[
  {
    "path": "LICENSE",
    "content": "                    GNU GENERAL PUBLIC LICENSE\n                       Version 2, June 1991\n\n Copyright (C) 1989, 1991 Free Software Foundation, Inc., <http://fsf.org/>\n 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA\n Everyone is permitted to copy and distribute verbatim copies\n of this license document, but changing it is not allowed.\n\n                            Preamble\n\n  The licenses for most software are designed to take away your\nfreedom to share and change it.  By contrast, the GNU General Public\nLicense is intended to guarantee your freedom to share and change free\nsoftware--to make sure the software is free for all its users.  This\nGeneral Public License applies to most of the Free Software\nFoundation's software and to any other program whose authors commit to\nusing it.  (Some other Free Software Foundation software is covered by\nthe GNU Lesser General Public License instead.)  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The Free Software Foundation may publish revised and/or new versions\nof the General Public License from time to time.  Such new versions will\nbe similar in spirit to the present version, but may differ in detail to\naddress new problems or concerns.\n\nEach version is given a distinguishing version number.  If the Program\nspecifies a version number of this License which applies to it and \"any\nlater version\", you have the option of following the terms and conditions\neither of that version or of any later version published by the Free\nSoftware Foundation.  If the Program does not specify a version number of\nthis License, you may choose any version ever published by the Free Software\nFoundation.\n\n  10. If you wish to incorporate parts of the Program into other free\nprograms whose distribution conditions are different, write to the author\nto ask for permission.  For software which is copyrighted by the Free\nSoftware Foundation, write to the Free Software Foundation; we sometimes\nmake exceptions for this.  Our decision will be guided by the two goals\nof preserving the free status of all derivatives of our free software and\nof promoting the sharing and reuse of software generally.\n\n                            NO WARRANTY\n\n  11. BECAUSE THE PROGRAM IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY\nFOR THE PROGRAM, TO THE EXTENT PERMITTED BY APPLICABLE LAW.  EXCEPT WHEN\nOTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES\nPROVIDE THE PROGRAM \"AS IS\" WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED\nOR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF\nMERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.  THE ENTIRE RISK AS\nTO THE QUALITY AND PERFORMANCE OF THE PROGRAM IS WITH YOU.  SHOULD THE\nPROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING,\nREPAIR OR CORRECTION.\n\n  12. IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING\nWILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY AND/OR\nREDISTRIBUTE THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES,\nINCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING\nOUT OF THE USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED\nTO LOSS OF DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY\nYOU OR THIRD PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER\nPROGRAMS), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE\nPOSSIBILITY OF SUCH DAMAGES.\n\n                     END OF TERMS AND CONDITIONS\n\n            How to Apply These Terms to Your New Programs\n\n  If you develop a new program, and you want it to be of the greatest\npossible use to the public, the best way to achieve this is to make it\nfree software which everyone can redistribute and change under these terms.\n\n  To do so, attach the following notices to the program.  It is safest\nto attach them to the start of each source file to most effectively\nconvey the exclusion of warranty; and each file should have at least\nthe \"copyright\" line and a pointer to where the full notice is found.