Repository: BafS/Java8-CheatSheet Branch: master Commit: 5e4a8c18389f Files: 4 Total size: 41.3 KB Directory structure: gitextract_tvnl2378/ ├── LICENSE ├── README.md ├── README_CN.md └── README_ES.md ================================================ FILE CONTENTS ================================================ ================================================ FILE: LICENSE ================================================ GNU GENERAL PUBLIC LICENSE Version 2, June 1991 Copyright (C) 1989, 1991 Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed. Preamble The licenses for most software are designed to take away your freedom to share and change it. By contrast, the GNU General Public License is intended to guarantee your freedom to share and change free software--to make sure the software is free for all its users. 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If this is what you want to do, use the GNU Lesser General Public License instead of this License. ================================================ FILE: README.md ================================================ # JAVA 8 - Cheat Sheet ## Lambda Expression ```java (int a) -> a * 2; // Calculate the double of a a -> a * 2; // or simply without type ``` ```java (a, b) -> a + b; // Sum of 2 parameters ``` If the lambda is more than one expression we can use `{ }` and `return` ```java (x, y) -> { int sum = x + y; int avg = sum / 2; return avg; } ``` A lambda expression cannot stand alone in Java, it need to be associated to a functional interface. ```java interface MyMath { int getDoubleOf(int a); } MyMath d = a -> a * 2; // associated to the interface d.getDoubleOf(4); // is 8 ``` --- All examples with "list" use : ```java List list = [Bohr, Darwin, Galilei, Tesla, Einstein, Newton] ``` ## Collections **sort** `sort(list, comparator)` ```java list.sort((a, b) -> a.length() - b.length()) list.sort(Comparator.comparing(n -> n.length())); // same list.sort(Comparator.comparing(String::length)); // same //> [Bohr, Tesla, Darwin, Newton, Galilei, Einstein] ``` **removeIf** ```java list.removeIf(w -> w.length() < 6); //> [Darwin, Galilei, Einstein, Newton] ``` **merge** `merge(key, value, remappingFunction)` ```java Map names = new HashMap<>(); names.put("Albert", "Ein?"); names.put("Marie", "Curie"); names.put("Max", "Plank"); // Value "Albert" exists // {Marie=Curie, Max=Plank, Albert=Einstein} names.merge("Albert", "stein", (old, val) -> old.substring(0, 3) + val); // Value "Newname" don't exists // {Marie=Curie, Newname=stein, Max=Plank, Albert=Einstein} names.merge("Newname", "stein", (old, val) -> old.substring(0, 3) + val); ``` ## Method Expressions `Class::staticMethod` Allows to reference methods (and constructors) without executing them ```java // Lambda Form: getPrimes(numbers, a -> StaticMethod.isPrime(a)); // Method Reference: getPrimes(numbers, StaticMethod::isPrime); ``` | Method Reference | Lambda Form | | ---------------- | ----------- | | `StaticMethod::isPrime` | `n -> StaticMethod.isPrime(n)` | | `String::toUpperCase` | `(String w) -> w.toUpperCase()` | | `String::compareTo` | `(String s, String t) -> s.compareTo(t)` | | `System.out::println` | `x -> System.out.println(x)` | | `Double::new` | `n -> new Double(n)` | | `String[]::new` | `(int n) -> new String[n]` | ## Streams Similar to collections, but - They don't store their own data - The data comes from elsewhere (collection, file, db, web, ...) - *immutable* (produce new streams) - *lazy* (only computes what is necessary !) ```java // Will compute just 3 "filter" Stream longNames = list .filter(n -> n.length() > 8) .limit(3); ``` **Create a new stream** ```java Stream stream = Stream.of(1, 2, 3, 5, 7, 11); Stream stream = Stream.of("Jazz", "Blues", "Rock"); Stream stream = Stream.of(myArray); // or from an array Stream stream = list.stream(); // or from a list // Infinit stream [0; inf[ Stream integers = Stream.iterate(0, n -> n + 1); ``` **Collecting results** ```java // Collect into an array (::new is the constructor reference) String[] myArray = stream.toArray(String[]::new); // Collect into a List or Set List myList = stream.collect(Collectors.toList()); Set mySet = stream.collect(Collectors.toSet()); // Collect into a String String str = list.collect(Collectors.joining(", ")); ``` **map** `map(mapper)`
