Nim for Java programmers

DISCLAIMER!

This page is unofficial and incomplete. Contributions are appreciated.

This guide assumes a good understanding of the Java language and JVM semantics. There might still be something for you if you don't use Java but use another JVM language like Scala, Kotlin, Groovy etc.

General learning resources: https://nim-lang.org/learn.html

Comparison

General

Feature	Java	Nim
Compilation	JVM bytecode	C/C++/Obj-C/JS source
Paradigms	class-based, object-oriented	procedural, compile-time
Metaprogramming	reflection, annotations	macros, templates, generics
Memory management	Garbage collected	Multiple strategies: garbage collected, manual, automatic, traced and untraced pointers

Syntax

Feature	Java	Nim
Comments	// single line, /* multiline */ (nestable)	# single line, #[multiline]# (nestable)
Blocks	Uses curly braces	Uses indents like Python, another option is statement list expression
Operators	Predefined set of symbols, 1+-1 easily parsed as 1 + (-1)	Custom operators supported for a limited set of characters, first character based precedence, strong spaces when using command call syntax and prefix operators
Operator overloading	None	Operators are user defined except = and ., can overload: subscripts, curly subscripts a{b}, experimental call and dot operators
If/else statement	if (a) { foo(); } else { bar(); }	if a: foo() else: bar()
If/else expression	a ? b : c	if a: b else: c
Switch/case expression/statement	switch (a) { case b -> c }	case a of b: c
Return statements	return x;	Multiple options: implicit return at the end of a proc, result variable, explicit return statement
Try/catch/finally	try { foo(); } catch (Exception ex) { bar(); } finally { baz(); }	try: foo() except Exception as ex: bar() finally: bar() - can omit as ex or Exception as ex
Throw exception	throw ex;	raise ex
Procedure definition	static T foo(U a, S b) {} in class body	proc foo(a: U, b: S): T = discard in module
Method definition	T foo(U a, S b) {} in class body	method foo(obj: Obj, a: U, b: S): T = discard in module
Calling procedure	ClassName.foo(a, b)	foo(a, b), foo a, b, a.foo(b), a.foo b
Calling method	obj.foo(a, b)	foo(obj, a, b), foo obj, a, b, obj.foo(a, b), obj.foo a, b
Method/procedure declarations are order-agnostic	Yes	No, can use forward declarations, experimental code reordering
String literals	"str", """str""" as of Java 13	"str", """str""", foo"str" (raw string literals)
Collection literals	new int[] {1, 2, 3}	array: [1, 2, 3], seq: @[1, 2, 3], bitset: {1, 2, 3}, tuple: (1, 2, 3), table constructor
Introduce new lexical scope	{}, label: {}, can't break (?)	block:, block label:, can break with break label
Increment	i++	Uses inc proc, inc i

