java8 stream

• Stream是java8 推出的一個處理集合類的一個全新的接口，在工作當中經常用到，并且他的預發規則與我們平臺的有一點不一樣，是流式的處理，非常類似RXJava的一個風格。
• java中對Stream的定義是：

A sequence of elements supporting sequential and parallel aggregate operations.

• 以上的的語義包含兩部分：

  • Stream是元素的集合，這點讓Stream看起來有一些類似Iterator
  • 可以支持順序和并行的對原Stream進行匯聚的操作

• 剛學習Stream時候可以當一個高級的Iterator，原本的Iterator，用戶只能逐個遍歷每個元素，但是Stream用戶只需要給出對其包含元素執行什么操作，也就是通過給Stream傳遞某一個的操作符或者方法，他會將集合中所有元素使用你給定的方法或者操作符進行修改。

• 以下總結是我工作中用到的一些，會給出具體案例，初衷只是為了給自己一個參考，因為經常是在使用的時候，會忘記他的語法規則，或者忘記他的一些參數，以此作為一個記錄。

• 以下用到的基礎對象：

/*** FM表Result*/ public class FmResult implements Serializable {public FmResult() {}public FmResult(Integer sortNo, String platform) {this.sortNo = sortNo;this.platform = platform;}/*** 序列化ID*/private static final long serialVersionUID = 7327233719573948963L;/*** 主鍵ID*/private Long id;/*** 排序值*/private Integer sortNo;/*** FM名稱*/private String fmName;/*** 開放平臺號，多個半角逗號隔開*/private String platform;public Long getId() {return id;}public void setId(Long id) {this.id = id;}public Integer getSortNo() {return sortNo;}public void setSortNo(Integer sortNo) {this.sortNo = sortNo;}public String getFmName() {return fmName;}public void setFmName(String fmName) {this.fmName = fmName;}public String getPlatform() {return platform;}public void setPlatform(String platform) {this.platform = platform;} }

Stream 流操作

/*** Stream 流操作** 類似Rxjava中的流操作* */public static void streamBuildList(){//stream 快速成成listList<Integer> ofList = Stream.of(1,2,3,4,5).collect(Collectors.toList());//安指定規律生成流List<Integer> iteraterList = Stream.iterate(1, (x) -> x+2).limit(100).collect(Collectors.toList());//生成隨機數流List<Double> generateList = Stream.generate(Math::random).limit(199).collect(Collectors.toList());}

Stream 流元素操作

/*** Stream 流中間操作* distinct 更具hashCode，equals 反復去重* filter過濾* skip 跳過n個元素* limit獲取n個元素* */public static void streamTest(){//生成隨機數流List<Double> generateList = Stream.generate(Math::random).limit(199).collect(Collectors.toList());List<Double> myDubleLIst = generateList.stream().filter(d -> d > 0.5).distinct().skip(4).limit(20).collect(Collectors.toList());myDubleLIst.forEach(System.out::println);}

Stream map表達式映射

/*** Stream map表達式映射* */public static void testStreamMap(List<FmResult> list){//傳遞一個函數給map，他會映射到每一個元素上Set<String> resultlist = list.stream().map(result -> result.getPlatform().replace(",", "")).collect(Collectors.toSet());//復雜表達式處理List<FmResult> fmResults = list.stream().map( result -> {result.setPlatform(result.getPlatform().substring(0,1));return result;}).collect(Collectors.toList());}

Stream peek消費元素

/*** Stream peek消費元素* peek 和map操作一樣，當peek接受的是一個Consumer 表達式沒有返回值，map接受的是Function表達式有返回值* */public static void testStreamPeek(){FmResult result1 = new FmResult(1,"aa");FmResult result2 = new FmResult(2,"bb");FmResult result3 = new FmResult(3,"aa");FmResult result4 = new FmResult(4,"dd");List<FmResult> fmResults = Arrays.asList(result1,result2, result3, result4);List<FmResult> newResults = fmResults.stream().peek(result -> result.setFmName("123123")).collect(Collectors.toList());for (FmResult newResult: newResults) {System.out.println(newResult.getSortNo() + ": "+ newResult.getPlatform() + ": "+ newResult.getFmName());}}

Stream flatMapToInt,flatMapToDouble,flatMapToLong

/*** Stream flatMapToInt,flatMapToDouble,flatMapToLong* 一對多，將一個元素拆分成多個元素，流元素變多，map是一對一* */public static void testStreamFlagMap(){FmResult result1 = new FmResult(1,"aa");FmResult result2 = new FmResult(2,"bb");FmResult result3 = new FmResult(3,"aa");FmResult result4 = new FmResult(4,"dd");List<FmResult> fmResults = Arrays.asList(result1,result2, result3, result4);IntStream platformStream = fmResults.stream().map(FmResult::getPlatform).flatMapToInt(String::chars);int[] ints = platformStream.toArray();System.out.println(ints.length +" :"+ ints[0]);}