Applying a function to each element ```java // Apply "toLowerCase" for each element Stream res = stream.map(w -> w.toLowerCase()); Stream res = stream.map(String::toLowerCase); //> bohr darwin galilei tesla einstein newton Stream res = Stream.of(1, 2, 3, 4, 5).map(x -> x + 1); //> 2 3 4 5 6 ``` **filter** `filter(predicate)`
Retains elements that match the predicate ```java // Filter elements that begin with "E" res = stream.filter(n -> n.substring(0, 1).equals("E")); //> Einstein res = Stream.of(1, 2, 3, 4, 5).filter(x -> x < 3); //> 1 2 ``` **reduce**
Reduce the elements to a single value ```java String reduced = stream.reduce("", (acc, el) -> acc + "|" + el); //> |Bohr|Darwin|Galilei|Tesla|Einstein|Newton ``` **limit** `limit(maxSize)`
Select the n first elements ```java res = stream.limit(3); //> Bohr Darwin Galilei ``` **skip**
Discarding the first n elements ```java res = stream.skip(2); // skip Bohr and Darwin //> Galilei Tesla Einstein Newton ``` **distinct**
Remove duplicated elemetns ```java res = Stream.of(1, 0, 0, 1, 0, 1).distinct(); //> 1 0 ``` **sorted**
Sort elements (must be *Comparable*) ```java res = stream.sorted(); //> Bohr Darwin Einstein Galilei Newton Tesla ``` **allMatch** / **noneMatch** ```java // Check if there is a "e" in each elements boolean res = words.allMatch(n -> n.contains("e")); ``` anyMatch: Check if there is a "e" in an element
noneMatch: Check if there is no "e" in elements **parallel**
Returns an equivalent stream that is parallel **findAny**
faster than findFirst on parallel streams ### Primitive-Type Streams Wrappers (like Stream) are inefficients. It requires a lot of unboxing and boxing for each element. Better to use `IntStream`, `DoubleStream`, etc. **Creation** ```java IntStream stream = IntStream.of(1, 2, 3, 5, 7); stream = IntStream.of(myArray); // from an array stream = IntStream.range(5, 80); // range from 5 to 80 Random gen = new Random(); IntStream rand = gen(1, 9); // stream of randoms ``` Use *mapToX* (mapToObj, mapToDouble, etc.) if the function yields Object, double, etc. values. ### Grouping Results **Collectors.groupingBy** ```java // Groupe by length Map> groups = stream.collect(Collectors.groupingBy(w -> w.length())); //> {4=[Bohr], 5=[Tesla], 6=[Darwin, Newton], 7=[Galilei], 8=[Einstein]} ``` **Collectors.toSet** ```java // Same as before but with Set Map> groups2 = stream.collect(Collectors.groupingBy(w -> w.substring(0, 1), Collectors.toSet())); //> {B=[Bohr], T=[Tesla], D=[Darwin], E=[Einstein], G=[Galilei], N=[Newton]} ``` **Collectors.counting**
Count the number of values in a group **Collectors.summing__**
`summingInt`, `summingLong`, `summingDouble` to sum group values **Collectors.averaging__**
`averagingInt`, `averagingLong`, ... to average group values ```java // Average length of each element of a group Collectors.averagingInt(String::length) ``` *PS*: Don't forget Optional (like `Map>`) with some Collection methods (like `Collectors.maxBy`). ### Parallel Streams **Creation** ```java Stream parStream = list.parallelStream(); Stream parStream = Stream.of(myArray).parallel(); ``` **unordered** Can speed up the `limit` or `distinct` ```java stream.parallelStream().unordered().distinct(); ``` *PS*: Work with the streams library. Eg. use `filter(x -> x.length() < 9)` instead of a `forEach` with an `if`. ## Optional In Java, it is common to use null to denote absence of result.
