JDK8 Stream

轉載：https://www.bilibili.com/video/BV1VQ4y1P7Fq?spm_id_from=333.1007.top_right_bar_window_history.content.click

概念

Stream是Java8 API的新成員，它允許以宣告性方式處理資料集合 ，

特點

（1）代碼簡潔：函式式編程寫出的代碼簡潔且意圖明確，使用stream介面讓你從此告別for回圈，

（2）多核友好：Java函式式編程使得撰寫并行程式從未如此簡單，你需要的全部就是呼叫一下方法，

流程

1）第一步：把集合轉換為流stream
2）第二步：操作stream流
stream流在管道中經過中間操作（intermediate operation）的處理，最后由最終操作(terminal operation)得到前面處理的結果

運算子

兩種：中間運算子、終止運算子

中間運算子

示例代碼：

1）filter

/**
 * 功能描述:根據條件過濾集合資料
 * @return : void
 */
@Test
public void filter(){
    List<String> strings = Arrays.asList("abc", "", "bc", "efg", "abcd","", "jkl");
    List<String> filtered = strings.stream().filter(string -> !string.isEmpty()).collect(Collectors.toList());
    out.println(filtered);
}

2）distinct

/**
 * 功能描述:去除集合中重復資料
 * @return : void
 */
@Test
public void distinct(){
    List<String> strings = Arrays.asList("abc", "abc", "bc", "efg", "abcd","jkl", "jkl");
    List<String> distincted = strings.stream().distinct().collect(Collectors.toList());
    out.println(distincted);
}

3）limit

/**
 * 功能描述:指定獲取集合前x條資料，重新構造一個新的集合
 * @return : void
 */
@Test
public void limit(){
    List<String> strings = Arrays.asList("abc", "abc", "bc", "efg", "abcd","jkl", "jkl");
    List<String> limited = strings.stream().limit(3).collect(Collectors.toList());
    out.println(limited);
}

4）skip

/**
 * 功能描述:排除集合前x條資料，把后面的資料重新構造一個新的集合
 * @return : void
 */
@Test
    public void skip(){
    List<String> strings = Arrays.asList("abc", "abc", "bc", "efg", "abcd","jkl", "jkl");
    List<String> skiped = strings.stream().skip(3).collect(Collectors.toList());
    out.println(skiped);
}

5）map

/**
 * 功能描述:對集合中所有元素統一處理
 * @return : void
 */
@Test
public void map(){
    List<String> strings = Arrays.asList("abc", "abc", "bc", "efg", "abcd","jkl", "jkl");
    List<String> mapped = strings.stream().map(str->str+"-itcast").collect(Collectors.toList());
    out.println(mapped);
}

6）flatMap

/**
 * 功能描述:對集合中所有元素統一處理
 * @return : void
 */
@Test
public void flatMap(){
    List<String> strings = Arrays.asList("abc", "abc", "bc", "efg", "abcd","jkl", "jkl");
    Stream<String> stringStream = strings.stream().map(x -> x);
    Stream<String> stringStream1 = strings.stream().flatMap(x -> Arrays.asList(x.split(" ")).stream());
}

7）sorted

/**
 * 功能描述 : 對集合進行排序
 * @return : void
 */
@Test
public void sorted(){
    List<String> strings1 = Arrays.asList("abc", "abd", "aba", "efg", "abcd","jkl", "jkl");
    List<String> strings2 = Arrays.asList("張三", "李四", "王五", "趙柳", "張哥","李哥", "王哥");
    List<Integer> strings3 = Arrays.asList(10, 2, 30, 22, 1,0, -9);
    List<String> sorted1 = strings1.stream().sorted().collect(Collectors.toList());
    List<String> sorted2 = strings2.stream().sorted(Collections.reverseOrder(Collator.getInstance(Locale.CHINA))).collect(Collectors.toList());
    List<Integer> sorted3 = strings3.stream().sorted().collect(Collectors.toList());
    out.println(sorted1);
    out.println(sorted2);
    out.println(sorted3);
}

Map、flatMap區別

map：對流中每一個元素進行處理
flatMap：流扁平化，讓你把一個流中的“每個值”都換成另一個流，然后把所有的流連接起來成為一個流 
總結：map是對一級元素進行操作，flatmap是對二級元素操作，

本質區別：map回傳一個值；flatmap回傳一個流，多個值，

應用場景：map對集合中每個元素加工,回傳加工后結果；flatmap對集合中每個元素加工后，做扁平化處理后（拆分層級，放到同一層）然后回傳

/**
 * 方法一
 * 功能描述:  通過使用map、flatMap把字串轉換為字符輸出對比區別
 * @return : void
 */
@Test
public void flatMap2Map(){
    List<String> strings = Arrays.asList("abc", "abc", "bc", "efg", "abcd","jkl", "jkl");
    final Stream<Character> flatMap = strings.stream().flatMap(Java8StreamTest::getCharacterByString);
    flatMap.forEach(System.out::println);
    //----------------------------------------------
    final Stream<Stream<Character>> mapStream = strings.stream().map(Java8StreamTest::getCharacterByString);
    //mapStream.forEach(System.out::println);
    out.println("------------------------------------------------");
    mapStream.forEach(stream-> {stream.forEach(character->{System.out.println(character);});});