\n\n    {description}\n    Copyright (C) {year}  {fullname}\n\n    This program is free software; you can redistribute it and/or modify\n    it under the terms of the GNU General Public License as published by\n    the Free Software Foundation; either version 2 of the License, or\n    (at your option) any later version.\n\n    This program is distributed in the hope that it will be useful,\n    but WITHOUT ANY WARRANTY; without even the implied warranty of\n    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the\n    GNU General Public License for more details.\n\n    You should have received a copy of the GNU General Public License along\n    with this program; if not, write to the Free Software Foundation, Inc.,\n    51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.\n\nAlso add information on how to contact you by electronic and paper mail.\n\nIf the program is interactive, make it output a short notice like this\nwhen it starts in an interactive mode:\n\n    Gnomovision version 69, Copyright (C) year name of author\n    Gnomovision comes with ABSOLUTELY NO WARRANTY; for details type `show w'.\n    This is free software, and you are welcome to redistribute it\n    under certain conditions; type `show c' for details.\n\nThe hypothetical commands `show w' and `show c' should show the appropriate\nparts of the General Public License.  Of course, the commands you use may\nbe called something other than `show w' and `show c'; they could even be\nmouse-clicks or menu items--whatever suits your program.\n\nYou should also get your employer (if you work as a programmer) or your\nschool, if any, to sign a \"copyright disclaimer\" for the program, if\nnecessary.  Here is a sample; alter the names:\n\n  Yoyodyne, Inc., hereby disclaims all copyright interest in the program\n  `Gnomovision' (which makes passes at compilers) written by James Hacker.\n\n  {signature of Ty Coon}, 1 April 1989\n  Ty Coon, President of Vice\n\nThis General Public License does not permit incorporating your program into\nproprietary programs.  If your program is a subroutine library, you may\nconsider it more useful to permit linking proprietary applications with the\nlibrary.  If this is what you want to do, use the GNU Lesser General\nPublic License instead of this License.\n\n"
  },
  {
    "path": "README.md",
    "content": "\n# JAVA 8 - Cheat Sheet\n\n## Lambda Expression\n\n```java\n(int a) -> a * 2; // Calculate the double of a\na -> a * 2; // or simply without type\n```\n\n```java\n(a, b) -> a + b; // Sum of 2 parameters\n```\n\nIf the lambda is more than one expression we can use `{ }` and `return`\n\n```java\n(x, y) -> {\n    int sum = x + y;\n    int avg = sum / 2;\n    return avg;\n}\n```\n\nA lambda expression cannot stand alone in Java, it need to be associated to a functional interface.\n\n```java\ninterface MyMath {\n    int getDoubleOf(int a);\n}\n\nMyMath d = a -> a * 2; // associated to the interface\nd.getDoubleOf(4); // is 8\n```\n\n---\n\nAll examples with \"list\" use :\n\n```java\nList<String> list = [Bohr, Darwin, Galilei, Tesla, Einstein, Newton]\n```\n\n\n## Collections\n\n**sort** `sort(list, comparator)`\n\n```java\nlist.sort((a, b) -> a.length() - b.length())\nlist.sort(Comparator.comparing(n -> n.length())); // same\nlist.sort(Comparator.comparing(String::length)); // same\n//> [Bohr, Tesla, Darwin, Newton, Galilei, Einstein]\n```\n\n**removeIf**\n\n```java\nlist.removeIf(w -> w.length() < 6);\n//> [Darwin, Galilei, Einstein, Newton]\n```\n\n**merge**\n`merge(key, value, remappingFunction)`\n\n```java\nMap<String, String> names = new HashMap<>();\nnames.put(\"Albert\", \"Ein?