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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Overriding explicit and implicit conversion in C# .NET

Custom implicit and explicit conversions for numeric types can be defined in C# quite easily. You need to be aware of the “implicit”, “explicit” and “operator” keywords.

Consider the following class:

public class Measurement
	public int Value { get; set; }

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Implementing an indexer for your object with C# .NET

Indexers are used extensively when accessing items in an array or List:

Friend f = friends[2];

It’s fairly easy to implement your own indexer. Imagine a table with guests sitting around. We could implement an indexer to easily access guest #n.

The Guest object is simple with only one property:

public class Guest
	public string Name { get; set; }

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How to hide the text entered in a .NET console application

You’ve probably encountered console applications that ask for a password. It’s very likely that the password will stay hidden otherwise other people viewing your screen can easily read it.

This short post will present a possible solution on how to achieve a hidden string input in a .NET console application.

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Getting notified when collection changes with ObservableCollection in C# .NET

Imagine that you’d like to be notified when something is changed in a collection, e.g. an item is added or removed. One possible solution is to use the built-in .NET generic collection type ObservableCollection of T which is located in the System.Collections.ObjectModel namespace. The ObservableCollection object has an event called CollectionChanged. You can hook up an event handler to be notified of the changes.

If you don’t know what events, event handlers and delegates mean then start here.

Let’s see a simple example with a collection of strings:

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Customise your list by overriding Collection of T with C# .NET

Imagine that you’d like to build a list type of collection where you want to restrict the insertion and/or deletion of items in some way. Let’s say we need an integer list with the following rules:

  • The allowed range of integers is between 0 and 10 inclusive
  • A user should not be able to remove an item at index 0
  • A user should not be able to remove all items at once

One possible solution is to derive from the Collection of T class. The generic Collection of T class in the System.Collections.ObjectModel namespace provides virtual methods that you can override in your custom collection.

The virtual InsertItem and SetItem methods are necessary to control the behaviour of the Collection.Add and the way items can be modified through an indexer:

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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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