Java8_Stream详解
JDK8 Stream详解
概念
Stream是Java8 API的新成员，它允许以声明性方式处理数据集合 。
特点
（1）代码简洁：函数式编程写出的代码简洁且意图明确，使用stream接口让你从此告别for循环。
（2）多核友好：Java函数式编程使得编写并行程序从未如此简单，你需要的全部就是调用一下方法。
流程
1）第一步：把集合转换为流stream
2）第二步：操作stream流
stream流在管道中经过中间操作（intermediate operation）的处理，最后由最终操作(terminal operation)得到前面处理的结果
操作符
两种：中间操作符、终止操作符
中间操作符
示例代码：
1）filter
/** * 功能描述:根据条件过滤集合数据 * @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:去除集合中重复数据 * @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:指定获取集合前x条数据，重新构造一个新的集合 * @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条数据，把后面的数据重新构造一个新的集合 * @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:对集合中所有元素统一处理 * @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:对集合中所有元素统一处理 * @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 : 对集合进行排序 * @return : void */ @Test public void sorted(){ List<String> strings1 = Arrays.asList("abc", "abd", "aba", "efg", "abcd","jkl", "jkl"); List<String> strings2 = Arrays.asList("张三", "李四", "王五", "赵柳", "张哥","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); }
8)Map、flatMap区别
map：对流中每一个元素进行处理
flatMap：流扁平化，让你把一个流中的“每个值”都换成另一个流，然后把所有的流连接起来成为一个流
总结：map是对一级元素进行操作，flatmap是对二级元素操作。
本质区别：map返回一个值；flatmap返回一个流，多个值。
应用场景：map对集合中每个元素加工,返回加工后结果；flatmap对集合中每个元素加工后，做扁平化处理后（拆分层级，放到同一层）然后返回
/** * 方法一 * 功能描述: 通过使用map、flatMap把字符串转换为字符输出对比区别 * @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（字符串转换为字符流）
/** * 功能描述:字符串转换为字符流 * @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(); }
终止操作符
示例代码
1）anyMatch
/** * 功能描述 : 判断集合中是否至少存在一个元素满足条件 * @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 : 判断集合中是否所有元素都满足条件 * @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 : 判断集合中是否所有元素都不满足条件 * @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 : 返回当前流中任意元素 * @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 : 返回当前流中第一个元素 * @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 : 遍历流 * @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 : 流转换为其他形式 * @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 = trings.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 : 将流中元素反复结合起来，得到一个值 * @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 : 返回流中元素总数 * @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较器，需要导入外部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>
