Posts in the “java” category

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

A Java "extract method" refactoring example

Summary: A Java "extract method" refactoring example is demonstrated.

If you don't own a copy of Martin Fowler's Refactoring book, I highly recommend it. The basic idea of refactoring source code is that the code "smells" in one way or another, and there are a variety of ways to improve smelly code. More specifically, Mr. Fowler describes refactoring as this:

A simple RxJava 2 “Hello, world” example

As a brief note, and assuming that you already know a little bit about RxJava, here’s the simplest possible RxJava 2 “Hello, world” example I think you can create:

package hello;

import io.reactivex.Observable;

public class HelloWorld {

    public static void main(String[] args) {
        Observable<String> observable = Observable.just("Hello, world");


A Java method that returns a random boolean value based on a probability

If you ever need a Java method that returns a boolean value based on a given probability, I can confirm that this method works:

 * `probability` should be given as a percentage, such as
 * 10.0 (10.0%) or 25.5 (25.5%). As an example, if `probability` 
 * is 60% (60.0), 100 calls to this function should return ~60 
 * `true` values.
 * (Note that Math.random returns a value >= 0.0 and < 1.0.)
static boolean getRandomBoolean(float probability) {
    double randomValue = Math.random()*100;  //0.0 to 99.9
    return randomValue <= probability;

Creating and populating a Java ArrayList (Java 9 and newer)

If you want to create and populate a Java ArrayList with Java 9, 11, and newer, you can use this syntax:

List<> ints = ArrayList<Integer>(List.of(1,2,3));

As shown, this uses the usual ArrayList constructor and the Java List.of method. Once you have an ArrayList like this you can continue to add new elements to it as usual:


Java: How to print elements in a List (without using a 'for' loop)

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.

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.

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 cheat sheet for my own programming purposes, and then thought I would share it here.

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, Perl, PHP, Ruby, Scala, and others. This means that your printf knowledge is reusable, which is a good thing.

How to control Java heap size (memory) allocation (xmx, xms)


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

Java RAM: Short answer

The short answer is that you use these java command-line parameters to help control the RAM use of application:

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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A Java deep clone (deep copy) example


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 5 for loop syntax example

Java 5 FAQ: Can you share some examples of the Java 5 for loop syntax?

Sure. As a bit of background, in the days before Java 5 you could create a for loop to iterate over a collection of strings like this:

// assumes there is a method named "getList()"
List list = getList();

for (Iterator it = list.iterator(); it.hasNext();) {
  String value=(String);

Java 5 for loop syntax

That’s not too bad, but with the release of Java 5 your for loops can now be a little tighter, like this:

Java JFrame size: How to set the JFrame size

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.