JDK8 Stream详解

JDK8 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()).List());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().List());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).List());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*/
@Testpublic void skip(){List<String> strings = Arrays.asList("abc", "abc", "bc", "efg", "abcd","jkl", "jkl");List<String> skiped = strings.stream().skip(3).List());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").List());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().List());List<String> sorted2 = strings2.stream().Instance(Locale.CHINA))).List());List<Integer> sorted3 = strings3.stream().sorted().List());out.println(sorted1);out.println(sorted2);out.println(sorted3);
}

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 : CharArray()) {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.());
}

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.());
}

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().Set());List<String> list = strings.stream().List());Map<String, String> map = strings.stream().Map(v -&at("_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>>() {@Overridepublic ArrayList<String> apply(ArrayList<String> acc, String item) {acc.add(item);return acc;}}, new BinaryOperator<ArrayList<String>>() {@Overridepublic ArrayList<String> apply(ArrayList<String> acc, ArrayList<String> item) {return acc;}});if(reduce1.isPresent())out.());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.apachemons</groupId><artifactId>commons-collections4</artifactId><version>4.4</version>
</dependency>