Problems when no checks: `NullPointerException`. ```java // Optional contains a string or nothing Optional res = stream .filter(w -> w.length() > 10) .findFirst(); // length of the value or "" if nothing int length = res.orElse("").length(); // run the lambda if there is a value res.ifPresent(v -> results.add(v)); ``` Return an Optional ```java Optional squareRoot(double x) { if (x >= 0) { return Optional.of(Math.sqrt(x)); } else { return Optional.empty(); } } ``` --- **Note on inferance limitations** ```java interface Pair { A first(); B second(); } ``` A steam of type `Stream>` : - `stream.sorted(Comparator.comparing(Pair::first)) // ok` - `stream.sorted(Comparator.comparing(Pair::first).thenComparing(Pair::second)) // dont work` Java cannot infer type for the `.comparing(Pair::first)` part and fallback to Object, on which `Pair::first` cannot be applied. The required type for the whole expression cannot be propagated through the method call (`.thenComparing`) and used to infer type of the first part. Type *must* be given explicitly. ```java stream.sorted( Comparator., String>comparing(Pair::first) .thenComparing(Pair::second) ) // ok ``` --- This cheat sheet was based on the lecture of Cay Horstmann http://horstmann.com/heig-vd/spring2015/poo/ ================================================ FILE: README_CN.md ================================================ # JAVA 8 - 备忘录 ## Lambda 表达式 ```java (int a) -> a * 2; // 求a乘以2后的值 a -> a * 2; // 或者更直接的去掉类型也是可以的 ``` ```java (a, b) -> a + b; // 相加 ``` 如果lambda里面的代码块超过1行，可以配合使用 `{ }` 加 `return`来处理 ```java (x, y) -> { int sum = x + y; int avg = sum / 2; return avg; } ``` 一个lamdba表达式必须依赖一个具体的功能接口而存在 ```java interface MyMath { int getDoubleOf(int a); } MyMath d = a -> a * 2; // 关联到具体的接口实现 d.getDoubleOf(4); // is 8 ``` --- 下面所有的测试都是用到这个`list` : ```java List list = [Bohr, Darwin, Galilei, Tesla, Einstein, Newton] ``` ## Collections 集合 **sort** `sort(list, comparator)` ```java list.sort((a, b) -> a.length() - b.length()) list.sort(Comparator.comparing(n -> n.length())); // 使用具体Comparator接口实现 list.sort(Comparator.comparing(String::length)); // 这样写和上面也是一样的 //> [Bohr, Tesla, Darwin, Newton, Galilei, Einstein] ``` **removeIf** ```java list.removeIf(w -> w.length() < 6); //> [Darwin, Galilei, Einstein, Newton] ``` **merge** `merge(key, value, remappingFunction)` ```java Map names = new HashMap<>(); names.put("Albert", "Ein?"); names.put("Marie", "Curie"); names.put("Max", "Plank"); // "Albert" 这个值是存在的 就命中了后面处理流程 // {Marie=Curie, Max=Plank, Albert=Einstein} names.merge("Albert", "stein", (old, val) -> old.substring(0, 3) + val); // "Newname" 这个值是不存在的 所以后面的流程就不处理 // {Marie=Curie, Newname=stein, Max=Plank, Albert=Einstein} names.merge("Newname", "stein", (old, val) -> old.substring(0, 3) + val); ``` ## 方法引用 `Class::staticMethod` 允许引用类方法或者构造函数，引用时候是不执行的 ```java //通过lamdba getPrimes(numbers, a -> StaticMethod.isPrime(a)); //通过应用方法: getPrimes(numbers, StaticMethod::isPrime); ``` | Method Reference | Lambda Form | | ---------------- | ----------- | | `StaticMethod::isPrime` | `n -> StaticMethod.isPrime(n)` | | `String::toUpperCase` | `(String w) -> w.toUpperCase()` | | `String::compareTo` | `(String s, String t) -> s.compareTo(t)` | | `System.out::println` | `x -> System.out.println(x)` | | `Double::new` | `n -> new Double(n)` | | `String[]::new` | `(int n) -> new String[n]` | ## Streams 流式处理 和`collections`类似, 但有所不同 - 不能储存数据 - 数据来源外部例如 (collection, file, db, web, ...) - `immutable`不可变性，不影响外部数据 (因为产生了一个新的stream) - `lazy`懒式处理 (只有在计算的时候才用到，不处理不用 !) ```java // 仅仅计算前3个"filter" Stream longNames = list .filter(n -> n.length() > 8) .limit(3); ``` **创建一个stream** ```java Stream stream = Stream.of(1, 2, 3, 5, 7, 11); Stream stream = Stream.of("Jazz", "Blues", "Rock"); Stream stream = Stream.of(myArray); // 通过数组 Stream stream = list.stream(); // 通过list // Infinit stream [0; inf[ Stream integers = Stream.iterate(0, n -> n + 1); ``` **集合结果集** ```java //返回成一个数组 (::new 是构造函数的引用) String[] myArray = stream.toArray(String[]::new); // 返回成list或set List myList = stream.collect(Collectors.toList()); Set mySet = stream.collect(Collectors.toSet()); // 返回成String String str = list.collect(Collectors.joining(", ")); // 返回成一个LinkedHashMap list.stream().collect(Collectors.toMap(k -> k, v -> v, (a, b) -> a, LinkedHashMap::new)); // 默认转换成HashMap list.stream().collect(Collectors.toMap(k -> k, v -> v)); ``` **map** `map(mapper)`
对每个元素进行类型转换 ```java // 对每个元素使用 "toLowerCase" 处理 Stream res = stream.map(w -> w.toLowerCase()); Stream res = stream.map(String::toLowerCase); //> bohr darwin galilei tesla einstein newton Stream res = Stream.of(1, 2, 3, 4, 5).map(x -> x + 1); //> 2 3 4 5 6 ``` **filter** `filter(predicate)`
过滤处理，只保留匹配到的元素 ```java // 过掉保留 "E" 开头的元素 res = stream.filter(n -> n.substring(0, 1).equals("E")); //> Einstein res = Stream.of(1, 2, 3, 4, 5).filter(x -> x < 3); //> 1 2 ``` **reduce**
汇聚处理成为单一返回结果 ```java String reduced = stream.reduce("", (acc, el) -> acc + "|" + el); //> |Bohr|Darwin|Galilei|Tesla|Einstein|Newton ``` **limit** `limit(maxSize)`
保留前`maxSize`个元素 ```java res = stream.limit(3); //> Bohr Darwin Galilei ``` **skip**
忽略掉前`n`个元素 ```java res = stream.skip(2); // 忽略 Bohr 和 Darwin //> Galilei Tesla Einstein Newton ``` **distinct**
去除重复的元素 ```java res = Stream.of(1, 0, 0, 1, 0, 1).distinct(); //> 1 0 ``` **sorted**
排序 (必须使用 *Comparable* 接口) ```java res = stream.sorted(); //> Bohr Darwin Einstein Galilei Newton Tesla ``` **allMatch** / **noneMatch** ```java // 检查是否每个元素都是“e”开头 boolean res = words.allMatch(n -> n.contains("e")); ``` `anyMatch`: 只要其中一个元素包含"e"即可
`noneMatch`: 元素里面是否没有"e" **parallel**
返回一个并行的stream **findAny**
在并行流上findFirst执行更快 ### 原始类型的 Streams 原子类型的stream自动封装是低效的 (例如 Stream) ，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. **初始化** ```java IntStream stream = IntStream.of(1, 2, 3, 5, 7); stream = IntStream.of(myArray); // 通过数组 stream = IntStream.range(5, 80); // 5 到 80范围 Random gen = new Random(); IntStream rand = gen(1, 9); // stream of randoms ``` 使用 *mapToX* (mapToObj, mapToDouble, mapToLong) 如果需要把字段转换成 Object, double, long的话. values. ### Grouping 数据集 **Collectors.groupingBy** ```java // 通过长度分组 Map> groups = stream.collect(Collectors.groupingBy(w -> w.length())); //> 4=[Bohr], 5=[Tesla], 6=[Darwin, Newton], ... ``` **Collectors.toSet** ```java // 和之前一样但是使用的是Set Map> groups2 = stream.collect(Collectors.groupingBy(w -> w.substring(0, 1), Collectors.toSet())); //> {B=[Bohr], T=[Tesla], D=[Darwin], E=[Einstein], G=[Galilei], N=[Newton]} ``` **Collectors.counting**
