Creating an Amazon Beanstalk wrapper around a Kinesis application in Java


Suppose that you have a Java Amazon Kinesis application which handles messages from the Amazon message queue handler Kinesis. This means that you have a method that starts a Worker object in the library.

If you are developing an application that is meant to process messages from Amazon Kinesis and you don’t know what I mean by a Kinesis app then check out the Amazon documentation on the Kinesis Client Library (KCL) here.

The starting point for this post is that you have a KCL application and want to host it somewhere. One possibility is to deploy it on Amazon Elastic Beanstalk. You cannot simply deploy a KCL application as it is. You’ll need to wrap it within a special Kinesis Beanstalk worker wrapper.

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How to calculate the message digest in Java

A message digest is an important concept in cryptography. A digest is an array of bytes created by a hashing formula. It is used to make sure that some digital information has not been tampered with. In a sense it is a footprint of an object, such as a file. If someone modifies the file then the footprint also changes. Then we know that the file has been changed. Another word for a message digest is checksum. There are various hashing algorithms to perform the calculation. SHA-256 and MD5 are the most common ones.

For an example you can check out the Apacha log4j2 download page here. You’ll see a column called “checksum” for various files. If you click on one of those you’ll see the MD5 hash of the file in a relatively human readable form, such as “31826c19fff94790957d798cb1caf29a”.

Java and other popular programming languages have built-in classes to construct a message digest. Let’s see an example from Java.

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Getting a result from a parallel task in Java using CompletableFuture

In this post we saw how to start several processes on different threads using the CompletableFuture class. The example concentrated on methods with no return value. We let CompletableFuture finish the tasks in parallel before continuing with another process.

In this post we’ll see a usage of CompletableFuture for functions with a return value. We’ll reuse several elements we saw in the post that concentrated on the Future class.

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Time zones in Java 8 Date and Time API


I know for a fact that all programmers love working with time zones. Chances are high that you, as a reader of this blog, are also a programmer so I bet you also just love time zones. Let’s see what Java 8 offers as far as time zones are concerned.

Time zones

So far in this series on the date and time in Java 8 we always worked with the local time zone found on your computer. All date-related classes, such as LocalTime or LocalDateTime allow you to easily set the time zone. Here’s an example with the LocalDateTime class:

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Conditionally remove elements from a List in Java 8

Java 8 introduces a new method available for Collection types: removeif(). It accepts a predicate which defines the condition on which the elements should be removed. It returns a boolean where a true response means that at least one item has been removed and false otherwise:

Collection<String> stringStack = new Stack<>();
stringStack.removeIf(s -> s.contains("ll"));

The above example will remove “Hello” from the list stack.

Note that not all collections support item removal. In that case the method will throw an UnsupportedOperationException in case an attempt is made to remove a matching element. The ArrayList is one such collection:

Collection<String> asList = Arrays.asList("hello", "my", "dear", "world");
asList.removeIf(s -> s.contains("ll"));

This will throw an exception unfortunately as the Array.asList method returns an ArrayList of type java.util.Arrays.ArrayList (which is read only and fixed size) and not the classic java.util.ArrayList (resizable and item-removable) – based on a comment by Juanito below.

View all posts related to Java here.

Waiting for background tasks to finish using the CompletableFuture class in Java


In this post we saw how to wait for a number background tasks to finish using the CountDownLatch class. The starting point for the discussion was the following situation:

Imagine that you execute a number of long running methods. Also, let’s say that the very last time consuming process depends on the previous processes, let’s call them prerequisites. The dependence is “sequential” meaning that the final stage should only run if the prerequisites have all completed and returned. The first implementation may very well be sequential where the long running methods are called one after the other and each of them blocks the main thread.

However, in case the prerequisites can be executed independently then there’s a much better solution: we can execute them in parallel instead. Independence in this case means that prerequisite A doesn’t need any return value from prerequisite B in which case parallel execution of A and B is not an option.

In this post we’ll look at an alternative solution using the CompletableFuture class. It is way more versatile than CountDownLatch which is really only sort of like a simple lock object. CompletableFuture offers a wide range of possibilities to organise your threads with a fluent API. Here we’ll start off easy with a simple application of this class.

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Lambda expressions in Java


If you’re familiar with .NET then you already know what Lambda expressions are and how useful they can be. They were not available in Java before version 8. Let’s investigate how they can be applied in Java.

First example: an interface method with a single parameter

Say you have the following Employee class:

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