美文网首页
Collectors简单使用

Collectors简单使用

作者: 冷月成双 | 来源:发表于2021-10-26 22:37 被阅读0次

Collectors

Collectors类API

1. public static <T, C extends Collection<T>> Collector<T, ?, C> toCollection(Supplier<C> collectionFactory);
    
2. public static <T> Collector<T, ?, List<T>> toList();
3. public static <T> Collector<T, ?, Set<T>> toSet();
    
4. public static <T, K, U> Collector<T, ?, Map<K,U>> toMap(Function<? super T, ? extends K> keyMapper,
                                    Function<? super T, ? extends U> valueMapper) ;
5. public static <T, K, U> Collector<T, ?, Map<K,U>> toMap(
                                    Function<? super T, ? extends K> keyMapper,
                                    Function<? super T, ? extends U> valueMapper,
                                    BinaryOperator<U> mergeFunction);
6. public static <T, K, U, M extends Map<K, U>> Collector<T, ?, M> toMap(
                                Function<? super T, ? extends K> keyMapper,
                                Function<? super T, ? extends U> valueMapper,
                                BinaryOperator<U> mergeFunction,
                                Supplier<M> mapSupplier);
    
7. public static <T, K> Collector<T, ?, Map<K, List<T>>>
    groupingBy(Function<? super T, ? extends K> classifier) ;
8. public static <T, K, A, D>
    Collector<T, ?, Map<K, D>> groupingBy(Function<? super T, ? extends K> classifier,
                                          Collector<? super T, A, D> downstream) ;
9. public static <T, K, D, A, M extends Map<K, D>>
    Collector<T, ?, M> groupingBy(Function<? super T, ? extends K> classifier,
                                  Supplier<M> mapFactory,
                                  Collector<? super T, A, D> downstream);
    
10. public static Collector<CharSequence, ?, String> joining();
11. public static Collector<CharSequence, ?, String> joining(CharSequence delimiter);
12. public static Collector<CharSequence, ?, String> joining(CharSequence delimiter,
                                                             CharSequence prefix,
                                                             CharSequence suffix) ;
13. public static <T, U, A, R>
    Collector<T, ?, R> mapping(Function<? super T, ? extends U> mapper,
                               Collector<? super U, A, R> downstream)

@Getter
@Setter
@ToString
@Builder
public class Student {

    private Long id;
    private String name;
    private Integer age;
    private String address;
}

toCollection方法

Collectors类给了toList()和toSet()方法,默认的是ArrayList和HashSet,所以要想使用其他的类型的集合就可以使用这个方法,例如使用LinkedList

public class CollectorsTest {

    public static void main(String[] args) {
    }
    
    private List<Student> list = new ArrayList<>();

    @Before
    public void init(){
        Student student1 = Student.builder().id(1L).name("张三").age(20).address("北京").build();
        Student student2 = Student.builder().id(2L).name("李四").age(21).address("上海").build();
        Student student3 = Student.builder().id(3L).name("王五").age(22).address("广州").build();
        Student student4 = Student.builder().id(4L).name("赵六").age(23).address("重庆").build();
        Student student5 = Student.builder().id(5L).name("钱七").age(24).address("武汉").build();
        list = Stream.of( student4,student5,student1, student2, student3).collect(Collectors.toList());
    }


    @Test
    public void testCollection(){
        System.out.println("list : ");
        System.out.println(JSON.toJSONString(list));
        LinkedList<Student> linkedList = list.stream().collect(Collectors.toCollection(LinkedList::new));
        System.out.println("linkedList : ");
        System.out.println(JSON.toJSONString(linkedList));
    }
}


list : 
[{"address":"北京","age":20,"id":1,"name":"张三"},{"address":"上海","age":21,"id":2,"name":"李四"},{"address":"广州","age":22,"id":3,"name":"王五"},{"address":"重庆","age":23,"id":4,"name":"赵六"},{"address":"武汉","age":24,"id":5,"name":"钱七"}]
linkedList : 
[{"address":"北京","age":20,"id":1,"name":"张三"},{"address":"上海","age":21,"id":2,"name":"李四"},{"address":"广州","age":22,"id":3,"name":"王五"},{"address":"重庆","age":23,"id":4,"name":"赵六"},{"address":"武汉","age":24,"id":5,"name":"钱七"}]

toList()方法

这个最常见和常用的方法

List<Student> list = Stream.of( student4,student5,student1, student2, student3).collect(Collectors.toList());

toSet()方法

这个也是最常见和常用的方法

Set<Student> set = Stream.of(student4, student5, student1, student2, student3).collect(Collectors.toSet());

toMap()方法

toMap()最少接收两个参数

    @Test
    public void testToMap(){
        Map<Long, Student> towParamMap = list.stream().collect(Collectors.toMap(Student::getId, Function.identity()));
        System.out.println(JSON.toJSONString(towParamMap));
    }