获取元素总算 **Collectors.summing__**
`summingInt`, `summingLong`, `summingDouble` 计算所有元素值相加后结果 **Collectors.averaging__**
`averagingInt`, `averagingLong`, ... 计算所有元素值的算术平均值 ```java // 计算平均数 Collectors.averagingInt(String::length) ``` *PS*: 另外不要忘记 Optional (例如 `Map>`) 有同样的处理方法 (例如 `Collectors.maxBy`). ### 并行 Streams **创建一个并行处理的stream** ```java Stream parStream = list.parallelStream(); Stream parStream = Stream.of(myArray).parallel(); ``` **unordered** 能提高计算 `limit`，`distinct`的速度 ```java stream.parallelStream().unordered().distinct(); ``` *PS*: 使用streams类库, 例如使用 `filter(x -> x.length() < 9)` 代替 `forEach` 和 `if` ## Optional 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`. ```java // Optional 包含一个string和空 Optional res = stream .filter(w -> w.length() > 10) .findFirst(); // 返回元素长度或者返回 "" 如果没有的话 int length = res.orElse("").length(); // 使用lambda作为一个返回值 res.ifPresent(v -> results.add(v)); ``` 返回一个 Optional ```java Optional squareRoot(double x) { if (x >= 0) { return Optional.of(Math.sqrt(x)); } else { return Optional.empty(); } } ``` --- **注意引用推测限制** ```java interface Pair { A first(); B second(); } ``` 一个 steam 类型 `Stream>` : - `stream.sorted(Comparator.comparing(Pair::first)) // 有效` - `stream.sorted(Comparator.comparing(Pair::first).thenComparing(Pair::second)) // 无效` Java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 如果需要使用泛型接口的话需要显示写清楚，否则无效 ```java stream.sorted( Comparator., String>comparing(Pair::first) .thenComparing(Pair::second) ) // 有效 ``` --- This cheat sheet was based on the lecture of Cay Horstmann http://horstmann.com/heig-vd/spring2015/poo/ ================================================ FILE: README_ES.md ================================================ # JAVA 8 - Chuleta ## Expresión Lambda ```java (int a) -> a * 2; // Calcula el docble de a a -> a * 2; // o simplemente sin tipo ``` ```java (a, b) -> a + b; // Suma 2 parametros ``` Si lambda tiene mas de una expresión podemos usar `{ }` y `return` ```java (x, y) -> { int sum = x + y; int avg = sum / 2; return avg; } ``` Una expresión lambda no puede usarse sola en Java, necesita estar asociada a un interfaz funcional ```java interface MyMath { int getDoubleOf(int a); } MyMath d = a -> a * 2; // asociado a un interfaz d.getDoubleOf(4); // es 8 ``` --- Todos los ejemplos con "list" usan: ```java List list = [Bohr, Darwin, Galilei, Tesla, Einstein, Newton] ``` ## Colecciones **sort** `sort(list, comparator)` ```java list.sort((a, b) -> a.length() - b.length()) list.sort(Comparator.comparing(n -> n.length())); // igual list.sort(Comparator.comparing(String::length)); // igual //> [Bohr, Tesla, Darwin, Newton, Galilei, Einstein] ``` **removeIf** ```java list.removeIf(w -> w.length() < 6); //> [Darwin, Galilei, Einstein, Newton] ``` **merge** `merge(key, value, remappingFunction)` ```java Map names = new HashMap<>(); names.put("Albert", "Ein?"); names.put("Marie", "Curie"); names.put("Max", "Plank"); // El valor "Albert" existe // {Marie=Curie, Max=Plank, Albert=Einstein} names.merge("Albert", "stein", (old, val) -> old.substring(0, 3) + val); // El valor "Newname" no existe // {Marie=Curie, Newname=stein, Max=Plank, Albert=Einstein} names.merge("Newname", "stein", (old, val) -> old.substring(0, 