关于java:Collectors简单使用

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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办法

Collectors 类给了 toList() 和toSet()办法, 默认的是 ArrayList 和HashSet, 所以要想应用其余的类型的汇合就能够应用这个办法, 例如应用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()办法

这个最常见和罕用的办法

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

toSet()办法

这个也是最常见和罕用的办法

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

toMap()办法

toMap()起码接管两个参数

    @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())这个办法碰到反复的 key 会抛出异样, 把张三和李四的 id 改成一样, 再次运行机会抛出异样

    @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)

所以应用这个办法时须要留神, 这时能够通过第三个参数来解决反复 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));
    }

后果为

{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"}}

能够看到李四笼罩了张三, 所以理论中应用时也要留神数据抉择. 留神一下这个函数 (a,b) -> b 也能够从新本人定义一个合并的函数 BinaryOperator<U> mergeFunction 来解决反复数据

还有四个参数的办法能够不应用默认的 HashMap 而应用其余的容器, 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));

后果

{1:"李四",3:"王五",4:"赵六",5:"钱七"}

groupingBy()办法

Collectors 类 groupingBy 办法

  1. 一个参数的办法

    一个参数的办法, 还是调用的两个参数的重载办法, 第二个参数默认调用 toList() 办法

    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));
    }
}

    后果

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

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

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

Function是函数式接口, 接管一个参数 T, 返回一个后果 R, 示例中能够示意为上面这样的, 先创立一个 Function 接口, 再将接口当作参数传进去.

   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()} 能够简化写法

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

  1. 两个参数的办法

    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. 三个参数的办法

    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()办法

总共默认了三个拼接办法

    @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

从外部实现来看, 还是有不一样, 不带参数的办法, 外部应用 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);
    }

带参数的办法应用 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()办法

mapping简略用法,mapping最简略的用法能够和 map 办法一样收集某一个字段的值, 大部分状况能够和 groupingBy, 配合应用, 例如分组之后只想取某一个字段的值, 就能够应用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 配合应用
        Map<Long, List<String>> mapping = list.stream().collect(Collectors.groupingBy(Student::getId, Collectors.mapping(Student::getName, Collectors.toList())));
        System.out.println(JSON.toJSONString(mapping));
    }

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java
发表至： java
2021-10-26
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