Stream 流的終止操作

/*** Stream 流的終止* allMatch：接收一個 Predicate 函數，當流中每個元素都符合該斷言時才返回true，否則返回false* noneMatch：接收一個 Predicate 函數，當流中每個元素都不符合該斷言時才返回true，否則返回false* anyMatch：接收一個 Predicate 函數，只要流中有一個元素滿足該斷言則返回true，否則返回false* findFirst：返回流中第一個元素* findAny：返回流中的任意元素* count：返回流中元素的總個數* max：返回流中元素最大值* min：返回流中元素最小值* */public static void testStreamTermal(){List<String> strList = Arrays.asList("a2","a1","b4","c1","d5","a6");boolean allMatch = strList.stream().allMatch(str -> str.equals("a1"));boolean noneMatch = strList.stream().noneMatch(str -> str.equals("x"));boolean anyMatch = strList.stream().anyMatch(str -> str.equals("a1"));String findFirst = strList.stream().findFirst().get();String findAny = strList.stream().findAny().get();Long count = strList.stream().count();String max = strList.stream().max(String::compareTo).get();String min = strList.stream().min(String::compareTo).get();}

Stream 排序處理

/*** Stream 排序處理* */public static void testStreamSort(){//String 自己已經實現了Compareable接口List<String> strList = Arrays.asList("a2","a1","b4","c1","d5","a6");strList.stream().sorted().forEach(System.out::print);System.out.println();//自實現Compareable接口FmResult result1 = new FmResult(1,"aa");FmResult result2 = new FmResult(2,"bb");FmResult result3 = new FmResult(3,"aa");FmResult result4 = new FmResult(4,"dd");List<FmResult> fmResults = Arrays.asList(result1,result2, result3, result4);List<FmResult> sortResults = fmResults.stream().sorted((k1,k2)->{if(k1.getPlatform().equals(k2.getPlatform())){return k1.getSortNo()-k2.getSortNo();}else {return k1.getPlatform().compareTo(k2.getPlatform());}}).collect(Collectors.toList());for (FmResult sortResult : sortResults) {System.out.println(sortResult.getSortNo() + ": "+ sortResult.getPlatform());}}

Stream 表達式list 轉 map

/*** Stream 表達式list -->map* (key1, key2) -> key1 作用在于如果存在相同id的數據，取第一個* */public static Map<Long, FmResult> streamListTOMap(List<FmResult> list){return list.stream().collect(Collectors.toMap(FmResult::getId, Function.identity(), (key1, key2) -> key1));}

Stream groupingBy分組

/*** Stream 表達式 list --> map groupingBy分組* */public static void streamGroupingBy(List<FmResult> list){Map<String, List<FmResult>> fmMap = list.stream().collect(Collectors.groupingBy(FmResult::getPlatform));//按排平臺分組，并返回數量Map<String, Long> fmCountMap = list.stream().collect(Collectors.groupingBy(FmResult::getPlatform, Collectors.counting()));}

Stream partitioningBy分區

/*** Stream partitioningBy分區* 按排序值 > 2 分組* */public static Map<Boolean, List<FmResult>> streamPartitioningBy(List<FmResult> list){return list.stream().collect(Collectors.partitioningBy(x -> x.getSortNo() > 2));}

Stream 連接 joining

/*** Stream 連接 joining* */public static void streamJoining(){FmResult result1 = new FmResult(1,"aa");FmResult result2 = new FmResult(2,"bb");FmResult result3 = new FmResult(3,"aa");FmResult result4 = new FmResult(4,"dd");List<FmResult> fmResults = Arrays.asList(result1,result2, result3, result4);String platform = fmResults.stream().map(result -> result.getPlatform()).collect(Collectors.joining(","));System.out.println(platform);}

Stream 規約 reducing

/*** Stream 規約 reducing* */public static void streamReducing(){FmResult result1 = new FmResult(1,"aa");FmResult result2 = new FmResult(2,"bb");FmResult result3 = new FmResult(3,"aa");FmResult result4 = new FmResult(4,"dd");List<FmResult> fmResults = Arrays.asList(result1,result2, result3, result4);Optional<Integer> sumSortNo = fmResults.stream().map(result ->result.getSortNo()).collect(Collectors.reducing(Integer::sum));System.out.println(sumSortNo.get());}

stream Collectors.averagingDouble()、Collectors.averagingInt()、Collectors.averagingLong() ： 計算平均數

/*** stream Collectors.averagingDouble()、Collectors.averagingInt()、Collectors.averagingLong() ： 計算平均數* */public static void streamAveraging(){FmResult result1 = new FmResult(1,"aa");FmResult result2 = new FmResult(2,"bb");FmResult result3 = new FmResult(3,"aa");FmResult result4 = new FmResult(4,"dd");List<FmResult> fmResults = Arrays.asList(result1,result2, result3, result4);Double avgSortNo = fmResults.stream().collect(Collectors.averagingInt(FmResult::getSortNo));}

其他api遇到了在補充

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