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bastie/JavApi4Swift 0.6.0
JavApi⁴Swift is a pure implementation of Java API in Swift under business friendly license.
⭐️ 1
🕓 2 weeks ago
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.package(url: "https://github.com/bastie/JavApi4Swift.git", from: "0.6.0")


This project provides a Java like API in 100%-pure Swift instead bridging to Java virtual machine. In result you do not need a Java virtual machine. On the otherside you can port Java code to Swift without Big-Bang step by step.

This project use MinD pattern to reduce dependency and eat other great resources.


This project use only business friendly permissive licenses.

Image: David A. Wheeler, et al., CC BY-SA 3.0 , via Wikimedia Commons

By default the MIT License is used. Parts of source code use the BSD Licenses and the Public Domain / The Unlicense.


Special thanks to contributors of


When working with XCode add dependency


When working with SwiftPM add dependency

.package(url: "https://github.com/bastie/JavApi4Swift.git", from: "0.6.0")


.Package(url: "https://github.com/bastie/JavApi4Swift.git", .upToNextMajor(from: "0.6.0"))


I'm implementing some porting projects to check JavApi⁴Swift releases and look for the next missing features. The use could start from version 0.4.2. Check out these projects and learn how to use JavApi⁴Swift.


  • ASCII-Data is a library to display tables and graphs on command line as ASCII or ANSI.


include other projects

To honor the work of the developer of other project let the history come inside.

To include other project with compatible license do

# Example for some types from a jzlib clone
# clone the other project local, jzlib clone are here https://github.com/kohsuke/jzlib.git
git clone https://github.com/kohsuke/jzlib.git otherProject

# go into project directory and remove the origin
cd otherProject
git remote rm origin

# filter project to remove all unwanted data and commit it
git filter-branch --subdirectory-filter src/main/java/com/jcraft/jzlib -- --all
git rm Deflat*.java GZIP*.java Inf*.java Z*.java Tree.java StaticTree.java JZl*.java
git commit -m "ready to import"

# go to JavApi project directory
cd ../JavApi4Swift

# add (temporary) the local other project as remote source and pull wanted data with history
git remote add importSource ../otherProject
git pull importSource master --no-rebase --allow-unrelated-histories
git remote rm importSource

# work on conflicts f.e. in .gitignore

# optional take a look into history
git log

JavApi style guide

  1. The ported Java source code should not to modify more than needed.
  2. I like short lines and so indent is 2 spaces.
  3. Types are in same name files like Java with the exception of java.lang types.
  4. The opend curly braces are in the same line.
  5. packages are mapped over enums in files named as java.basepackagename.packagename.swift in result of compiler problems with more than one identical files names.
  6. Exported parameters of Java translated, non-private function parameter are marked with underline.
  7. Exceptions are mapped over extension of Error enumeration.
  8. Default methods are in extensions implemented.
  9. Swiftify code are implemented in extensions.
  10. All previous points are non-binding recommendations with no binding effect.

How to translate Java source code to Swift

abstract classes

Abstract classes are implemented as interface with default methods.


The Java length property of an array is mapped over a readonly computed Swift property with result of Swift count property.


Instead of assert use the guard with function fatalError.

blocks of statements

In Java blocks are between curly braces {}. Swift need the do keyword before the open brace.

byte type

Swift use UInt8 instead byte but typealias are exported.


To cast in Swift use as! keyword. If in Java is first check with instanceof you should use as?.


Not often but here Swift need explicite declaration and double quotes, for example

let chars : [Character] : "a string to char array".toCharArray()
let char : Character = "c"

In extension you can compare Character with Int value with ==. If you need the Int value of character use asDigit() function.

do while loop

Instead of do use keyword repeat.

exception handling

In Java exception can seperated in "RuntimeException" without explicit naming and "the other exception". The other exception must be declared and catched. RuntimeException and the super type Throwable can be declared and catched - or use public static void main [] throws Throwable {}. Java use a try block with catch NamedException instead to Swift with "normal" do block with catch. Swift also provide the error enum inside the catch block, Java has the NamedException instance.

How translation is relized by example:

// Java
try {
  // do something throwable
catch (NullPointerException npe) {}
catch (Exception e) {}
catch (Throwable t) {}

/// Swift
do {
  // do something throwable
catch {
  switch error {
    case Throwable.NullPointerException: do {}
    case Throwable.Exception : do {}
    default : do {}



First let let the replacement be for final. For functions / method parameters the semantic of Java and Swift are "different". In Java you can be declare a parameter as not changeable with final. In todays Swift versions all parameteres are final by default, you can use inout but the semantic is little different.


The Swift keyword is is. If it is only a check before casting use as? to optional cast.


Java interfaces are mapped with Swift protocols. With created package like structure (see package section) a typealias is declerated in this enum with reference of protocol implementation outside the enum. In the protocol implementation a associatedtype is declerated to the typealias name in the structure. This is f.e. needed to declerated function the result type of interface. Default methods need to declerated in a extendsion of protocol like normal Swift protocols.