\");\nnames.put(\"Marie\", \"Curie\");\nnames.put(\"Max\", \"Plank\");\n\n// Value \"Albert\" exists\n// {Marie=Curie, Max=Plank, Albert=Einstein}\nnames.merge(\"Albert\", \"stein\", (old, val) -> old.substring(0, 3) + val);\n\n// Value \"Newname\" don't exists\n// {Marie=Curie, Newname=stein, Max=Plank, Albert=Einstein}\nnames.merge(\"Newname\", \"stein\", (old, val) -> old.substring(0, 3) + val);\n```\n\n\n## Method Expressions `Class::staticMethod`\n\nAllows to reference methods (and constructors) without executing them\n\n```java\n// Lambda Form:\ngetPrimes(numbers, a -> StaticMethod.isPrime(a));\n\n// Method Reference:\ngetPrimes(numbers, StaticMethod::isPrime);\n```\n\n| Method Reference | Lambda Form |\n| ---------------- | ----------- |\n| `StaticMethod::isPrime` | `n -> StaticMethod.isPrime(n)` |\n| `String::toUpperCase`   | `(String w) -> w.toUpperCase()` |\n| `String::compareTo`     | `(String s, String t) -> s.compareTo(t)` |\n| `System.out::println`   | `x -> System.out.println(x)` |\n| `Double::new`           | `n -> new Double(n)` |\n| `String[]::new`         | `(int n) -> new String[n]` |\n\n\n## Streams\n\nSimilar to collections, but\n\n - They don't store their own data\n - The data comes from elsewhere (collection, file, db, web, ...)\n - *immutable* (produce new streams)\n - *lazy* (only computes what is necessary !)\n \n```java\n// Will compute just 3 \"filter\"\nStream<String> longNames = list\n   .filter(n -> n.length() > 8)\n   .limit(3);\n```\n\n**Create a new stream**\n\n```java\nStream<Integer> stream = Stream.of(1, 2, 3, 5, 7, 11);\nStream<String> stream = Stream.of(\"Jazz\", \"Blues\", \"Rock\");\nStream<String> stream = Stream.of(myArray); // or from an array\nStream<String> stream = list.stream(); // or from a list\n\n// Infinit stream [0; inf[\nStream<Integer> integers = Stream.iterate(0, n -> n + 1);\n```\n\n**Collecting results**\n\n```java\n// Collect into an array (::new is the constructor reference)\nString[] myArray = stream.toArray(String[]::new);\n\n// Collect into a List or Set\nList<String> myList = stream.collect(Collectors.toList());\nSet<String> mySet = stream.collect(Collectors.toSet());\n\n// Collect into a String\nString str = list.collect(Collectors.joining(\", \"));\n```\n\n**map** `map(mapper)`<br>\nApplying a function to each element\n\n```java\n// Apply \"toLowerCase\" for each element\nStream<String> res = stream.map(w -> w.toLowerCase());\nStream<String> res = stream.map(String::toLowerCase);\n//> bohr darwin galilei tesla einstein newton\n\nStream<Integer> res = Stream.of(1, 2, 3, 4, 5).map(x -> x + 1);\n//> 2 3 4 5 6\n```\n\n**filter** `filter(predicate)`<br>\nRetains elements that match the predicate\n\n```java\n// Filter elements that begin with \"E\"\nres = stream.filter(n -> n.substring(0, 1).equals(\"E\"));\n//> Einstein\n\nres = Stream.of(1, 2, 3, 4, 5).filter(x -> x < 3);\n//> 1 2\n```\n\n**reduce**<br>\nReduce the elements to a single value\n\n```java\nString reduced = stream.reduce(\"\", (acc, el) -> acc + \"|\" + el);\n//> |Bohr|Darwin|Galilei|Tesla|Einstein|Newton\n```\n\n**limit** `limit(maxSize)`<br>\nSelect the n first elements\n\n```java\nres = stream.limit(3);\n//> Bohr Darwin Galilei\n```\n\n**skip**<br>\nDiscarding the first n elements\n\n```java\nres = stream.skip(2); // skip Bohr and Darwin\n//> Galilei Tesla Einstein Newton\n```\n\n**distinct**<br>\nRemove duplicated elemetns\n\n```java\nres = Stream.of(1, 0, 0, 1, 0, 1).distinct();\n//> 1 0\n```\n\n**sorted**<br>\nSort elements (must be *Comparable*)\n\n```java\nres = stream.sorted();\n//> Bohr Darwin Einstein Galilei Newton Tesla \n```\n\n**allMatch** / **noneMatch**\n\n```java\n// Check if there is a \"e\" in each elements\nboolean res = words.allMatch(n -> n.contains(\"e\"));\n```\n\nanyMatch: Check if there is a \"e\" in an element<br>\nnoneMatch: Check if there is no \"e\" in elements\n\n**parallel**<br>\nReturns an equivalent stream that is parallel\n\n**findAny**<br>\nfaster than findFirst on parallel streams\n\n### Primitive-Type Streams\n\nWrappers (like Stream<Integer>) are inefficients. It requires a lot of unboxing and boxing for each element. Better to use `IntStream`, `DoubleStream`, etc.