Reduce:规约；Collector:收集、判断性终止函数、组函数、分组、分区

Stream 的终止操作

 一、规约

 * reduce(T iden, BinaryOperator b)     可以将流中元素反复结合起来，得到一个值。 返回 T
 * reduce(BinaryOperator b)             可以将流中元素反复结合起来，得到一个值。 返回 Optional<T>

public static void test02() {
        //01.数值求和
        List<Integer> list = Arrays.asList(1,2,3,4,5,6,7,8,9,10);
        /**
         *0 是起始x,0 + y[0] 结果赋值给x， 再➕y[1] 结果再赋值给x 再➕y[2] 。。。。。。
         * 因为有起始数值，所以不可能为空，所以返回值为 泛型的类型
         */

        Integer reduce = list.stream().reduce(0, (x, y) -> x + y);
        System.out.println(reduce);

        System.out.println("--------------------");

        //02.求JavaBean的工资属性求和
        //没有起始值，可能为空，所以返回值类型设为了Optional，为了避免空指针异常，有可能为空的情况就用这个
        Optional<Double> reduce2 = emps.stream()
                .map(Employee::getSalary)
                .reduce(Double::sum);
        System.out.println(reduce2.get());

    }

二、收集

方法	描述
collect(Collector c)	将流转换为其他形式。接收一个 Collector接口的 实现，用于给Stream中元素做汇总的方法

Collector 接口中方法的实现决定了如何对流执行收集操作(如收 集到 List、Set、Map)。但是 Collectors 实用类提供了很多静态 方法，可以方便地创建常见收集器实例，具体方法与实例如下表:

/**
     * 收集
     */
    public static  void collect() {
        List<String> list = emps.stream()
                .map(Employee::getName)
                .collect(Collectors.toList());//Collectors.toSet();
        list.forEach(System.out::println);

        System.out.println("----------------------");

        TreeSet<String> treeSet = emps.stream()
                .map(Employee::getName)
                .collect(Collectors.toCollection(TreeSet::new));    //定制化容器类

        treeSet.forEach(System.out::println);
    }

 三、判断性终止函数

public static void test01() {
        boolean allMatch = emps.stream().allMatch(x -> x.getStatus().equals(Status.BUSY));
        boolean anyMatch = emps.stream().anyMatch(x -> x.getStatus().equals(Status.BUSY));
        boolean noneMatch = emps.stream().noneMatch(x -> x.getStatus().equals(Status.BUSY));
        //Optional是一个新的集合类，为了防止空指针异常
        Optional<Employee> first = emps.stream().sorted((x1,x2) -> Double.compare(x1.getSalary(),x2.getSalary())).findFirst();
        Employee employee = first.get();

        //并行执行 sorted和findAny
        Optional<Employee> anyEmployeeOptional = emps.parallelStream().sorted((x1,x2) -> Double.compare(x1.getSalary(),x2.getSalary())).findAny();
        Employee anyEmployee = first.get();

        long count = emps.stream().count();

        Optional<Employee> max = emps.stream().max((x, y) -> Double.compare(x.getSalary(), y.getSalary()));
        Employee maxSalaryEmployee = max.get();

        Optional<Double> minSalary = emps.stream().map(Employee::getSalary)
                .min(Double::compare);
        Double maxSalary = minSalary.get();

        System.out.println(allMatch);
        System.out.println(anyMatch);
        System.out.println(noneMatch);
        System.out.println(anyEmployee);
        System.out.println(first);
        System.out.println(count);
        System.out.println(maxSalaryEmployee);
        System.out.println(minSalary);
    }

 

 四、组函数

　　/**
     * 组函数
     */
    public static  void groupFunction() {

        //总数
        long count0 = emps.stream().count();
        Long count1 = emps.stream().collect(Collectors.counting());

        System.out.println("总数1：" + count0);
        System.out.println("总数2：" + count1);

        //平均值
        Double avg = emps.stream().collect(Collectors.averagingDouble(Employee::getSalary));
        System.out.println("平均值：" + avg);

