Author: Dmitry Egorov

Multithreading is the most complex part of Java. Happens-before is a relation that gives a guarantee of allowing the writing of predictable code in multithreading a reality. Such code is also known as thread-safe code. Unfortunately, Oracle Java documentation about this notion is hard to read. So in this article, I'll mainly explain what happens-before is in human language and provide detailed examples.

Happens-Before Solves the Main Multithreading Problem

Before we start learning the happens-before notion, we have to understand the reason for creating it. Once multithreading comes to the scene, your code might become inconsistent because shared objects between threads might have different and unpredictable values. Let's review a simple example. In that example, we will update values in one thread and read and print them in another.

What Index Is and How To Use It in Java

An Index is a structure that facilitates search in your data. Is a pretty general term and doesn't belong to some language or database. In order to explain it let's create a way to search users for specific criteria like name, id, birth date, etc.

Quickly Searching Among Millions of User Entries

Let's assume that we have 1 million users. As a requirement, we have to expose API to search users by specific criteria. Obviously, simple iteration will take too much time. So let's sort this problem using indexes (or simply saying by pre-generating some data that accelerate lookup during API calls). So first place let's define our User class and Search interface:

Articles To Read Before We Start

I've already written articles about memory estimation. The first one explains how to calculate memory for objects, arrays, etc. The second one shows tools for how to estimate recursively any object size. Before you begin this article, I strongly recommend you read the first ones. Otherwise, it would be a bit difficult to understand how I calculate the size of objects.

This article will cover the next tricks and topics:

Documentation Sounds Like an Annoying, Boring Thing

Maybe you feel that documenting the project or your code is an extra annoying step. Often, documentation for projects is outdated and irrelevant. In such cases, most of the expertise is hidden in analysts or just developers. In this article I'll try to explain why having a good documentation culture will make projects much more stable and successful.

Writing Good Documentation Often Makes a Big Difference

There is no specific rule on how to make perfect documentation, but obviously plain text without details is much worse than documentation with screenshots, diagrams, and images. It takes time to write a quality manual, but it's worth it because it solves many problems at once, including slow onboarding processes, slow communication, single-person projects, and lack of team support.