\n\n**Creation**\n\n```java\nIntStream stream = IntStream.of(1, 2, 3, 5, 7);\nstream = IntStream.of(myArray); // from an array\nstream = IntStream.range(5, 80); // range from 5 to 80\n\nRandom gen = new Random();\nIntStream rand = gen(1, 9); // stream of randoms\n```\n\nUse *mapToX* (mapToObj, mapToDouble, etc.) if the function yields Object, double, etc. values.\n\n### Grouping Results\n\n**Collectors.groupingBy**\n\n```java\n// Groupe by length\nMap<Integer, List<String>> groups = stream.collect(Collectors.groupingBy(w -> w.length()));\n//> {4=[Bohr], 5=[Tesla], 6=[Darwin, Newton], 7=[Galilei], 8=[Einstein]}\n```\n\n**Collectors.toSet**\n\n```java\n// Same as before but with Set\nMap<String, Set<String>> groups2 = stream.collect(Collectors.groupingBy(w -> w.substring(0, 1), Collectors.toSet()));\n//> {B=[Bohr], T=[Tesla], D=[Darwin], E=[Einstein], G=[Galilei], N=[Newton]}\n```\n\n**Collectors.counting**<br>\nCount the number of values in a group\n\n**Collectors.summing__**<br>\n`summingInt`, `summingLong`, `summingDouble` to sum group values\n\n**Collectors.averaging__**<br>\n`averagingInt`, `averagingLong`, ... to average group values\n\n```java\n// Average length of each element of a group\nCollectors.averagingInt(String::length)\n```\n\n*PS*: Don't forget Optional (like `Map<T, Optional<T>>`) with some Collection methods (like `Collectors.maxBy`).\n\n\n### Parallel Streams\n\n**Creation**\n\n```java\nStream<String> parStream = list.parallelStream();\nStream<String> parStream = Stream.of(myArray).parallel();\n```\n\n**unordered**\nCan speed up the `limit` or `distinct`\n\n```java\nstream.parallelStream().unordered().distinct();\n```\n\n*PS*: Work with the streams library. Eg. use `filter(x -> x.length() < 9)` instead of a `forEach` with an `if`.\n\n\n## Optional\n\nIn Java, it is common to use null to denote absence of result.<br>\nProblems when no checks: `NullPointerException`.\n\n```java\n// Optional<String> contains a string or nothing\nOptional<String> res = stream\n   .filter(w -> w.length() > 10)\n   .findFirst();\n\n// length of the value or \"\" if nothing\nint length = res.orElse(\"\").length();\n\n// run the lambda if there is a value\nres.ifPresent(v -> results.add(v));\n```\n\nReturn an Optional\n\n```java\nOptional<Double> squareRoot(double x) {\n   if (x >= 0) { return Optional.of(Math.sqrt(x)); }\n   else { return Optional.empty(); }\n}\n```\n\n---\n\n**Note on inferance limitations**\n\n```java\ninterface Pair<A, B> {\n    A first();\n    B second();\n}\n```\n\nA steam of type `Stream<Pair<String, Long>>` :\n- `stream.sorted(Comparator.comparing(Pair::first)) // ok`\n- `stream.sorted(Comparator.comparing(Pair::first).thenComparing(Pair::second)) // dont work`\n\nJava cannot infer type for the `.comparing(Pair::first)` part and fallback to Object, on which `Pair::first` cannot be applied.\n\nThe required type for the whole expression cannot be propagated through the method call (`.thenComparing`) and used to infer type of the first part.\n\nType *must* be given explicitly.\n\n```java\nstream.sorted(\n    Comparator.<Pair<String, Long>, String>comparing(Pair::first)\n    .thenComparing(Pair::second)\n) // ok\n```\n\n---\n\nThis cheat sheet was based on the lecture of Cay Horstmann\nhttp://horstmann.com/heig-vd/spring2015/poo/\n"
  },
  {
    "path": "README_CN.md",
    "content": "\n# JAVA 8 - 备忘录\n\n## Lambda 表达式\n\n```java\n(int a) -> a * 2; // 求a乘以2后的值\na -> a * 2; // 或者更直接的去掉类型也是可以的\n```\n\n```java\n(a, b) -> a + b; // 相加\n```\n\n如果lambda里面的代码块超过1行，可以配合使用 `{ }` 加 `return`来处理\n\n```java\n(x, y) -> {\n    int sum = x + y;\n    int avg = sum / 2;\n    return avg;\n}\n```\n\n一个lamdba表达式必须依赖一个具体的功能接口而存在\n\n```java\ninterface MyMath {\n    int getDoubleOf(int a);\n}\n\nMyMath d = a -> a * 2; // 关联到具体的接口实现\nd.getDoubleOf(4); // is 8\n```\n\n---\n\n下面所有的测试都是用到这个`list` :\n\n```java\nList<String> list = [Bohr, Darwin, Galilei, Tesla, Einstein, Newton]\n```\n\n\n## Collections 集合\n\n**sort** `sort(list, comparator)`\n\n```java\nlist.sort((a, b) -> a.length() - b.length())\nlist.sort(Comparator.comparing(n -> n.length())); // 使用具体Comparator接口实现\nlist.sort(Comparator.comparing(String::length)); // 这样写和上面也是一样的\n//> [Bohr, Tesla, Darwin, Newton, Galilei, Einstein]\n```\n\n**removeIf**\n\n```java\nlist.removeIf(w -> w.length() < 6);\n//> [Darwin, Galilei, Einstein, Newton]\n```\n\n**merge**\n`merge(key, value, remappingFunction)`\n\n```java\nMap<String, String> names = new HashMap<>();\nnames.put(\"Albert\", \"Ein?