本文介绍java 8 Stream流的常用高频api，通过实战级别的案例进行演示。实现结合实际业务、开发需要来应用技术，不让技术讲解枯燥无味，带来技术落地成生产力的价值。

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1. 思考，stream 的多个操作，相当于几个for循环，实际执行了几次循环？

List<String> collect = strings.stream()

// peek1

.peek(e -> out.println("Original Element: " + e))

// filter

.filter(e -> e.length() > 3)

// peek2

.peek(e -> out.println("Filtered value: " + e))

//map

.map(String::toUpperCase)

//peek3

.peek(e -> out.println("Mapped value: " + e))

// collect

.collect(Collectors.toList());

2. Lambda 的 peek 和 forEach 区别

• peek 返回值是和之前流泛型相同的流, 多用于打印中间操作时元素数据

• forEach 没有返回值, 无法继续使用点语法操作

/**

* @author zhangcq

* @version 1.0.0

* @title

* @date 2022/6/9 13:50

*/

public class PeekAndForEachExample {

public static void main(String[] args) {

PrintStream out = System.out;

final List<String> strings

= Arrays.asList("one", "two", "three", "four");

List<String> result = new ArrayList<>();

/**~~~~~~~重点看这里~~~~~~~~~~~~~~~~~~~~**/

for (String e : strings) {

// peek1

out.println("Original Element: " + e);

// filter

if (e.length() > 3) {

// peek2

out.println("Filtered value: " + e);

//map

e = e.toUpperCase();

//peek3

out.println("Mapped value: " + e);

// collect

result.add(e);

}

}

System.out.println("stream collect: " + result);

out.println("Final Results: " + strings);

out.println("~~~~~~~~~~~~~~~~~~~~~");

List<String> collect = strings.stream()

// peek1

.peek(e -> out.println("Original Element: " + e))

// filter

.filter(e -> e.length() > 3)

// peek2

.peek(e -> out.println("Filtered value: " + e))

//map

.map(String::toUpperCase)

//peek3

.peek(e -> out.println("Mapped value: " + e))

// collect

.collect(Collectors.toList());

System.out.println("stream collect: " + collect);

out.println("Final Results: " + strings);

}

}

3. Reduce用法之BigDecimal求和

/**

* @author zhangcq

* @version 1.0.0

* @title

* @date 2022/6/9 16:46

*/

public class ReduceExample {

public static void main(String[] args) {

List<BigDecimal> list = new ArrayList<>();

list.add(BigDecimal.valueOf(1.1));

list.add(BigDecimal.valueOf(1.2));

list.add(BigDecimal.valueOf(1.3));

list.add(BigDecimal.valueOf(1.4));

BigDecimal decimal = list.stream().reduce(BigDecimal.ZERO, BigDecimal::add);

System.out.println("reduce: " + decimal);

DoubleSummaryStatistics doubleSummaryStatistics = list.stream().mapToDouble(o -> o.doubleValue()).summaryStatistics();

System.out.println("doubleSummaryStatistics: " + doubleSummaryStatistics.getAverage());

System.out.println("doubleSummaryStatistics: " + doubleSummaryStatistics.getSum());

}

}

4 map 和 flatMap用法

/**

* @author zhangcq

* @version 1.0.0

* @title

* @date 2022/6/9 17:21

*/

public class MapAndFlatMapExample {

public static void main(String[] args) {

UserAbAutoHandinDo do1 = UserAbAutoHandinDo.builder().list(Arrays.asList(1, 2, 3)).build();

UserAbAutoHandinDo do2 = UserAbAutoHandinDo.builder().list(Arrays.asList(4, 5, 6)).build();

UserAbAutoHandinDo do3 = UserAbAutoHandinDo.builder().list(Arrays.asList(7, 8, 9)).build();

UserAbAutoHandinDo do4 = UserAbAutoHandinDo.builder().list(Arrays.asList(10, 11, 12)).build();

List<UserAbAutoHandinDo> userAbAutoHandinDos = Arrays.asList(do1, do2, do3, do4);

userAbAutoHandinDos.stream().map(o -> o.getList());

Stream<Integer> integerStream = userAbAutoHandinDos.stream().flatMap(o -> o.getList().stream());

System.out.println(integerStream.collect(Collectors.toList()));

String[] strings = {"Hello", "World"};

List<String> collect = Arrays.stream(strings)

.map(str -> str.split(""))

.flatMap(splitArray -> Arrays.stream(splitArray))

.collect(Collectors.toList());

System.out.println(collect);

}

}

5. sortedExample

/**

* @author zhangcq

* @version 1.0.0

* @title

* @date 2022/6/9 17:21

*/

public class SortedExample {

public static void main(String[] args) {

UserAbAutoHandinDo do1 = UserAbAutoHandinDo.builder().list(Arrays.asList(1, 2, 3)).build();

UserAbAutoHandinDo do2 = UserAbAutoHandinDo.builder().list(Arrays.asList(4, 5, 6)).build();

UserAbAutoHandinDo do3 = UserAbAutoHandinDo.builder().list(Arrays.asList(7, 8, 9)).build();

