see your interface, not your implementation. Put another way, does it matter to anyone else if you used a LinkedList or an ArrayList? If it doesn't matter, then return a List, or perhaps even a Collection.

I just needed to add a manifest file to a Java JAR file, and this command worked for me:

Summary: A simple Java JScrollPane source code example.

When you're first learning how to use Java Swing components, like a JScrollPane it can be a little hard to figure out how to get started. To that end I am trying to share several complete Java source code examples that demonstrate how to work with components like these.

Java JFrame FAQ: How do I properly create and display a JFrame? While you're at it, how do I center a JFrame?

In this Java tutorial I'll demonstrate how to create and display a JFrame Other than the comments in the source code, I'm going to keep the code as simple as possible, so I can demonstrate how this works.

I've been getting back into the Java/Swing world during the last few days, and I thought I'd share some example code here. In my current application, one of the things I just worked on was setting the initial JFrame size to a value I liked.

Here's a quick line of Java code that takes a given input string, strips all the characters from that string other than lowercase and uppercase letters, and returns whatever is left:

Problem: In a Java program, you want to determine whether a String contains a certain pattern. The pattern can be a simple String, or a more complicated regular expression (regex).