Collectors.toMap(Student::getId, Function.identity())这个方法碰到重复的key会抛出异常,把张三和李四的id改成一样,再次运行机会抛出异常

    @Before
    public void init(){
        Student student1 = Student.builder().id(1L).name("张三").age(20).address("北京").build();
        Student student2 = Student.builder().id(1L).name("李四").age(21).address("上海").build();
        Student student3 = Student.builder().id(3L).name("王五").age(22).address("广州").build();
        Student student4 = Student.builder().id(4L).name("赵六").age(23).address("重庆").build();
        Student student5 = Student.builder().id(5L).name("钱七").age(24).address("武汉").build();
        list = Stream.of(student1, student2, student3,student4,student5).collect(Collectors.toList());
    }

    @Test
    public void testToMap(){
        Map<Long, Student> towParamMap = list.stream().collect(Collectors.toMap(Student::getId, Function.identity()));
        System.out.println(JSON.toJSONString(towParamMap));
    }

java.lang.IllegalStateException: Duplicate key Student(id=1, name=张三, age=20, address=北京)

所以使用这个方法时需要注意,这时可以通过第三个参数来解决重复key的问题

    @Before
    public void init(){
        Student student1 = Student.builder().id(1L).name("张三").age(20).address("北京").build();
        Student student2 = Student.builder().id(1L).name("李四").age(21).address("上海").build();
        Student student3 = Student.builder().id(3L).name("王五").age(22).address("广州").build();
        Student student4 = Student.builder().id(4L).name("赵六").age(23).address("重庆").build();
        Student student5 = Student.builder().id(5L).name("钱七").age(24).address("武汉").build();
        list = Stream.of(student1, student2, student3,student4,student5).collect(Collectors.toList());
    }

    @Test
    public void testToMap(){
        Map<Long, Student> threeParamMap = list.stream().collect(Collectors.toMap(Student::getId, Function.identity(), (a,b) -> b));
        System.out.println(JSON.toJSONString(threeParamMap));
    }

结果为

{1:{"address":"上海","age":21,"id":1,"name":"李四"},3:{"address":"广州","age":22,"id":3,"name":"王五"},4:{"address":"重庆","age":23,"id":4,"name":"赵六"},5:{"address":"武汉","age":24,"id":5,"name":"钱七"}}

可以看到李四覆盖了张三,所以实际中使用时也要注意数据选择.注意一下这个函数(a,b) -> b也可以重新自己定义一个合并的函数BinaryOperator<U> mergeFunction来处理重复数据

还有四个参数的方法可以不使用默认的HashMap而使用其他的容器,例如TreeMap

Map<Long, String> fourParamMap = list.stream().collect(Collectors.toMap(Student::getId, student -> student.getName(), (a,b) -> b, TreeMap::new));
System.out.println(JSON.toJSONString(fourParamMap));

结果

{1:"李四",3:"王五",4:"赵六",5:"钱七"}

groupingBy()方法

Collectors类 groupingBy 方法

  1. 一个参数的方法

    一个参数的方法,还是调用的两个参数的重载方法,第二个参数默认调用 toList() 方法

    public static <T, K> Collector<T, ?, Map<K, List<T>>>
    groupingBy(Function<? super T, ? extends K> classifier) {
        return groupingBy(classifier, toList());
    }

    示例:

    public class GroupingByTest {

    public static void main(String[] args) {
        List<Student> students = new ArrayList<>();
        Student student = new Student();
        student.setId(1L);
        student.setName("小明");
        students.add(student);
        Student student2 = new Student();
        student2.setId(2L);
        student2.setName("小红");
        students.add(student2);
        Map<Long, List<Student>> collect = students.stream().collect(Collectors.groupingBy(s -> {
            return s.getId();
        }));
        System.out.println(JSON.toJSONString(collect));
    }
}

    结果

    {1:[{"id":1,"name":"小明"}],2:[{"id":2,"name":"小红"}]}

    groupingBy 方法参数Function<? super T, ? extends K> classifier

@FunctionalInterface
public interface Function<T, R> {
R apply(T t);
}

   
`Function`是函数式接口,接收一个参数T,返回一个结果R,示例中可以表示为下面这样的,先创建一个Function接口,再将接口当作参数传进去.
   
