## 三、Trait Traversable
Scala中所有集合的顶层实现是`Traversable` 。它唯一的抽象方法是`foreach`：
```scala
def foreach[U](f: Elem => U)
```
实现Traversable的集合类只需要定义这个方法，所有其他方法都可以从Traversable继承。Traversable中定义了很多方法，如下:
> 下面表格来源于官方文档：https://docs.scala-lang.org/overviews/collections/trait-traversable.html
| 方法 | 描述 |
| ----------------------------------- | ------------------------------------------------------------ |
| **Abstract Method:** | |
| `xs foreach f` | Executes function `f` for every element of `xs`. |
| **Addition:** | |
| `xs ++ ys` | A collection consisting of the elements of both `xs` and `ys`. `ys` is a [TraversableOnce](http://www.scala-lang.org/api/current/scala/collection/TraversableOnce.html) collection, i.e., either a [Traversable](http://www.scala-lang.org/api/current/scala/collection/Traversable.html) or an [Iterator](http://www.scala-lang.org/api/current/scala/collection/Iterator.html). |
| **Maps:** | |
| `xs map f` | The collection obtained from applying the function f to every element in `xs`. |
| `xs flatMap f` | The collection obtained from applying the collection-valued function `f` to every element in `xs` and concatenating the results. |
| `xs collect f` | The collection obtained from applying the partial function `f` to every element in `xs` for which it is defined and collecting the results. |
| **Conversions:** | |
| `xs.toArray` | Converts the collection to an array. |
| `xs.toList` | Converts the collection to a list. |
| `xs.toIterable` | Converts the collection to an iterable. |
| `xs.toSeq` | Converts the collection to a sequence. |
| `xs.toIndexedSeq` | Converts the collection to an indexed sequence. |
| `xs.toStream` | Converts the collection to a lazily computed stream. |
| `xs.toSet` | Converts the collection to a set. |
| `xs.toMap` | Converts the collection of key/value pairs to a map. If the collection does not have pairs as elements, calling this operation results in a static type error. |
| **Copying:** | |
| `xs copyToBuffer buf` | Copies all elements of the collection to buffer `buf`. |
| `xs copyToArray(arr, s, n)` | Copies at most `n` elements of the collection to array `arr` starting at index `s`. The last two arguments are optional. |
| **Size info:** | |
| `xs.isEmpty` | Tests whether the collection is empty. |
| `xs.nonEmpty` | Tests whether the collection contains elements. |
| `xs.size` | The number of elements in the collection. |
| `xs.hasDefiniteSize` | True if `xs` is known to have finite size. |
| **Element Retrieval:** | |
| `xs.head` | The first element of the collection (or, some element, if no order is defined). |
| `xs.headOption` | The first element of `xs` in an option value, or None if `xs` is empty. |
| `xs.last` | The last element of the collection (or, some element, if no order is defined). |
| `xs.lastOption` | The last element of `xs` in an option value, or None if `xs` is empty. |
| `xs find p` | An option containing the first element in `xs` that satisfies `p`, or `None` if no element qualifies. |
| **Subcollections:** | |
| `xs.tail` | The rest of the collection except `xs.head`. |
| `xs.init` | The rest of the collection except `xs.last`. |
| `xs slice (from, to)` | A collection consisting of elements in some index range of `xs` (from `from` up to, and excluding `to`). |
| `xs take n` | A collection consisting of the first `n` elements of `xs` (or, some arbitrary `n` elements, if no order is defined). |
| `xs drop n` | The rest of the collection except `xs take n`. |
| `xs takeWhile p` | The longest prefix of elements in the collection that all satisfy `p`. |
| `xs dropWhile p` | The collection without the longest prefix of elements that all satisfy `p`. |
| `xs filter p` | The collection consisting of those elements of xs that satisfy the predicate `p`. |
| `xs withFilter p` | A non-strict filter of this collection. Subsequent calls to `map`, `flatMap`, `foreach`, and `withFilter` will only apply to those elements of `xs` for which the condition `p` is true. |
| `xs filterNot p` | The collection consisting of those elements of `xs` that do not satisfy the predicate `p`. |
| **Subdivisions:** | |
| `xs splitAt n` | Split `xs` at a position, giving the pair of collections `(xs take n, xs drop n)`. |
| `xs span p` | Split `xs` according to a predicate, giving the pair of collections `(xs takeWhile p, xs.dropWhile p)`. |
| `xs partition p` | Split `xs` into a pair of collections; one with elements that satisfy the predicate `p`, the other with elements that do not, giving the pair of collections `(xs filter p, xs.filterNot p)` |
| `xs groupBy f` | Partition `xs` into a map of collections according to a discriminator function `f`. |
