Java 8 中的 Stream API 实操

程序代码

public class AnswerApp {
    public static void main(String[] args) {
        List<String> names = Lists.newArrayList("Answer", "AnswerAIL", "AI", "Answer", "Answer@AI@CODER", "KOBE@IVERSION@JORDAN");

        // 根据条件对集合元素进行过滤
        List<String> collect1 = names.stream().filter(e -> e.equals("Answer")).collect(Collectors.toList());
        System.out.println("根据条件对集合元素进行过滤...");
        System.out.println("before ==> " + names);
        System.out.println("after ==> " + collect1);
        System.out.println();

        // 集合元素去重
        List<String> collect2 = names.stream().distinct().collect(Collectors.toList());
        System.out.println("集合元素去重...");
        System.out.println("before ==> " + names);
        System.out.println("after ==> " + collect2);
        System.out.println();

        // 跳过集合中前 n 个元素
        List<String> collect3 = names.stream().skip(2).collect(Collectors.toList());
        System.out.println("跳过集合中前 n 个元素...");
        System.out.println("before ==> " + names);
        System.out.println("after ==> " + collect3);
        System.out.println();

        // 扁平化集合元素
        List<String> collect4 = names.stream().flatMap(e -> Arrays.stream(e.split("@"))).collect(Collectors.toList());
        System.out.println("扁平化集合元素...");
        System.out.println("before ==> " + names);
        System.out.println("after ==> " + collect4);
        System.out.println();

        // 对集合元素进行分组
        Map<String, List<String>> collect5 = collect4.stream().collect(Collectors.groupingBy(e -> e));
        System.out.println("对集合元素进行分组...");
        System.out.println("before ==> " + names);
        System.out.println("after ==> " + collect5);
        System.out.println();

        // 集合转字符串
        String collect6 = names.stream().collect(Collectors.joining("#"));
        System.out.println("集合转字符串...");
        System.out.println("before ==> " + names);
        System.out.println("after ==> " + collect6);
        System.out.println();

        // map
        List<String> collect7 = names.stream().map(e -> MessageFormat.format("Prefix:{0}", e)).collect(Collectors.toList());
        System.out.println("map...");
        System.out.println("before ==> " + names);
        System.out.println("after ==> " + collect7);
        System.out.println();

        // reduce
        System.out.println("reduce...");
        List<Integer> scores = com.google.common.collect.Lists.newArrayList(130, 150, 143, 108, 100, 80);
        Optional<Integer> totalScore = scores.stream().reduce((x, y) -> x + y);
        System.out.println("totalScore ==> " + totalScore.orElse(0));
        Optional<Integer> maxScore  = scores.stream().reduce(Integer::max);
        System.out.println("maxScore ==> " + maxScore.orElse(0));
        Optional<Integer> minScore  = scores.stream().reduce(Integer::min);
        System.out.println("minScore ==> " + minScore.orElse(0));
    }
}

​​程序运行结果​​

根据条件对集合元素进行过滤...
before ==> [Answer, AnswerAIL, AI, Answer, Answer@AI@CODER, KOBE@IVERSION@JORDAN]
after ==> [Answer, Answer]

集合元素去重...
before ==> [Answer, AnswerAIL, AI, Answer, Answer@AI@CODER, KOBE@IVERSION@JORDAN]
after ==> [Answer, AnswerAIL, AI, Answer@AI@CODER, KOBE@IVERSION@JORDAN]

跳过集合中前 n 个元素...
before ==> [Answer, AnswerAIL, AI, Answer, Answer@AI@CODER, KOBE@IVERSION@JORDAN]
after ==> [AI, Answer, Answer@AI@CODER, KOBE@IVERSION@JORDAN]

扁平化集合元素...
before ==> [Answer, AnswerAIL, AI, Answer, Answer@AI@CODER, KOBE@IVERSION@JORDAN]
after ==> [Answer, AnswerAIL, AI, Answer, Answer, AI, CODER, KOBE, IVERSION, JORDAN]

对集合元素进行分组...
before ==> [Answer, AnswerAIL, AI, Answer, Answer@AI@CODER, KOBE@IVERSION@JORDAN]
after ==> {CODER=[CODER], Answer=[Answer, Answer, Answer], JORDAN=[JORDAN], KOBE=[KOBE], AnswerAIL=[AnswerAIL], AI=[AI, AI], IVERSION=[IVERSION]}

集合转字符串...
before ==> [Answer, AnswerAIL, AI, Answer, Answer@AI@CODER, KOBE@IVERSION@JORDAN]
after ==> Answer#AnswerAIL#AI#Answer#Answer@AI@CODER#KOBE@IVERSION@JORDAN

map...
before ==> [Answer, AnswerAIL, AI, Answer, Answer@AI@CODER, KOBE@IVERSION@JORDAN]
after ==> [Prefix:Answer, Prefix:AnswerAIL, Prefix:AI, Prefix:Answer, Prefix:Answer@AI@CODER, Prefix:KOBE@IVERSION@JORDAN]

reduce...
totalScore ==> 711
maxScore ==> 150
minScore ==> 80

list.stream().peek() 与 list.stream().map()区别： peek用来修改数据,map用来转换数据类型

 