For example:

extension myrootpackage.mysubpackage {
  public typealias JavaInterfaceName = JavApi.JavaInterfaceName

public protocol JavaInterfaceName {
  associatedtype JavaInterfaceName: myrootpackage.mysubpackage.JavaInterfaceName
  func toDo (_ with : Int) -> myrootpackage.mysubpackage.JavaInterfaceName

extension JavaInterfaceName {
  func toDo (_ with : Int) -> myrootpackage.mysubpackage.JavaInterfaceName {
    return self

java.lang types

All types in package java.lang doesn't need to import in Java. One solution is to set the full package name before. In result of Java-Swift name overlap it seems better to implement these types outside the java.lang enum structure (see package section).

Also if default Swing type exists it is extended instead Java implementation is ported, see String type as example.

keyword masking

The best way is to rename variables and types in Java before collision with Swift keywords. But you can also variable names mask with backtick like:

let in : Int


Java Map is similar to dictionary type in Swift

java.util.Map<String, Double> variable;    to    var variable : [String: Double]


Java methods in classes can be mapped as function of struct or classes. Java method define

visible returnType name (parameterType parameterName = defaultParameterValue, ...) throws namedException {}

Swift mapping is

visible name (_ parameterName : parameterType = defaultParameterValue, ...) throws {}

Swift prefered named parameter call of function instead of Java nameless function call. The underline adress this, but you need to add extra code in Swift (version 5.9) if parameterType is modifyable. In Swift you add a var modifier to parameter, but it doesn't work with underline. The target to make call of public functions do not make different, the var keyword need to be manual converted.

For example normal Swift code:

public func toDo (var with : Int) -> Int64 {
  if with < 5 {
    with = 5
  return Int64 (with)

For example to translate Java code:

public long toDo (int with) {
  if (with < 5) {
    with = 5;
  return with;

For example Swift translated Java code:

public func toDo (_ _with : Int) -> Int64 {
  var with = _with
  if with < 5 {
    with = 5
  return Int64(with)

I assume there is more than one call to a function, so the call is the same for a single line change/add. The other solution is to set the parameter with (var with : Int). Then all calls must be changed, including calls from other developers. Therefore, the underline solution is preferred.

The translation has a Optional problem because Java reference types are implicite nullable instead of Swift with explicite nilable. In result it exists not the only one way. Take a closer look to the method implementation and the documentation (for black box reimplementation). One way is to declerated some methods with (all) optional and non-optional variants of method. See System.arraycopy as example in the source files.

default methods

Default methods are implemented in a extension, because the mapping of Java interfaces in a Java package like structures need a bit more code.

new instance

We do not need the new keyword to create an instance of type.

operator >>>

The operator >>> is implemented. Composite operator like >>>= need to separated


In Swing not exists Java operators are implemented in java.lang

Composite operators must be separated before they can be used. Also operator statements in loops must be separated. The spaced must be placed. Unknown operators need to be translated. Assignment with = operator need to be separated.


i++            translated to    i += 1   /// unknown operator
i--            translated to    i -= 1   /// unknown operator
i+=1           translated to    i += 1   /// spaces placed, same for other operators
int i, j = 1   translated to    var i = 1; var j = 1;   /// separation of assignment operator
int i, j = 1   translated to    var i : Int = 1; var j : Int = 1;   /// alternative solution

Other example: result = (first + second++) to result = (first + second); second += 1

In loops operator position are relevant. The operator before the variable name tells first do the operator statement and than test. The operator after the variable name tells test first and then do the operator statement.

For example:

while (++i < 5) {}   translated to   i += 1; while (i < 5) {i += 1}       /// 1234
while (i++ < 5) {}   translated to   i += 1; while (i < 5 + 1) {i += 1}   /// 12345
while (--i > 0) {}   translated to   i -= 1; while (i > 5) {i -= 1}       /// 4321
while (i-- > 0) {}   translated to   i -= 1; while (i > 5 + 1) {i -= 1}   /// 43210

Of course this are the easiest variants.


Java packages can be mapped as enum hierachy. Take a look to the CryptoKit, for example MD5 is placed in the enum Insecure and must called with Insecure.MD5 Too add a new package create an enum with same name. Subpackages can be add directly in the enum or maybe better as extension of these enum.


Java shortcuts for types need to be translated like

1d    to    1.0         // double shortcut
1l    to    Int64(1)    // long shortcut


Replace String.format with Swift placeholders like from

String.format("Text %d, more text %s", intValue, stringValue)


"Text \(intValue), more text \(stringValue)"


Swiftify is a JavApi extension. It make additional implementations of Java methods with syntax more like Swift code. It can also implements additional Swift type to use the Java ported code like Swift in other real Swift code.


Java cases fallthrough by default and need explicite break if not. But Java need no default. Also newer Java can use -> instead of :.


Swift visiblilities are:

  1. private: like Java - symbol visible within the current declaration only.
  2. fileprivate: symbol visible within the current file.
  3. internal: like .net - symbol visible within the current module or default access modifier.
  4. public: for classes is not identical with Java - symbol visible outside the current module.
  5. open: like public in Java - for class or function to be subclassed or overridden outside the current module.

In result of these Java ported classes are by default open except they are final then public.


Stars: 1
Last commit: 1 week ago
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Release Notes

Extends char sequence implementation and collections
2 weeks ago

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