\");\nnames.put(\"Marie\", \"Curie\");\nnames.put(\"Max\", \"Plank\");\n\n//  \"Albert\" 这个值是存在的 就命中了后面处理流程\n// {Marie=Curie, Max=Plank, Albert=Einstein}\nnames.merge(\"Albert\", \"stein\", (old, val) -> old.substring(0, 3) + val);\n\n// \"Newname\" 这个值是不存在的 所以后面的流程就不处理\n// {Marie=Curie, Newname=stein, Max=Plank, Albert=Einstein}\nnames.merge(\"Newname\", \"stein\", (old, val) -> old.substring(0, 3) + val);\n```\n\n\n## 方法引用 `Class::staticMethod` \n\n允许引用类方法或者构造函数，引用时候是不执行的\n\n```java\n//通过lamdba\ngetPrimes(numbers, a -> StaticMethod.isPrime(a));\n\n//通过应用方法:\ngetPrimes(numbers, StaticMethod::isPrime);\n```\n\n| Method Reference | Lambda Form |\n| ---------------- | ----------- |\n| `StaticMethod::isPrime` | `n -> StaticMethod.isPrime(n)` |\n| `String::toUpperCase`   | `(String w) -> w.toUpperCase()` |\n| `String::compareTo`     | `(String s, String t) -> s.compareTo(t)` |\n| `System.out::println`   | `x -> System.out.println(x)` |\n| `Double::new`           | `n -> new Double(n)` |\n| `String[]::new`         | `(int n) -> new String[n]` |\n\n\n## Streams 流式处理\n\n和`collections`类似, 但有所不同\n\n - 不能储存数据\n - 数据来源外部例如 (collection, file, db, web, ...)\n - `immutable`不可变性，不影响外部数据 (因为产生了一个新的stream)\n - `lazy`懒式处理 (只有在计算的时候才用到，不处理不用 !)\n \n```java\n// 仅仅计算前3个\"filter\"\nStream<String> longNames = list\n   .filter(n -> n.length() > 8)\n   .limit(3);\n```\n\n**创建一个stream**\n\n```java\nStream<Integer> stream = Stream.of(1, 2, 3, 5, 7, 11);\nStream<String> stream = Stream.of(\"Jazz\", \"Blues\", \"Rock\");\nStream<String> stream = Stream.of(myArray); // 通过数组\nStream<String> stream = list.stream(); // 通过list\n\n// Infinit stream [0; inf[\nStream<Integer> integers = Stream.iterate(0, n -> n + 1);\n```\n\n**集合结果集**\n\n```java\n//返回成一个数组 (::new 是构造函数的引用)\nString[] myArray = stream.toArray(String[]::new);\n\n// 返回成list或set\nList<String> myList = stream.collect(Collectors.toList());\nSet<String> mySet = stream.collect(Collectors.toSet());\n\n// 返回成String\nString str = list.collect(Collectors.joining(\", \"));\n\n// 返回成一个LinkedHashMap\nlist.stream().collect(Collectors.toMap(k -> k, v -> v, (a, b) -> a, LinkedHashMap::new));\n// 默认转换成HashMap\nlist.stream().collect(Collectors.toMap(k -> k, v -> v));\n```\n\n**map** `map(mapper)`<br>\n对每个元素进行类型转换\n\n```java\n// 对每个元素使用 \"toLowerCase\" 处理\nStream<String> res = stream.map(w -> w.toLowerCase());\nStream<String> res = stream.map(String::toLowerCase);\n//> bohr darwin galilei tesla einstein newton\n\nStream<Integer> res = Stream.of(1, 2, 3, 4, 5).map(x -> x + 1);\n//> 2 3 4 5 6\n```\n\n**filter** `filter(predicate)`<br>\n过滤处理，只保留匹配到的元素\n\n```java\n// 过掉保留 \"E\" 开头的元素\nres = stream.filter(n -> n.substring(0, 1).equals(\"E\"));\n//> Einstein\n\nres = Stream.of(1, 2, 3, 4, 5).filter(x -> x < 3);\n//> 1 2\n```\n\n**reduce**<br>\n汇聚处理成为单一返回结果\n\n```java\nString reduced = stream.reduce(\"\", (acc, el) -> acc + \"|\" + el);\n//> |Bohr|Darwin|Galilei|Tesla|Einstein|Newton\n```\n\n**limit** `limit(maxSize)`<br>\n保留前`maxSize`个元素\n\n```java\nres = stream.limit(3);\n//> Bohr Darwin Galilei\n```\n\n**skip**<br>\n忽略掉前`n`个元素\n\n```java\nres = stream.skip(2); // 忽略 Bohr 和 Darwin\n//> Galilei Tesla Einstein Newton\n```\n\n**distinct**<br>\n去除重复的元素\n\n```java\nres = Stream.of(1, 0, 0, 1, 0, 1).distinct();\n//> 1 0\n```\n\n**sorted**<br>\n排序 (必须使用 *Comparable* 接口)\n\n```java\nres = stream.sorted();\n//> Bohr Darwin Einstein Galilei Newton Tesla \n```\n\n**allMatch** / **noneMatch**\n\n```java\n// 检查是否每个元素都是“e”开头\nboolean res = words.allMatch(n -> n.contains(\"e\"));\n```\n\n`anyMatch`: 只要其中一个元素包含\"e\"即可 <br>\n`noneMatch`: 元素里面是否没有\"e\" \n\n**parallel**<br>\n返回一个并行的stream\n\n**findAny**<br>\n在并行流上findFirst执行更快\n\n### 原始类型的 Streams\n\n原子类型的stream自动封装是低效的 (例如 Stream<Integer>) ，因为它需要对每个元素进行大量拆箱和装箱. 