匹配 ***Match

public class AnswerApp {
    public static void main(String[] args) throws Exception {
        List<String> names = Lists.newArrayList("Answer", "AnswerAIL", "AI", "Answer", "Answer@AI@CODER", "KOBE@IVERSION@JORDAN");

        // 全部元素匹配
        boolean allMatch = names.stream().allMatch(e -> e.startsWith("Ans"));
        System.out.println("全部元素匹配...");
        System.out.println(MessageFormat.format("allMatch result {0}", allMatch));
        System.out.println();

        // 部分元素匹配
        boolean anyMatch = names.stream().anyMatch(e -> e.startsWith("Ans"));
        System.out.println("部分元素匹配...");
        System.out.println(MessageFormat.format("anyMatch result {0}", anyMatch));
        System.out.println();

        // 所有元素都不匹配
        boolean noneMatch = names.stream().noneMatch(e -> e.startsWith("Ans"));
        System.out.println("所有元素都不匹配...");
        System.out.println(MessageFormat.format("noneMatch result {0}", noneMatch));
        System.out.println();
    }
}

​​程序运行结果​​

全部元素匹配...
allMatch result false

部分元素匹配...
anyMatch result true

所有元素都不匹配...
noneMatch result false

 

List<String> names = Lists.newArrayList("Answer", "AI", "BlackManba");
Map<String, String> nameMap = ImmutableMap.of("Answer", "A1", "AI", "A2", "BlackManba", "A3");
System.out.println(names.stream().reduce((x, y) -> x + "#" + y).orElse(""));
System.out.println(String.join("#", names));
System.out.println(names.stream().map(e -> nameMap.getOrDefault(e, "")).collect(Collectors.joining("#", "[", "]")));

程序运行输出

Answer#AI#BlackManba
Answer#AI#BlackManba
[A1#A2#A3]

 

排序sorted

public class AnswerApp {
  public static void main(String[] args) {
        List<Student> students = Lists.newArrayList();

        for (int i = 0; i < 10; i++) {
            students.add(new Student().setStuNo((long) i).setStuName("Jaemon-" + i).setIndex(i));
        }

        // 使用该方式进行排序, Student 类必须实现 Comparable<Student> 接口(o.getStuName().compareTo(this.getStuName()))
        List<Student> students1 = students.stream().sorted().collect(Collectors.toList());
        System.out.println(students1);
        System.out.println();

        // 根据 index 进行升序排序
        List<Student> students2 = students.stream().sorted(Comparator.comparingInt(Student::getIndex)).collect(Collectors.toList());
        System.out.println(students2);
        System.out.println();

        // 根据 stuName 进行降序排序
        List<Student> students3 = students.stream().sorted((p, q) -> q.getStuName().compareTo(p.getStuName())).collect(Collectors.toList());
        System.out.println(students3);
        System.out.println(); 
  }
}

Student实体类

@Data
@Accessors(chain = true)
public class Student {
    private Long stuNo;
    private String stuName;
    private int index;
}

 

reduce用法

List -> List<List> -> List

@Data
  public class Pair {
        private Long id;
        private Long transactionId;
        private String key;
        private String value;
  }
  
  @Data
  public class TxInfo {
    private List<Pair > pairs;
  }
  
  List<Pair> pairs = 
            txInfos
                    .stream()
                    // 过滤掉pairs为空的
                    .filter(e -> e.getPairs() != null)
                    .map(e -> {
                        for (Pair pair : e.getPairs()) {
                            pair.setTxId(e.getId());
                        }
                        return e.getPairs();
                    })
                    // 合并pairs
                    .reduce((x, y) -> {
                        x.addAll(y);
                        return x;
                    })
                    .get();

 

max/min

public class App {
    public static void main(String[] args) {
        List<Report> list = new ArrayList<>();
        list.add(new Report(6, "java"));
        list.add(new Report(5, "springboot"));
        list.add(new Report(7, "spring"));
        list.add(new Report(9, "mysql"));
        list.add(new Report(2, "mq"));
        list.add(new Report(8, "redis"));

        // 集合中seq字段值最大的对象对应的text
        System.out.println(
                "max seq text: " + list.stream().max(Comparator.comparingInt(Report::getSeq)).map(Report::getText).get()
        );
        // 集合中seq字段值最小的对象对应的text
        System.out.println(
                "min seq text: " + list.stream().min(Comparator.comparingInt(Report::getSeq)).map(Report::getText).get()
        );
    }

    @NoArgsConstructor
    @AllArgsConstructor
    @Data
    public static class Report {
        private int seq;
        private String text;
    }
}

程序运行输出

max seq text: mysql
min seq text: mq

 

解决map key重复问题

Map<Integer, String> map = list.stream().collect(Collectors.toMap(Person::getId, Person::getName, (oldValue, newValue) -> newValue));