声明

这一系列文章旨在帮助大家理解 Collector 的执行流程，至于实现的是否高效、是否优雅、是否合理等暂且不论。

现在来实现一个和 java.util.stream.Collectors#groupingBy() 一样效果的 GroupByCollector。

当然此处的实现肯定没有jdk那样灵活，效率就先不提了，至少贫道的这个实现不支持多级分组V_V。

若对 Collector 的各个流程有疑问请移驾此处：

简单实现

// 这个是从jdk8源码里提出来的一个 Characteristics// 只包含一个恒等函数的Characteristicsstatic final Set
    
      CH_ID        = Collections.unmodifiableSet(EnumSet.of(Collector.Characteristics.IDENTITY_FINISH));

static class GroupByCollector
    
      implements Collector
     
      >, Map
      
       >> {    Function
       
         classifier;    public GroupByCollector(Function
        
          classifier) {        this.classifier = classifier;    }    @Override    public Supplier
         
          
           >> supplier() { return () -> new HashMap<>(); } @Override public BiConsumer
           
            
             >, T> accumulator() { return (map, e) -> { K key = classifier.apply(e); List
             
               value = map.get(key); if (map.get(key) == null) { map.put(key, Lists.newArrayList()); } map.get(key).add(e); }; } @Override public BinaryOperator
              
               
                >> combiner() { return (map1, map2) -> { map2.forEach((k, v) -> { if (map1.get(k) == null) { map1.put(k, v); } else { map1.get(k).addAll(v); } }); return map1; }; } @Override public Function
                
                 
                  >, Map
                  
                   >> finisher() { return Function.identity(); } @Override public Set
                   
                     characteristics() { return CH_ID; }}

简化代码

static class GroupByCollector2
    
      implements Collector
     
      >, Map
      
       >> {    Function
       
         classifier;    public GroupByCollector2(Function
        
          classifier) {        this.classifier = classifier;    }    @Override    public Supplier
         
          
           >> supplier() { return HashMap::new; } @Override public BiConsumer
           
            
             >, T> accumulator() { return (map, e) -> { K key = classifier.apply(e); List
             
               value = Optional.ofNullable(map) .map(m -> m.get(key)).orElse(Lists.newArrayList()); value.add(e); map.put(key, value); }; } @Override public BinaryOperator
              
               
                >> combiner() { return (m1, m2) -> { m2.forEach((k, v) -> { List
                
                  value = Optional.ofNullable(m1) .map(m -> m.get(k)).orElse(Lists.newArrayList()); value.addAll(v); m1.put(k, value); }); return m1; }; } @Override public Function
                 
                  
                   >, Map
                   
                    >> finisher() { return Function.identity(); } @Override public Set
                    
                      characteristics() { return CH_ID; }}

使用自定义Collector

static class User {    private Integer id;    private String name;    private Integer gender;    private Integer age;    User(Integer id, String name, Integer gender, Integer age) {        this.id = id;        this.name = name;        this.gender = gender;        this.age = age;    }    // getter,setter}List
    
      users = Lists.newArrayList(        new User(1, "java", 1, 25),        new User(2, "C", 1, 22),        new User(3, "scala", 0, 23),        new User(4, "C++", 0, 11),        new User(5, "Spark", 1, 25),        new User(6, "PHP", 0, 45),        new User(7, "Python", 1, 89),        new User(8, "JavaScript", 0, 110),        new User(9, "C#", 1, 33));void printUserMap(Map
     
      > map) {    map.forEach((k, v) -> {        System.out.println(k);        v.forEach(e-> System.out.println("\t"+e));    });}@Testpublic void test2() {    Map
      
       > map = users.stream()      .collect(Collectors.groupingBy(User::getGender));    printUserMap(map);    map = users.stream().      collect(new GroupByCollector<>(User::getGender));    printUserMap(map);    map = users.stream().collect(new GroupByCollector2<>(User::getGender));    printUserMap(map);}

输出效果

0    User{id=3, name='scala', gender=0, age=23}    User{id=4, name='C++', gender=0, age=11}    User{id=6, name='PHP', gender=0, age=45}    User{id=8, name='JavaScript', gender=0, age=110}1    User{id=1, name='java', gender=1, age=25}    User{id=2, name='C', gender=1, age=22}    User{id=5, name='Spark', gender=1, age=25}    User{id=7, name='Python', gender=1, age=89}    User{id=9, name='C#', gender=1, age=33}