所以最好使用 `IntStream`, `DoubleStream`, 等等.\n\n**初始化**\n\n```java\nIntStream stream = IntStream.of(1, 2, 3, 5, 7);\nstream = IntStream.of(myArray); // 通过数组\nstream = IntStream.range(5, 80); //  5 到 80范围\n\nRandom gen = new Random();\nIntStream rand = gen(1, 9); // stream of randoms\n```\n\n使用 *mapToX* (mapToObj, mapToDouble, mapToLong) 如果需要把字段转换成 Object, double, long的话. values.\n\n### Grouping 数据集\n\n**Collectors.groupingBy**\n\n```java\n// 通过长度分组\nMap<Integer, List<String>> groups = stream.collect(Collectors.groupingBy(w -> w.length()));\n//> 4=[Bohr], 5=[Tesla], 6=[Darwin, Newton], ...\n```\n\n**Collectors.toSet**\n\n```java\n// 和之前一样但是使用的是Set\nMap<String, Set<String>> groups2 = stream.collect(Collectors.groupingBy(w -> w.substring(0, 1), Collectors.toSet()));\n//> {B=[Bohr], T=[Tesla], D=[Darwin], E=[Einstein], G=[Galilei], N=[Newton]}\n```\n\n**Collectors.counting**<br>\n获取元素总算\n\n**Collectors.summing__**<br>\n`summingInt`, `summingLong`, `summingDouble` 计算所有元素值相加后结果\n\n**Collectors.averaging__**<br>\n`averagingInt`, `averagingLong`, ... 计算所有元素值的算术平均值\n\n```java\n// 计算平均数\nCollectors.averagingInt(String::length)\n```\n\n*PS*: 另外不要忘记 Optional (例如 `Map<T, Optional<T>>`) 有同样的处理方法 (例如 `Collectors.maxBy`).\n\n\n### 并行 Streams \n\n**创建一个并行处理的stream**\n\n```java\nStream<String> parStream = list.parallelStream();\nStream<String> parStream = Stream.of(myArray).parallel();\n```\n\n**unordered** \n能提高计算 `limit`，`distinct`的速度\n\n```java\nstream.parallelStream().unordered().distinct();\n```\n\n*PS*: 使用streams类库, 例如使用 `filter(x -> x.length() < 9)` 代替 `forEach` 和 `if`\n\n\n## Optional\n\n在Java, 通常使用`null`表示没有结果，但是如果不检查的话很容易出现`NullPointerException`.\n\n```java\n// Optional<String> 包含一个string和空\nOptional<String> res = stream\n   .filter(w -> w.length() > 10)\n   .findFirst();\n\n// 返回元素长度或者返回 \"\" 如果没有的话\nint length = res.orElse(\"\").length();\n\n// 使用lambda作为一个返回值\nres.ifPresent(v -> results.add(v));\n```\n\n返回一个 Optional\n\n```java\nOptional<Double> squareRoot(double x) {\n   if (x >= 0) { return Optional.of(Math.sqrt(x)); }\n   else { return Optional.empty(); }\n}\n```\n\n---\n\n**注意引用推测限制**\n\n```java\ninterface Pair<A, B> {\n    A first();\n    B second();\n}\n```\n\n一个 steam 类型 `Stream<Pair<String, Long>>` :\n- `stream.sorted(Comparator.comparing(Pair::first)) // 有效`\n- `stream.sorted(Comparator.comparing(Pair::first).thenComparing(Pair::second)) // 无效`\n\nJava不能通过 `.comparing(Pair::first)`回调过来的数据来判断类型, 故 `Pair::first` 就不能这样用了\n\n如果需要使用泛型接口的话需要显示写清楚，否则无效\n\n```java\nstream.sorted(\n    Comparator.<Pair<String, Long>, String>comparing(Pair::first)\n    .thenComparing(Pair::second)\n) // 有效\n```\n\n---\n\nThis cheat sheet was based on the lecture of Cay Horstmann\nhttp://horstmann.com/heig-vd/spring2015/poo/\n"
  },
  {
    "path": "README_ES.md",
    "content": "\n# JAVA 8 - Chuleta\n\n## Expresión Lambda\n\n```java\n(int a) -> a * 2; // Calcula el docble de a\na -> a * 2; // o simplemente sin tipo\n```\n\n```java\n(a, b) -> a + b; // Suma 2 parametros\n```\n\nSi lambda tiene mas de una expresión podemos usar `{ }` y `return`\n\n```java\n(x, y) -> {\n    int sum = x + y;\n    int avg = sum / 2;\n    return avg;\n}\n```\n\nUna expresión lambda no puede usarse sola en Java, necesita estar asociada a un interfaz funcional\n\n```java\ninterface MyMath {\n    int getDoubleOf(int a);\n}\n\nMyMath d = a -> a * 2; // asociado a un interfaz\nd.getDoubleOf(4); // es 8\n```\n\n---\n\nTodos los ejemplos con \"list\" usan:\n\n```java\nList<String> list = [Bohr, Darwin, Galilei, Tesla, Einstein, Newton]\n```\n\n\n## Colecciones\n\n**sort** `sort(list, comparator)`\n\n```java\nlist.sort((a, b) -> a.length() - b.length())\nlist.sort(Comparator.comparing(n -> n.length())); // igual\nlist.sort(Comparator.comparing(String::length)); // igual\n//> [Bohr, Tesla, Darwin, Newton, Galilei, Einstein]\n```\n\n**removeIf**\n\n```java\nlist.removeIf(w -> w.length() < 6);\n//> [Darwin, Galilei, Einstein, Newton]\n```\n\n**merge**\n`merge(key, value, remappingFunction)`\n\n```java\nMap<String, String> names = new HashMap<>();\nnames.put(\"Albert\", \"Ein?