What should I know before getting started?

JVM / Class file format

General concepts

A basic understanding of the JVM / class file format is highly reccomended before contributing. Here are some articles that should bring you up to speed:

Terminology

Qualified name: Package separators using the . character. These are names used by runtime functions like Class.forName(name).

For example:

java.lang.String
com.example.MyClass.InnerClass

Internal name: Package separators using the / character. Inner classes specified with the $ character. These are names how classes are specified internally in the class file.

For example:

java/lang/String
com/example/MyClass$InnerClass

Primitives (Not the boxed types) use single characters:

Primitive	Internal
`long`	`J`
`int`	`I`
`short`	`S`
`byte`	`B`
`boolean`	`Z`
`float`	`F`
`double`	`D`
`void`	`V`

Descriptor: Used to describe field and method types. These are essentially the same as internal names, but class names are wrapped in a prefix (L) and suffix character (;).

For example:

Ljava/lang/String;
I (primitives stay the same)

Method descriptors are formatted like so:

double method(int i, String s) = (ILjava/lang/String;)D
void method() = ()V

Arrays are prefixed with a [ for each level of the array.

int[] = [I
String[][] = [[Ljava/lang/String;

Quirks

Wide types: double and long typed variables take up two slots (On the stack and in the local variable table). For example, declaring two doubles in a static method will use slots 0, then 2. Slots 0-3 are all in-use.

Lambdas: The content of a lambda is defined in compiler-generated hidden methods and are invoked with INVOKEDYNAMIC. Decompilers will in-line the code so that it looks more similar to the source representation.