UserAbAutoHandinDo do4 = UserAbAutoHandinDo.builder().list(Arrays.asList(10, 11, 12)).build();

List<UserAbAutoHandinDo> userAbAutoHandinDos = Arrays.asList(do1, do2, do3, do4);

System.out.println("~~~~~~~没有短路操作不执行~~~~");

userAbAutoHandinDos.stream().sorted((x, y) -> x.getList().get(0).compareTo(y.getList().get(0))).peek(

o -> System.out.println(o.getList().get(0))

);

System.out.println("~~~~~~~有短路操作执行~~~~Comparator.comparing(keySelector）");

userAbAutoHandinDos.stream().sorted(Comparator.comparing(x -> x.getList().get(0)))

.peek(o -> System.out.println(o.getList().get(0)))

.collect(Collectors.toList());

System.out.println("~~~~~乱序之后~~有短路操作执行~~~~Comparator.comparing(keySelector)");

userAbAutoHandinDos = Arrays.asList(do1, do4, do2, do3);

userAbAutoHandinDos.stream().sorted(Comparator.comparing(x -> x.getList().get(0))).peek(

o -> System.out.println(o.getList().get(0))

).collect(Collectors.toList());

}

}

6 Collecotrs之toMap 和 groupingBy

/**

* @author zhangcq

* @version 1.0.0

* @title

* @date 2022/6/9 17:53

*/

public class CollectorsExample1 {

public static void main(String[] args) {

UserAbAutoHandinDo do1 = UserAbAutoHandinDo.builder().userId("aaaa").list(Arrays.asList(1, 2, 3)).build();

UserAbAutoHandinDo do2 = UserAbAutoHandinDo.builder().userId("bbbb").list(Arrays.asList(4, 5, 6)).build();

UserAbAutoHandinDo do3 = UserAbAutoHandinDo.builder().userId("cccc").list(Arrays.asList(7, 8, 9)).build();

UserAbAutoHandinDo do4 = UserAbAutoHandinDo.builder().userId("dddd").list(Arrays.asList(10, 11, 12)).build();

System.out.println("~~~~~~~maxBy~~~~");

List<UserAbAutoHandinDo> userAbAutoHandinDos = Arrays.asList(do1, do2, do3, do4);

Optional<UserAbAutoHandinDo> collect = userAbAutoHandinDos.stream().collect(Collectors.maxBy(Comparator.comparing(x -> x.getList().get(0))));

System.out.println(collect.get().getList());

System.out.println("~~~~~~~ToMap~~~~");

Map<Integer, String> collect2 = userAbAutoHandinDos.stream().collect(Collectors

.toMap(x -> x.getList().get(0) % 2, //keyMapper

x -> x.getUserId(), // valueMapper

(x, y) -> x + "+" + y) // mergeFunction

);

System.out.println(JSON.toJSONString(collect2, true));

System.out.println("~~~~~~~groupingBy~~~~");

Map<Integer, List<UserAbAutoHandinDo>> collect1 = userAbAutoHandinDos.stream().collect(Collectors

.groupingBy(x -> x.getList().get(0) % 2));

System.out.println(JSON.toJSONString(collect1, true));

}

}

~~~~~~~maxBy~~~~

[10, 11, 12]

~~~~~~~ToMap~~~~

{

0:"bbbb+dddd",

1:"aaaa+cccc"

}

~~~~~~~groupingBy~~~~

{

"0": [

{

"list": [

4,

5,

6

],

"userId": "bbbb"

},

{

"list": [

10,

11,

12

],

"userId": "dddd"

}

],

"1": [

{

"list": [

1,

2,

3

],

"userId": "aaaa"

},

{

"list": [

7,

8,

9

],

"userId": "cccc"

}

]

}

7. Collectors之partitionBy

/**

* @author zhangcq

* @version 1.0.0

* @title

* @date 2022/6/9 17:53

*/

public class CollectorsExample2 {

public static void main(String[] args) {

UserAbAutoHandinDo do1 = UserAbAutoHandinDo.builder().userId("aaaa").list(Arrays.asList(1, 2, 3)).build();

UserAbAutoHandinDo do2 = UserAbAutoHandinDo.builder().userId("bbb").list(Arrays.asList(4, 5, 6)).build();

UserAbAutoHandinDo do3 = UserAbAutoHandinDo.builder().userId("ccc").list(Arrays.asList(7, 8, 9)).build();

UserAbAutoHandinDo do4 = UserAbAutoHandinDo.builder().userId("dddd").list(Arrays.asList(10, 11, 12)).build();

System.out.println("~~~~~~~partitioningBy~~~~");

List<UserAbAutoHandinDo> userAbAutoHandinDos = Arrays.asList(do1, do2, do3, do4);

Map<Boolean, Map<Integer, List<UserAbAutoHandinDo>>> collect = userAbAutoHandinDos.stream().collect(Collectors.partitioningBy(o -> o.getUserId().length() > 3, Collectors.groupingBy(o -> o.getList().get(0) % 2))

);

System.out.println(JSON.toJSONString(collect, true));

}

}