草稿 Stream

概念

• 顺序遍历每一个进行操作 和 对集合的聚合操作 可以类比数据库的聚合函数 sequential and parallel aggregate operations

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顶层接口

• BaseStream

  • 终端操作
    
    • Iterator<T> iterator() 返回集合的迭代器
    • Spliterator<T> spliterator() 返回集合的分割器 没搞懂怎么用，干啥用的？

  • 中间状态操作

    • S sequential() 返回一个顺序流

              int[] array = {1,2,4,6,3,1,3};
              Stream<int[]> sequential = Stream.of(array).sequential();

    • S parallel() 返回一个并行流？？
    • S unordered() 返回一个无序流 ？？

  • 其他操作

    • boolean isParallel() 判断当前流是否可执行终端操作
    • S onClose(Runnable closeHandler) 当流被关闭时触发的动作。

• Collector

  • Supplier<A> supplier() 返回一个可变的容器？？
  • BiConsumer<A,T> accumulator() 把多个值折叠入一个可变容器？？？
  • BinaryOperator<A> combiner() 组合两个结果
  • Function<A,R> finisher() 把一个中间结果变成一个最终结果
  • Set<Collector.Characteristics> characteristics() 特性集合？？？
  • static <T,R> Collector<T,R,R> of(Supplier<R> supplier,
BiConsumer<R,T> accumulator,
BinaryOperator<R> combiner,
Collector.Characteristics... characteristics) Collector的构造器

       Collector<Widget, ?, TreeSet<Widget>> intoSet =
           Collector.of(TreeSet::new, TreeSet::add,
                        (left, right) -> { left.addAll(right); return left; });

       Collector<Employee, ?, Integer> summingSalaries
           = Collectors.summingInt(Employee::getSalary))

  
   Collector<Employee, ?, Map<Department, Integer>> summingSalariesByDept
       = Collectors.groupingBy(Employee::getDepartment, summingSalaries);

• DoubleStream double相关的所有的你能想到的操作

  • DoubleStream peek(DoubleConsumer action)
    

         DoubleStream.of(1, 2, 3, 4)
             .filter(e -> e > 2)
             .peek(e -> System.out.println("Filtered value: " + e))
             .map(e -> e * e)
             .peek(e -> System.out.println("Mapped value: " + e))
             .sum();

  
       double sum = widgets.stream()
                           .filter(w -> w.getColor() == RED)
                           .mapToDouble(w -> w.getWeight())
                           .sum();

• Collectors

   // Accumulate names into a List
       List<String> list = people.stream().map(Person::getName).collect(Collectors.toList());
  
       // Accumulate names into a TreeSet
       Set<String> set = people.stream().map(Person::getName).collect(Collectors.toCollection(TreeSet::new));
  
       // Convert elements to strings and concatenate them, separated by commas
       String joined = things.stream()
                             .map(Object::toString)
                             .collect(Collectors.joining(", "));
  
       // Compute sum of salaries of employee
       int total = employees.stream()
                            .collect(Collectors.summingInt(Employee::getSalary)));
  
       // Group employees by department
       Map<Department, List<Employee>> byDept
           = employees.stream()
                      .collect(Collectors.groupingBy(Employee::getDepartment));
  
       // Compute sum of salaries by department
       Map<Department, Integer> totalByDept
           = employees.stream()
                      .collect(Collectors.groupingBy(Employee::getDepartment,
                                                     Collectors.summingInt(Employee::getSalary)));
  
       // Partition students into passing and failing
       Map<Boolean, List<Student>> passingFailing =
           students.stream()
                   .collect(Collectors.partitioningBy(s -> s.getGrade() >= PASS_THRESHOLD));

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用例

 - sum 求和

     int sum = widgets.stream()
                      .filter(w -> w.getColor() == RED)
                      .mapToInt(w -> w.getWeight())
                      .sum();