Java Notes

A summary of the syntax, and the one thing about Java's version that trips everyone.

Two classes do the work, and three String methods hide them for simple cases.

The single most common mistake in beginner Java, and the reason it is so persistent.

Strings are immutable, so every concatenation makes a new one.

Almost always one line.

One method, and three behaviours that surprise people.

Two indices walking toward each other. The whole exercise is where they stop.

Comparable is how a class sorts itself. Comparator is every other order you might want.

Find the smallest, put it first, repeat. The simplest sort to reason about.

Use the library. It is better than what you would write, for reasons worth knowing.

Java has no true 2D array. It has arrays whose elements are arrays.

Fixed length, decided when you create it, and never changing.

A growable array. The default choice for a list, and usually the right one.

== asks whether two variables point at the same thing. Usually you meant something else.

Operating on individual bits. Rarer than it used to be, still occasionally exactly right.

Integers with no size limit, at the cost of every operation being a method call.

Optional for a single statement. Use them anyway.

A separate channel for reporting that something went wrong, so the normal path stays readable.

Three expressions, any of which may be empty.

Use it whenever you can, which is most of the time.

A method that belongs to the class rather than to any object of it.

Two forms now: the old one with break, and the arrow one without.

A shorthand for names. It does not load or include anything.

The parent is always constructed first, whether or not you say so.

Runs once, when the object is created, to put it into a valid state.

A bug found at its cause is cheap. The same bug found three layers away is not.

How the cost grows as the input grows. Not how long anything takes.

Pseudo-random: deterministic given the seed, which is a feature.

Halve the range each step. Requires sorted data, and that requirement is the whole cost.

Windows uses two characters, Unix one. Mostly you should let the library decide.

A map of strings the JVM keeps about its environment.

Three classes, two of them called Timer, and the difference matters.

Use java.time. The older classes are still there and should not be.

The obvious approach is the one that does not work, for a reason worth understanding.

A handful of arrangements that turn up in every desktop application.

A menu bar, menus, and items. Three classes and a couple of conventions.

Setting coordinates by hand. Tempting, and wrong in four specific ways.

Building a Java interface is not hard. Building one that is easy to maintain and does not look dated is the hard part, and several approaches try to solve it.

Move the state of the screen out of the screen.

Keep what the program knows separate from how it is shown.

Downward by method call, upward by event. Never the other way round.

Enough of an application to meet every part of a Swing program at once.

Event-driven drawing in Swing: mouse listeners, paintComponent, and a BufferedImage that makes the picture persist.