```java
   Function<Student, Long> groupingByFun = s -> {return s.getId()};
           Map<Long, List<Student>> collect = students.stream().collect(Collectors.groupingBy(groupingByFun
           ));

优化s -> {return s.getId()} 可以简化写法

Function<Student, Long> groupingByFun = s -> {return s.getId()};
// 可以简化成
Function<Student, Long> groupingByFun = s ->  s.getId();
// 再一次简化
Function<Student, Long> groupingByFun = Student::getId;

  1. 两个参数的方法

    public static <T, K, A, D>
    Collector<T, ?, Map<K, D>> groupingBy(Function<? super T, ? extends K> classifier,
                                          Collector<? super T, A, D> downstream) {
        return groupingBy(classifier, HashMap::new, downstream);
    }

    示例

    Map<Long, Long> countMap = students.stream().collect(Collectors.groupingBy(Student::getId, Collectors.counting()));
        System.out.println(JSON.toJSONString(countMap));

// {1:1,2:1}

  1. 三个参数的方法

    public static <T, K, D, A, M extends Map<K, D>>
    Collector<T, ?, M> groupingBy(Function<? super T, ? extends K> classifier,
                                  Supplier<M> mapFactory,
                                  Collector<? super T, A, D> downstream) {
    
}

    示例

    TreeMap<Long, Set<Student>> treeMapSet = students.stream().collect(Collectors.groupingBy(Student::getId, TreeMap::new, Collectors.toSet()));

System.out.println(JSON.toJSONString(treeMapSet));
// {1:[{"id":1,"name":"小明"}],2:[{"id":2,"name":"小红"}]}

joining()方法

总共默认了三个拼接方法

    @Test
    public void testJoin(){
        String join = Stream.of("hello", "world", "hello", "java").collect(Collectors.joining());
        System.out.println(join);
        String join1 = Stream.of("hello", "world", "hello", "java").collect(Collectors.joining(","));
        System.out.println(join1);
        String join2 = Stream.of("hello", "world", "hello", "java").collect(Collectors.joining(",","",""));
        System.out.println(join2);
    }
// helloworldhellojava
//hello,world,hello,java
//hello,world,hello,java

从内部实现来看,还是有不一样,不带参数的方法,内部使用StringBuilder实现

public static Collector<CharSequence, ?, String> joining() {
        return new CollectorImpl<CharSequence, StringBuilder, String>(
                StringBuilder::new, StringBuilder::append,
                (r1, r2) -> { r1.append(r2); return r1; },
                StringBuilder::toString, CH_NOID);
    }

带参数的方法使用StringJoiner实现

public static Collector<CharSequence, ?, String> joining(CharSequence delimiter,
                                                             CharSequence prefix,
                                                             CharSequence suffix) {
        return new CollectorImpl<>(
                () -> new StringJoiner(delimiter, prefix, suffix),
                StringJoiner::add, StringJoiner::merge,
                StringJoiner::toString, CH_NOID);
    }

mapping()方法

mapping简单用法,mapping最简单的用法可以和map方法一样收集某一个字段的值,大部分情况可以和groupingBy,配合使用,例如分组之后只想取某一个字段的值,就可以使用Collectors.mapping配合

    private List<Student> list = new ArrayList<>();

    @Before
    public void init(){
        Student student1 = Student.builder().id(1L).name("张三").age(20).address("北京").build();
        Student student2 = Student.builder().id(1L).name("李四").age(21).address("上海").build();
        Student student3 = Student.builder().id(3L).name("王五").age(22).address("广州").build();
        Student student4 = Student.builder().id(4L).name("赵六").age(23).address("重庆").build();
        Student student5 = Student.builder().id(5L).name("钱七").age(24).address("武汉").build();
        list = Stream.of(student1, student2, student3,student4,student5).collect(Collectors.toList());
    }

    @Test
    public void testMapping(){
        // 收集单个字段
         List<Long> mappingList = list.stream().collect(Collectors.mapping(Student::getId, Collectors.toList()));
        System.out.println(JSON.toJSONString(mappingList));
        // 和groupingBy配合使用
        Map<Long, List<String>> mapping = list.stream().collect(Collectors.groupingBy(Student::getId, Collectors.mapping(Student::getName, Collectors.toList())));
        System.out.println(JSON.toJSONString(mapping));
    }

本文由博客一文多发平台 OpenWrite 发布！

相关文章

网友评论

      本文标题：Collectors简单使用

      本文链接：https://www.haomeiwen.com/subject/ypfualtx.html

      延伸阅读

      深度阅读

      • 您也可以注册成为美文阅读网的作者，发表您的原创作品、分享您的心情！

      栏目导航

      热点阅读

      关于我们|服务条款|联系我们|Collectors简单使用|投稿指南|网站地图|RSS订阅|排版工具|手机版

      提供经典美文摘抄,优美散文欣赏,现代诗歌精选,短篇小说,心情随笔,表白情书范文,故事会在线阅读欣赏

      Copyright © 2014-2023 Haomeiwen.com All Rights Reserved. 好美文阅读网 版权所有

      备案信息：桂公网安备 45052102000051号 · 桂ICP备13007215号-3

      本站所收录作品、热点评论等信息部分来源互联网，目的只是为了系统归纳学习和传递资讯

      所有作品版权归原创作者所有，与本站立场无关，如不慎侵犯了你的权益，请联系我们告知，我们将做删除处理！