3) + val); ``` ## Expresión de metodo `Class::staticMethod` Permite referenciar métodos (y constructores) sin ejecutarlos ```java // Con Lambda: getPrimes(numbers, a -> StaticMethod.isPrime(a)); // Metodo Referenciado: getPrimes(numbers, StaticMethod::isPrime); ``` | Método Referenciado | Forma de Lambda | | ------------------- | --------------- | | `StaticMethod::isPrime` | `n -> StaticMethod.isPrime(n)` | | `String::toUpperCase` | `(String w) -> w.toUpperCase()` | | `String::compareTo` | `(String s, String t) -> s.compareTo(t)` | | `System.out::println` | `x -> System.out.println(x)` | | `Double::new` | `n -> new Double(n)` | | `String[]::new` | `(int n) -> new String[n]` | ## Streams Similares a las colecciones, pero: - No almacenan su propia información - La información viene de otra parte (colleciones, archivos, db, web, ...) - *immutable* (crean un nuevo stream) - *lazy* (solo computa lo que es necesario !) ```java // Computara 3 "filter" Stream longNames = list .filter(n -> n.length() > 8) .limit(3); ``` **Crea un nuevo stream** ```java Stream stream = Stream.of(1, 2, 3, 5, 7, 11); Stream stream = Stream.of("Jazz", "Blues", "Rock"); Stream stream = Stream.of(myArray); // or from an array Stream stream = list.stream(); // or from a list // Stream infinito [0; inf[ Stream integers = Stream.iterate(0, n -> n + 1); ``` **Resultado de las colecciones** ```java // Collect into an array (::new is the constructor reference) // Colecciona en un array (::new es la referencia del contructor) String[] myArray = stream.toArray(String[]::new); // Collecciona en un List o Set List myList = stream.collect(Collectors.toList()); Set mySet = stream.collect(Collectors.toSet()); // Collecciona en un String String str = list.collect(Collectors.joining(", ")); ``` **map** `map(mapper)`
Aplica una función a cada elemento ```java // Aplica "toLowerCase" a cada elemento Stream res = stream.map(w -> w.toLowerCase()); Stream res = stream.map(String::toLowerCase); //> bohr darwin galilei tesla einstein newton Stream res = Stream.of(1, 2, 3, 4, 5).map(x -> x + 1); //> 2 3 4 5 6 ``` **filter** `filter(predicado)`
Retiene elementos que coinciden con el predicado ```java // Filtra elementos que empiecen con "E" res = stream.filter(n -> n.substring(0, 1).equals("E")); //> Einstein res = Stream.of(1, 2, 3, 4, 5).filter(x -> x < 3); //> 1 2 ``` **reduce**
Reduce los elementos a un unico valor ```java String reduced = stream.reduce("", (acc, el) -> acc + "|" + el); //> |Bohr|Darwin|Galilei|Tesla|Einstein|Newton ``` **limit** `limit(maxSize)`
Los n primeros elementos ```java res = stream.limit(3); //> Bohr Darwin Galilei ``` **skip**
Descarta los primeros n elementos ```java res = strem.skip(2); // skip Bohr and Darwin //> Galilei Tesla Einstein Newton ``` **distinct**
Borra los elementos repetidos ```java res = Stream.of(1, 0, 0, 1, 0, 1).distinct(); //> 1 0 ``` **sorted**
Ordena elementos (debe ser *Comparable*) ```java res = stream.sorted(); //> Bohr Darwin Einstein Galilei Newton Tesla ``` **allMatch** / **noneMatch** ```java // Comprueba si hay una "e" en cada elemento boolean res = words.allMatch(n -> n.contains("e")); ``` anyMatch: Comprueba si hay una "e" en algun elemento
