The labs on this site need JavaScript to run. The same content is written out below.

Fundamentals

Before looking at how a concept behaves in detail, these labs show what the concept looks like in code and what happens without it.

Java

Variables and types — What is a variable, and what does a type decide?
Methods — What is a method, and how is declaring it different from calling it?
Classes and objects — How is a class different from an object?
References — What does it mean to store an object in a variable?
null — What is null, and why does NullPointerException happen?
Constructors — What is a constructor, and when does it run?
static — What is static, and what changes when you add it?
Inheritance — What does a class inherit through extends?
Interfaces — What does an interface look like in code, and what does it decide?
Exceptions — What is an exception, and what does try-catch do?
Generics — What does the <String> in List<String> do?
List and Map — What do List and Map hold, and how do you get things out?
Lambdas — What is a lambda a shorter way to write?
Streams — What is a stream, and how is it different from a for loop?
Annotations — What is an annotation, and what happens when you add one?
Threads — What is a thread, and what does start() do?
Locks (synchronized) — What is a lock, and what happens without one?
Thread pools — What is a thread pool, and how is it different from creating a thread every time?
Compilation and the JVM — How does Java code run?
File I/O — What do you exchange when you write to and read from a file?
Libraries and dependencies — What is a library, and what happens when it is missing?

Beans — What is a bean, and how does it differ from an object you just created?
Dependency injection — What does dependency injection do for you?
AOP and proxies — What is a proxy, and how does Spring step in before and after my method?
Servlets — Which class is called a servlet?
Controllers — Which class is called a controller?
Configuration files — What is a configuration file, and how is it different from writing values in the code?
JDBC — How does Java code send SQL to the database?
@Transactional — What does @Transactional group together?
Spring Boot — What does Spring Boot do for you?
Test code — What is test code, and who decides whether a test passes or fails?

Entities — What is an entity, and how does it differ from an ordinary class?
Persistence context — What is a persistence context, and what does it do for you?
Repositories — What is a repository, and who creates the implementation?
Associations — What is an association, and what does @ManyToOne do for you?
Lazy fetching — When is an associated object read from the database?
JPQL — What is JPQL, and how is it different from SQL?

Tables, rows, and columns — What does a table look like, and what are rows and columns?
Primary keys — What is a primary key, and what changes without one?
Foreign keys — What is a foreign key, and what changes without one?
WHERE — What does WHERE decide, and what happens without it?
Joins — What does a join attach, and what does ON decide?
GROUP BY and aggregation — What does GROUP BY group, and what do COUNT and SUM count?
Indexes — What is an index, and what changes when there is none?
Transactions — What does a transaction group together, and what do COMMIT and ROLLBACK do?
Row locks — What happens when two transactions try to change the same row?
Storage and durability — Does committed data stay after the server is turned off?

IP addresses and ports — What do an IP address and a port each point to?
TCP connections — What does it mean to open a connection, and what happens if you send without one?
URLs — What parts is a URL made of?
DNS — What does DNS turn a name into?
HTTP requests — What does an HTTP request look like as text?
HTTP methods — How is GET different from POST?
HTTP responses — What does an HTTP response look like as text?
Cookies — What is a cookie, and who stores it and who sends it?
JSON — What does JSON look like, and what mistakes make it unreadable?
HTTPS — How is HTTPS different from HTTP?
Proxy servers — What is a proxy server, and what changes when it sits in between?

Authentication and authorization — How are authentication and authorization different?
Sessions — What is a session, and where is the logged-in state remembered?
Tokens — What is a token, and what does the server look at to trust it?
Hashing — What is a hash, and why do we store passwords as hashes?
Encryption — What is encryption, and what happens without the key?
Origins — What is an origin, and why does the browser tell different origins apart?
Input and injection — What happens when text typed by a user is read as code?

Processes — What is a process, and what is separate when you run a program twice?
Calling another server — How is a server calling another server different from calling a method?
Load balancers — If you run several servers, which server does a request go to?
Caches — What is a cache, and what do you gain and lose by using one?
Message queues — What is a message queue, and how is it different from calling a server directly?
Deployment — When does a running server change after you edit the code?
Git commits and branches — What is a commit, and what does a branch point to?

Predict the result before running