Language semantics

Feature	Java	Nim
Namespacing & importing	public/private/package-scope modifiers, can import: single classes or all classes from a package, single or all static members from a class	export/* postfix based, can import: exported symbols from modules with import mod, only import specific symbols from a module with from mod import foo, bar, force all symbols from a module to be qualified with from mod import nil, don't import specific symbols from a module with import mod except foo, bar, can alias imports for both symbols and modules with import mod as m. include file is also possible, embeds source files inside modules
FFI with C	JNI	Compiles to C, only needs some pragmas
Immutability	final, getter and setter encapsulation	let, getter and setter encapsulation, enforced by let for value types
Discarding method return values	foo(); is a valid statement if foo() returns a value	foo() is not a valid statement if foo() returns a value, you have to use the discard statement.
Type inference	var type for locals, diamond operator, lambda inference	let/const/var don't need type annotation if initialized, routine generics can be inferred from arguments, auto return type. No inference for object constructor generics or anonymous procs (though generic lambdas are automatically specialized). Empty collections (@[], [], {}) need type annotation
Typeclasses	Interfaces	Experimental concepts, intersection/union types, openarray, range
Multiple inheritance	Interfaces only	No
Method polymorphism	Instance methods are polymorphic on the instance only	Multimethods are polymorphic on each possible argument
Generics	Erased and only involves classes (subject to change with Project Valhalla, uses angle brackets like C++	Similar to C++ templates, uses square brackets like Scala
Variance in generics	? extends T/? super T, generics invariant by default	out T/in T, experimental, generics invariant by default
Type aliases	None	type Foo = Bar
Dependent types	No	Yes
Named, order agnostic parameters	No	Yes
Default parameters	No	Yes
Varargs	Yes	Yes, can also use a transformation proc
Constructor overloading	Yes, public Obj() {}	No, convention is to use newObj() for reference types and initObj() for value types
Pass by reference	No	x: var T
Closures/function references	Classes that extend "functional interfaces" (interfaces that act as function signatures)	Pointers in conjunction with proc types that have calling convention information
Destructors	finalize(), discouraged, try with resources/Closable is preferred	=destroy, withFoo templates/defer: can be used to emulate Closable behaviour
Exceptions	Exception and Error implement Throwable, Error isn't supposed to be caught	CatchableError and Defect extend Exception, Defect isn't supposed to be caught
Exception tracking	Forced explicit checked exceptions with throws	Inferred or explicit with raises pragma
Effect tracking	None	Inferred or explicit with tags pragma
Null checking	Boolean check is x == null. Dereferencing null is NullPointerException, no compile-time mechanisms except Java EE @NotNull, other JVM languages have better null semantics and nullable types, Optional<T>	Boolean check is isNil(x). Dereferencing nil is a segmentation fault with --nilchecks:off, experimental not nil annotation, Option[T] in options module
Bounds checking	Yes, IndexOutOfBoundsException	Yes, IndexError, can be turned off
Overflow checking	No	Yes, OverflowError, can be turned off

Data types

Feature	JVM	Nim
Number types	Signed integers of 8, 16, 32 and 64 bits (8 and 16 bits are actually 32 bits in bytecode), floats of 32 and 64 bits	Unsigned and signed integers with bitsize in type name e.g. int32, int has platform-dependent size, float is always float64, compatibility types like cint available and use C defined types.
Boolean type	boolean, one of true or false, 32 bit integer	bool, enum of true or false, 8 bit integer
Char type	char, 32-bit integer, treated as unsigned 16-bit integer	char, unsigned 8 bit integer, cchar and cuchar, cschar types for compatibilty with C
Enums	Implemented as ordered singleton instances of a class	8 or 16 bit enumerations like C, can be used as array indices
Strings	Immutable wrapper around char[]. Concatenated with +	Similar to seq[char], mutable and growable, compatible with openarray[char]. Concatenated with &, but you can do var x = "abc"; x.add("def"). ref string is essentially StringBuilder
Collection types	Variable-length arrays in the core language, List, Set, Map interfaces in the standard library	array[I, T] where I is a compile time integer or range type, UncheckedArray[T], seq[T], Pascal-style bitset type set[T], slice type with a..b, tuples and named tuples, standard library: tables, sets and more
Structured types	Classes, reference type with extra data	Objects, nominal value type with optional inheritance, have no extra data without inheritance, named tuples have no inheritance but are structurally typed instead of nominal, any type can be made a reference or pointer (traced or untraced pointer)
Tagged unions	No, inheritance is used instead, enums can implement abstract methods/interfaces, Scala/Kotlin use sealed types	Object variants like Pascal and Ada
Pointer access	Incubating foreign memory access API	Yes, dereference operator is foo[], pointer arithmetic needs casting to uint

Documentation, libraries & popular tooling

Feature	Java	Nim
IDE support	Eclipse, Android Studio, IntelliJ IDEA, almost universally supported	Most of the community uses VS Code, see editor support
Package manager / build tool	Ant + Ivy, Maven, Gradle, sbt (Scala), Leiningen (Clojure)	Nimble
Library format	.jar binaries	Source code, unused code is not included in binary (dead code elimination), can also compile to a shared library
Style guide	Last one by Oracle from 1999, Google style guide, Twitter style guide	NEP-1
Doc comments	/// foo, /** line1<newline> * line2<newline> */, HTML formatted	## foo, ##[lines here don't matter]##, RST formatted
Doc generator	javadoc	nim doc
Unit testing	JUnit, Spock	Standard library unittest module