noneMatch: Comprueba si no hay una "e" en ningun elemento **parallel**
Devuelve un stream equivalente que es paralelo **findAny**
Mas rapido que findFirst en un stream paralelo ### Streams de tipo primitivo Los wrappers (como Stream) son ineficientes. Requieren de embalar y desembalar cada elemento demasiado. Mejor usar `IntStream`, `DoubleStream`, etc. **Creacion** ```java IntStream stream = IntStream.of(1, 2, 3, 5, 7); stream = IntStream.of(myArray); // de un array stream = IntStream.range(5, 80); // rango de 5 a 80 Random gen = new Random(); IntStream rand = gen(1, 9); // stream de aleatorios ``` Usa *mapToX* (mapToObj, mapToDouble, etc.) si la función produce un valor Object, double, etc. ### Resultados agrupados **Collectors.groupingBy** ```java // Agrupados por longitud Map> groups = stream.collect(Collectors.groupingBy(w -> w.length())); //> {4=[Bohr], 5=[Tesla], 6=[Darwin, Newton], 7=[Galilei], 8=[Einstein]} ``` **Collectors.toSet** ```java // Igual que antes pero en un Set Map> groups2 = stream.collect(Collectors.groupingBy(w -> w.substring(0, 1), Collectors.toSet())); //> {B=[Bohr], T=[Tesla], D=[Darwin], E=[Einstein], G=[Galilei], N=[Newton]} ``` **Collectors.counting**
Cuenta el numero de elementos en una coleccion **Collectors.summing__**
`summingInt`, `summingLong`, `summingDouble` para sumar valores de un grupo **Collectors.averaging__**
`averagingInt`, `averagingLong`, ... ```java // Longitud promedio de cada elemento de un grupo Collectors.averagingInt(String::length) ``` *PS*: No olvides Optional (como `Map>`) con algunos metodos de Colecciones (like `Collectors.maxBy`). ### Streams Paralelos **Creacion** ```java Stream parStream = list.parallelStream(); Stream parStream = Stream.of(myArray).parallel(); ``` **unordered** Pueden acelerar el `limit` o `distinct` ```java stream.parallelStream().unordered().distinct(); ``` *PS*: Trabaja con la librería de streams. Pe: usa `filter(x -> x.length() < 9)` en vez de `forEach` con un`if`. ## Optional En Java, es común usar null para denotar ausencia de resultado.
Problemas cuando no se comprueban: `NullPointerException`. ```java // Optional contiene un string o nada Optional res = stream .filter(w -> w.length() > 10) .findFirst(); // longitud de res o "", si no trae nada int length = res.orElse("").length(); // lanza el lambda si no trae nada res.ifPresent(v -> results.add(v)); ``` Devuelve un Optional ```java Optional squareRoot(double x) { if (x >= 0) { return Optional.of(Math.sqrt(x)); } else { return Optional.empty(); } } ``` --- **Note en la inferencia de la limitación** ```java interface Pair { A first(); B second(); } ``` Un stream de tipo `Stream>` : - `stream.sorted(Comparator.comparing(Pair::first)) // vale` - `stream.sorted(Comparator.comparing(Pair::first).thenComparing(Pair::second)) // no funciona` Java 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. El 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. El tipo *debe* ser dado explicitamente ```java stream.sorted( Comparator., String>comparing(Pair::first) .thenComparing(Pair::second) ) // ok ``` --- Esta cheat sheet esta basada en la lección de Cay Horstmann http://horstmann.com/heig-vd/spring2015/poo/