}

公共方法（字串轉換為字符流）

/**
* 功能描述:字串轉換為字符流
* @param str
* @return : java.util.stream.Stream<java.lang.Character>
*/
public static Stream<Character> getCharacterByString(String str) {
    List<Character> characterList = new ArrayList<>();
    for (Character character : str.toCharArray()) {
    	characterList.add(character);
    }
    return characterList.stream();
}

終止運算子

示例代碼

1）anyMatch

/**
 * 功能描述 : 判斷集合中是否至少存在一個元素滿足條件
 * @return : void
 */
@Test
public void anyMatch(){
    List<String> strings = Arrays.asList("abc", "abd", "aba", "efg", "abcd","jkl", "jkl");
    boolean b = strings.stream().anyMatch(s -> s == "abc");
    out.println(b);
}

2）allMatch

/**
 * 功能描述 : 判斷集合中是否所有元素都滿足條件
 * @return : void
 */
@Test
public void allMatch(){
    List<String> strings = Arrays.asList("abc", "abd", "aba", "efg", "abcd","jkl", "jkl");
    boolean b = strings.stream().allMatch(s -> s == "abc");
    out.println(b);
}

3）noneMatch

/**
 * 功能描述 : 判斷集合中是否所有元素都不滿足條件
 * @return : void
 */
@Test
public void noneMatch(){
    List<String> strings = Arrays.asList("abc", "abd", "aba", "efg", "abcd","jkl", "jkl");
    boolean b = strings.stream().noneMatch(s -> s == "abc");
    out.println(b);
}

4）findAny

/**
 * 功能描述 : 回傳當前流中任意元素
 * @return : void
 */
@Test
public void findAny(){
    List<String> strings = Arrays.asList("cv", "abd", "aba", "efg", "abcd","jkl", "jkl");
    Optional<String> any = strings.stream().findAny();
    if(any.isPresent()) out.println(any.get());
}

5）findFirst

/**
 * 功能描述 : 回傳當前流中第一個元素
 * @return : void
 */
@Test
public void findFirst(){
    List<String> strings = Arrays.asList("cv", "abd", "aba", "efg", "abcd","jkl", "jkl");
    Optional<String> first = strings.stream().findFirst();
    if(first.isPresent()) out.println(first.get());
}

6）forEach java

/**
 * 功能描述 : 遍歷流
 * @return : void
 */
@Test
public void foreach(){
    List<String> strings = Arrays.asList("cv", "abd", "aba", "efg", "abcd","jkl", "jkl");
    strings.stream().forEach(s -> out.println(s));
}

7）collect

/**
 * 功能描述 : 流轉換為其他形式
 * @return : void
 */
@Test
public void collect(){
    List<String> strings = Arrays.asList("cv", "abd", "aba", "efg", "abcd","jkl", "jkl");
    Set<String> set = strings.stream().collect(Collectors.toSet());
    List<String> list = strings.stream().collect(Collectors.toList());
    Map<String, String> map = strings.stream().collect(Collectors.toMap(v ->v.concat("_name"), v1 -> v1, (v1, v2) -> v1));
    out.println(set);
    out.println(list);
    out.println(map);
}

8）reduce

/**
 * 功能描述 : 將流中元素反復結合起來，得到一個值
 * @return : void
 */
@Test
public void reduce(){
    List<String> strings = Arrays.asList("cv", "abd", "aba", "efg", "abcd","jkl", "jkl");
    //reduce方法一
    Optional<String> reduce1 = strings.stream().reduce((acc,item) -> {return acc+item;});
    //reduce方法二
    String reduce2 = strings.stream().reduce("itcast", (acc, item) -> {
    	return acc + item;
    });
    //reduce方法三
    ArrayList<String> reduce3 = strings.stream().reduce(
        new ArrayList<String>(),
   		new BiFunction<ArrayList<String>, String, ArrayList<String>>() {
    		@Override
    		public ArrayList<String> apply(ArrayList<String> acc, String item) {
    			acc.add(item);
    			return acc;
    		}
    	}, 
        new BinaryOperator<ArrayList<String>>() {
   			@Override
    		public ArrayList<String> apply(ArrayList<String> acc, ArrayList<String> item) {
    		return acc;
    		}
    	}
    );
    if(reduce1.isPresent())out.println(reduce1.get());
    out.println(reduce2);
    out.println(reduce3);
}

9）count

/**
* 功能描述 : 回傳流中元素總數
* @return : void
*/
@Test
public void count(){
    List<String> strings = Arrays.asList("cv", "abd", "aba", "efg", "abcd","jkl", "jkl");
    long count = strings.stream().count();
    out.println(count);
}

注意：文章中因排序部分用到外部比較器，需要匯入外部jar包

<!--apache集合操作工具包-->
<dependency>
    <groupId>org.apache.commons</groupId>
    <artifactId>commons-collections4</artifactId>
    <version>4.4</version>
</dependency>

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