\");\nnames.put(\"Marie\", \"Curie\");\nnames.put(\"Max\", \"Plank\");\n\n// El valor \"Albert\" existe\n// {Marie=Curie, Max=Plank, Albert=Einstein}\nnames.merge(\"Albert\", \"stein\", (old, val) -> old.substring(0, 3) + val);\n\n// El valor  \"Newname\" no existe\n// {Marie=Curie, Newname=stein, Max=Plank, Albert=Einstein}\nnames.merge(\"Newname\", \"stein\", (old, val) -> old.substring(0, 3) + val);\n```\n\n\n## Expresión de metodo `Class::staticMethod`\n\nPermite referenciar métodos (y constructores) sin ejecutarlos\n\n```java\n// Con Lambda:\ngetPrimes(numbers, a -> StaticMethod.isPrime(a));\n\n// Metodo Referenciado:\ngetPrimes(numbers, StaticMethod::isPrime);\n```\n\n| Método Referenciado | Forma de Lambda |\n| ------------------- | --------------- |\n| `StaticMethod::isPrime` | `n -> StaticMethod.isPrime(n)` |\n| `String::toUpperCase`   | `(String w) -> w.toUpperCase()` |\n| `String::compareTo`     | `(String s, String t) -> s.compareTo(t)` |\n| `System.out::println`   | `x -> System.out.println(x)` |\n| `Double::new`           | `n -> new Double(n)` |\n| `String[]::new`         | `(int n) -> new String[n]` |\n\n\n## Streams\n\nSimilares a las colecciones, pero:\n\n - No almacenan su propia información\n - La información viene de otra parte (colleciones, archivos, db, web, ...)\n - *immutable* (crean un nuevo stream)\n - *lazy* (solo computa lo que es necesario !)\n \n```java\n// Computara 3 \"filter\"\nStream<String> longNames = list\n   .filter(n -> n.length() > 8)\n   .limit(3);\n```\n\n**Crea un nuevo stream**\n\n```java\nStream<Integer> stream = Stream.of(1, 2, 3, 5, 7, 11);\nStream<String> stream = Stream.of(\"Jazz\", \"Blues\", \"Rock\");\nStream<String> stream = Stream.of(myArray); // or from an array\nStream<String> stream = list.stream(); // or from a list\n\n// Stream infinito [0; inf[\nStream<Integer> integers = Stream.iterate(0, n -> n + 1);\n```\n\n**Resultado de las colecciones**\n\n```java\n// Collect into an array (::new is the constructor reference)\n// Colecciona en un array (::new es la referencia del contructor)\nString[] myArray = stream.toArray(String[]::new);\n\n// Collecciona en un List o Set\nList<String> myList = stream.collect(Collectors.toList());\nSet<String> mySet = stream.collect(Collectors.toSet());\n\n// Collecciona en un String\nString str = list.collect(Collectors.joining(\", \"));\n```\n\n**map** `map(mapper)`<br>\nAplica una función a cada elemento\n\n```java\n// Aplica \"toLowerCase\" a cada elemento\nStream<String> res = stream.map(w -> w.toLowerCase());\nStream<String> res = stream.map(String::toLowerCase);\n//> bohr darwin galilei tesla einstein newton\n\nStream<Integer> res = Stream.of(1, 2, 3, 4, 5).map(x -> x + 1);\n//> 2 3 4 5 6\n```\n\n**filter** `filter(predicado)`<br>\nRetiene elementos que coinciden con el predicado\n\n```java\n// Filtra elementos que empiecen con \"E\"\nres = stream.filter(n -> n.substring(0, 1).equals(\"E\"));\n//> Einstein\n\nres = Stream.of(1, 2, 3, 4, 5).filter(x -> x < 3);\n//> 1 2\n```\n\n**reduce**<br>\nReduce los elementos a un unico valor\n\n```java\nString reduced = stream.reduce(\"\", (acc, el) -> acc + \"|\" + el);\n//> |Bohr|Darwin|Galilei|Tesla|Einstein|Newton\n```\n\n**limit** `limit(maxSize)`<br>\nLos n primeros elementos\n\n```java\nres = stream.limit(3);\n//> Bohr Darwin Galilei\n```\n\n**skip**<br>\nDescarta los primeros n elementos\n\n```java\nres = strem.skip(2); // skip Bohr and Darwin\n//> Galilei Tesla Einstein Newton\n```\n\n**distinct**<br>\nBorra los elementos repetidos\n\n```java\nres = Stream.of(1, 0, 0, 1, 0, 1).distinct();\n//> 1 0\n```\n\n**sorted**<br>\nOrdena elementos (debe ser *Comparable*)\n\n```java\nres = stream.sorted();\n//> Bohr Darwin Einstein Galilei Newton Tesla \n```\n\n**allMatch** / **noneMatch**\n\n```java\n// Comprueba si hay una \"e\" en cada elemento\nboolean res = words.allMatch(n -> n.contains(\"e\"));\n```\n\nanyMatch: Comprueba si hay una \"e\" en algun elemento<br>\nnoneMatch: Comprueba si no hay una \"e\" en ningun elemento\n\n**parallel**<br>\nDevuelve un stream equivalente que es paralelo\n\n**findAny**<br>\nMas rapido que findFirst en un stream paralelo\n\n### Streams de tipo primitivo\n\nLos wrappers (como Stream<Integer>) son ineficientes. Requieren de embalar y desembalar cada elemento demasiado. Mejor usar `IntStream`, `DoubleStream`, etc.\n\n**Creacion**\n\n```java\nIntStream stream = IntStream.of(1, 2, 3, 5, 7);\nstream = IntStream.of(myArray); // de un array\nstream = IntStream.range(5, 80); // rango de 5 a 80\n\nRandom gen = new Random();\nIntStream rand = gen(1, 9); // stream de aleatorios\n```\n\nUsa *mapToX* (mapToObj, mapToDouble, etc.) si la función produce un valor Object, double, etc.\n\n### Resultados agrupados\n\n**Collectors.groupingBy**\n\n```java\n// Agrupados por longitud\nMap<Integer, List<String>> groups = stream.collect(Collectors.groupingBy(w -> w.length()));\n//> {4=[Bohr], 5=[Tesla], 6=[Darwin, Newton], 7=[Galilei], 8=[Einstein]}\n```\n\n**Collectors.toSet**\n\n```java\n// Igual que antes pero en un Set\nMap<String, Set<String>> groups2 = stream.collect(Collectors.groupingBy(w -> w.substring(0, 1), Collectors.toSet()));\n//> {B=[Bohr], T=[Tesla], D=[Darwin], E=[Einstein], G=[Galilei], N=[Newton]}\n```\n\n**Collectors.counting**<br>\nCuenta el numero de elementos en una coleccion\n\n**Collectors.summing__**<br>\n`summingInt`, `summingLong`, `summingDouble` para sumar valores de un grupo\n\n**Collectors.averaging__**<br>\n`averagingInt`, `averagingLong`, ... \n\n```java\n// Longitud promedio de cada elemento de un grupo\nCollectors.averagingInt(String::length)\n```\n\n*PS*: No olvides Optional (como `Map<T, Optional<T>>`) con algunos metodos de Colecciones (like `Collectors.maxBy`).\n\n\n### Streams Paralelos\n\n**Creacion**\n\n```java\nStream<String> parStream = list.parallelStream();\nStream<String> parStream = Stream.of(myArray).parallel();\n```\n\n**unordered**\nPueden acelerar el `limit` o `distinct`\n\n```java\nstream.parallelStream().unordered().distinct();\n```\n\n\n*PS*: Trabaja con la librería de streams. Pe: usa `filter(x -> x.length() < 9)` en vez de `forEach` con un`if`.\n\n## Optional\n\nEn Java, es común usar null para denotar ausencia de resultado.<br>\nProblemas cuando no se comprueban: `NullPointerException`.\n\n```java\n// Optional<String> contiene un string o nada\nOptional<String> res = stream\n   .filter(w -> w.length() > 10)\n   .findFirst();\n\n// longitud de res o \"\", si no trae nada\nint length = res.orElse(\"\").length();\n\n// lanza el lambda si no trae nada\nres.ifPresent(v -> results.add(v));\n```\n\nDevuelve un Optional\n\n```java\nOptional<Double> squareRoot(double x) {\n   if (x >= 0) { return Optional.of(Math.sqrt(x)); }\n   else { return Optional.empty(); }\n}\n```\n\n---\n\n**Note en la inferencia de la limitación**\n\n```java\ninterface Pair<A, B> {\n    A first();\n    B second();\n}\n```\n\nUn stream de tipo `Stream<Pair<String, Long>>` :\n- `stream.sorted(Comparator.comparing(Pair::first)) // vale`\n- `stream.sorted(Comparator.comparing(Pair::first).thenComparing(Pair::second)) // no funciona`\n\nJava no puede inferir el tipo para la parte de `.comparing(Pair::first)`y devolver el Objeto, por lo que `Pair::first` no podría ser aplicado.\n\nEl tipo requerido para toda la expresión no puede ser propagada a través de la llamada del método (`.thenComparing`) y  ser usada para inferir el tipo de la primera parte.\n\nEl tipo *debe* ser dado explicitamente\n\n```java\nstream.sorted(\n    Comparator.<Pair<String, Long>, String>comparing(Pair::first)\n    .thenComparing(Pair::second)\n) // ok\n```\n\n---\n\nEsta cheat sheet esta basada en la lección de Cay Horstmann\nhttp://horstmann.com/heig-vd/spring2015/poo/\n"
  }
]