| **Element Conditions:** | |
| `xs forall p` | A boolean indicating whether the predicate `p` holds for all elements of `xs`. |
| `xs exists p` | A boolean indicating whether the predicate `p` holds for some element in `xs`. |
| `xs count p` | The number of elements in `xs` that satisfy the predicate `p`. |
| **Folds:** | |
| `(z /: xs)(op)` | Apply binary operation `op` between successive elements of `xs`, going left to right and starting with `z`. |
| `(xs :\ z)(op)` | Apply binary operation `op` between successive elements of `xs`, going right to left and starting with `z`. |
| `xs.foldLeft(z)(op)` | Same as `(z /: xs)(op)`. |
| `xs.foldRight(z)(op)` | Same as `(xs :\ z)(op)`. |
| `xs reduceLeft op` | Apply binary operation `op` between successive elements of non-empty collection `xs`, going left to right. |
| `xs reduceRight op` | Apply binary operation `op` between successive elements of non-empty collection `xs`, going right to left. |
| **Specific Folds:** | |
| `xs.sum` | The sum of the numeric element values of collection `xs`. |
| `xs.product` | The product of the numeric element values of collection `xs`. |
| `xs.min` | The minimum of the ordered element values of collection `xs`. |
| `xs.max` | The maximum of the ordered element values of collection `xs`. |
| **Strings:** | |
| `xs addString (b, start, sep, end)` | Adds a string to `StringBuilder` `b` that shows all elements of `xs` between separators `sep` enclosed in strings `start` and `end`. `start`, `sep`, `end` are all optional. |
| `xs mkString (start, sep, end)` | Converts the collection to a string that shows all elements of `xs` between separators `sep` enclosed in strings `start` and `end`. `start`, `sep`, `end` are all optional. |
| `xs.stringPrefix` | The collection name at the beginning of the string returned from `xs.toString`. |
| **Views:** | |
| `xs.view` | Produces a view over `xs`. |
| `xs view (from, to)` | Produces a view that represents the elements in some index range of `xs`. |
## 四、Trait Iterable
Iterable拓展了Traversable，并定义了一些额外的方法，Scala中所有的集合都直接或者间接实现了Iterable。
> 下面表格来源于官方文档：https://docs.scala-lang.org/overviews/collections/trait-iterable.html
| 方法 | 描述 |
| ---------------------- | ------------------------------------------------------------ |
| **Abstract Method:** | |
| `xs.iterator` | An `iterator` that yields every element in `xs`, in the same order as `foreach` traverses elements. |
| **Other Iterators:** | |
| `xs grouped size` | An iterator that yields fixed-sized “chunks” of this collection. |
| `xs sliding size` | An iterator that yields a sliding fixed-sized window of elements in this collection. |
| **Subcollections:** | |
| `xs takeRight n` | A collection consisting of the last `n` elements of `xs` (or, some arbitrary `n` elements, if no order is defined). |
| `xs dropRight n` | The rest of the collection except `xs takeRight n`. |
| **Zippers:** | |
| `xs zip ys` | An iterable of pairs of corresponding elements from `xs` and `ys`. |
| `xs zipAll (ys, x, y)` | An iterable of pairs of corresponding elements from `xs` and `ys`, where the shorter sequence is extended to match the longer one by appending elements `x` or `y`. |
| `xs.zipWithIndex` | An iterable of pairs of elements from `xs` with their indices. |
| **Comparison:** | |
| `xs sameElements ys` | A test whether `xs` and `ys` contain the same elements in the same order |
## 五、修改集合
当你想对集合添加或者删除元素，需要根据不同的集合类型选择不同的操作符号。
| 操作符 | 描述 | 集合类型 |
| ------------------------------------------------------------ | ------------------------------------------------- | --------------------- |
| coll(k) 即coll.apply(k) | 获取指定位置的元素 | Seq, Map |
| coll :+ elem elem +: coll | 向集合末尾或者集合头增加元素 | Seq |
| coll + elem coll + (e1, e2, ...) | 追加元素 | Seq, Map |
| coll - elem coll - (e1, e2, ...) | 删除元素 | Set, Map, ArrayBuffer |
| coll ++ coll2 coll2 ++: coll | 合并集合 | Iterable |
| coll -- coll2 | 移除coll中包含的coll2中的元素 | Set, Map, ArrayBuffer |
| elem :: lst lst2 :: lst | 把指定列表(lst2)或者元素(elem)添加到列表(lst)头部 | List |
| list ::: list2 | 合并List | List |
| set \| set2 set & set2 set &~ set2 | 并集、交集、差集 | Set |
| coll += elem coll += (e1, e2, ...) coll ++= coll2 coll -= elem coll -= (e1, e2, ...) coll --= coll2 | 添加或者删除元素，并将修改后的结果赋值给集合本身 | 可变集合 |
| elem +=: coll coll2 ++=: coll | 在集合头部追加元素或集合 | ArrayBuffer |
## 参考资料
1. Martin Odersky . Scala编程(第3版)[M] . 电子工业出版社 . 2018-1-1
2. 凯.S.霍斯特曼 . 快学Scala(第2版)[M] . 电子工业出版社 . 2017-7
3. https://docs.scala-lang.org/overviews/collections/overview.html
4. https://docs.scala-lang.org/overviews/collections/trait-traversable.html
5. https://docs.scala-lang.org/overviews/collections/trait-iterable.html