        //总和
        Double collectSum = emps.stream().collect(Collectors.summingDouble(Employee::getSalary));
        double reduceSum = emps.stream().mapToDouble(Employee::getSalary).reduce(0, (x, y) -> x + y);
        System.out.println("总和1：" + collectSum);
        System.out.println("总和2：" + reduceSum);

        //最大值
        Optional<Double> collectMax = emps.stream().map(Employee::getSalary).collect(Collectors.maxBy((x1, x2) -> x1.compareTo(x2)));
        Optional<Double> max = emps.stream().map(Employee::getSalary).max((x1, x2) -> x1.compareTo(x2));

        System.out.println("最大值1：" + collectMax.get());
        System.out.println("最大值2：" + max.get());

        //最小值
        Optional<Double> collectMin = emps.stream().map(Employee::getSalary).collect(Collectors.minBy((x1, x2) -> x1.compareTo(x2)));
        Optional<Double> min = emps.stream().map(Employee::getSalary).min((x1, x2) -> x1.compareTo(x2));

        System.out.println("最小值1：" + collectMin.get());
        System.out.println("最小值2：" + min.get());
    }

 五、分组

    /**
     * 分组
     */
    public static void group() {
        System.out.println("按照单个字段分组-----------------");
        Map<Status, List<Employee>> collectGroupMap = emps.stream().collect(Collectors.groupingBy(Employee::getStatus));
        collectGroupMap.forEach((key,value) -> System.out.println(key + " | " + value));

        System.out.println("\n先按单个字段分组，每个分组里面再按自定义规则分组-----------------");

        Map<Status, Map<String, List<Employee>>> innerGroupMapByIdentify = emps.stream().collect(Collectors.groupingBy(Employee::getStatus, Collectors.groupingBy(x -> {
            if (((Employee) x).getAge() < 35) {
                return "青年";
            } else if (((Employee) x).getAge() < 55) {
                return "中年";
            } else {
                return "老年";
            }
        })));
        innerGroupMapByIdentify.forEach((key,value) -> System.out.println(key + " | " + value));

        System.out.println("\n先按单个字段分组，每个分组里面再按另外一字段分组-----------------");
        Map<Status, Map<String, List<Employee>>> innerGroupMapByColum = emps.stream()
                .collect(Collectors.groupingBy(Employee::getStatus,
                                Collectors.groupingBy(Employee::getName)
                            )
                 );

        innerGroupMapByColum.forEach((key,value) -> System.out.println(key + " | " + value));

        System.out.println("\n先按2个字段一起分组-----------------");
        Map<String, List<Employee>> columsMap = emps.stream()
                .collect(Collectors.groupingBy(x -> ((Employee) x).getName() + " ： " + ((Employee) x).getStatus().toString()));
        columsMap.forEach((key,value) -> System.out.println(key + " | " + value));
    }

六、分区

    /**
     * 分区
     */
    public static void partition() {
        //一层分区
        Map<Boolean, List<Employee>> partitionMap = emps.stream().collect(Collectors.partitioningBy(x -> x.getAge() > 35));
        //多层级分区,二层按照一层相同的规则分区
        Map<Boolean, Map<Boolean, List<Employee>>> collect = emps.stream().collect(Collectors.partitioningBy(x -> x.getAge() > 35, Collectors.partitioningBy(x -> x.getSalary() < 6000.0)));

        //多层级分区,二层按照一层不同的规则分区
        Map<Boolean, Map<String, List<Employee>>> collect1 = emps.stream().collect(Collectors.partitioningBy(x -> x.getAge() > 35, Collectors.groupingBy(Employee::getName)));

        partitionMap.forEach((key,value) -> System.out.println(key + " | " + value));
        collect.forEach((key,value) -> System.out.println(key + " | " + value));
        collect1.forEach((key,value) -> System.out.println(key + " | " + value));
    }

七、字符串拼接：join 

/**
     * 字符串拼接：join
     */
    public static void join(){
        String joinResult = emps.stream().map(Employee::getName).collect(Collectors.joining(",", " ===start=== ", " ===end=== "));
        System.out.println(joinResult);
    }