Posts in the “java” category

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.

Square a number by multiplying it by itself

Here’s how to square a number by multiplying it by itself:

i = 2
int square = i * i

In that case, if you print the value of square, it will be 4.

Java String array FAQ: Can you share some Java array examples, specifically some String array examples, as well as the Java 5 for loop syntax?

Sure. In this tutorial, we'll show how to declare, populate, and iterate through a String array, including the Java 5 for loop syntax. Because creating a String array is just like creating and using any other Java object array, these examples can also work as more generic object array examples.

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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.

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

For a long time I appended Java strings together using the + operator to combine them into the output format I wanted, but I always knew that approach was pretty ugly. Fortunately at some point I learned how to use the Java String format method to format my text, and as you’ll see, the resulting code is much cleaner this way.

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 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.

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

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).

Today I needed a Java method to remove all the blank characters from a String. I started to write some code with a StringBuffer or a StringBuilder, then thought there must be some other way to do this. It’s not that the StringBuilder/StringBuffer approach is hard, but just that someone must have already solved this problem.

The most common reason to use a StringBuffer instead of a String in your Java programs is when you need to make a lot of changes to a string of characters. For instance, a lot of times you’ll be creating a string of characters that is growing, as shown in this sample example program:

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

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

Java memory FAQ: How do I control the amount of memory my Java program uses (i.e., Java RAM usage)?

One of the Linux servers that I use is a little starved for memory, but I need to run a Java program on it periodically to run some utility tasks. However, every time I try to run the program I get this Java heap size error message:

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.

While interviewing for computer programming positions in Boulder, Colorado, I found that most interviewers ask questions about Java serialization. After being asked about serialization for the third time recently, I suddenly remembered an old Java deep clone hack that takes advantage of serialization.

The basic idea is this: