Java

What javac produces, what java consumes, and why one file runs on every machine.

What a variable actually holds, and how to choose the type that holds it.

What the symbols do when the types are not the ones you assumed.

Deciding once and repeating without copying, and the boundaries that go wrong.

Packaging behaviour under a name, and what a method can and cannot change for its caller.

A fixed run of values under one name, and the index that throws.

Text that never changes, and what that costs when you build it in a loop.

Turning a description into instances, and what this and static each belong to.

What a subclass gets from its parent, and the order in which things happen.

One call, several bodies, and which one the JVM picks.

Naming a capability without saying how it is met.

Keeping state where only its owner can reach it.

Failing loudly, recovering deliberately, and swallowing neither.

Where a value lives, what keeps it alive, and what the two memory errors mean.

The three methods every object inherits, and what breaks when two of them disagree.