[游戏开发]Stream流的相关使用

学习笔记：Stream流的创建、中间以及终端操作

一、Stream流的创建

??惰性操作，每当访问到流中的一个元素，才在此元素上执行相关操作，Stream不保存数据，故每个Stream流只能使用一次，分为中间操作和终止操作，中间操作：返回结果为Stream，可有多个中间操作，终止操作：返回数据，只能有一个。

 * Stream的实例化
 * 创建方式：
 *      方式一：通过集合创建
 *      方式二:通过数组
 *      方式三:通过Stream的of()、iterate()、generate()

    //方式一:通过集合
    @Test
    public void test1(){
        List<Employee> employees = EmployeeData.getEmployees();
        //Stream中的静态方法：default Stream<E> stream() 返回一个顺序流
        Stream<Employee> stream = employees.stream();
        //Stream中的静态方法：default Stream<E> parallelStream() 返回一个并行流
        Stream<Employee> employeeStream = employees.parallelStream();
    }
    //方式二:通过数组
    @Test
    public void test2(){
        int[] arr = new int[]{1,2,3,4,5};
        //调用Array类的static <T> Stream<T> stream(T[] array):返回一个流
        IntStream stream = Arrays.stream(arr);
        System.out.println(stream);
    }

    //方式三:通过Stream的of()、iterate()、generate()
    @Test
    public void test3(){
        Stream<Integer> stream = Stream.of(1, 2, 3, 4, 5);

        Stream<Integer> stream1 = Stream.iterate(0, x -> x + 3).limit(4);
        stream1.forEach(t-> System.out.println());
//        stream1.forEach(System.out::println);

        Stream<Double> stream2 = Stream.generate(Math::random).limit(4);
        stream2.forEach(System.out::println);
    }

二、中间操作

在这里所用的EmployeeData数据

public class EmployeeData {
	public static List<Employee> getEmployees(){
		List<Employee> list = new ArrayList<>();
		list.add(new Employee(1001, "马化腾", 34, 6000.38));
		list.add(new Employee(1002, "马云", 12, 9876.12));
		list.add(new Employee(1003, "刘强东", 33, 3000.82));
		list.add(new Employee(1004, "雷军", 26, 7657.37));
		list.add(new Employee(1005, "李彦宏", 65, 5555.32));
		list.add(new Employee(1006, "比尔盖茨", 42, 9500.43));
		list.add(new Employee(1007, "任正非", 26, 4333.32));
		list.add(new Employee(1008, "扎克伯格", 35, 2500.32));
		return list;
	}
}

filter

        List<Employee> list = EmployeeData.getEmployees();
        //filter(Predicate p)--接收Lambda,从流中排除某些元素
        Stream<Employee> stream = list.stream();
        //查询员工表中薪资大于8000的员工信息
        stream.filter(e -> e.getSalary() > 8000).forEach(System.out::println);
        System.out.println("~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~");

limit

        //limit(n)  截断流，使元素不超过给定数量
		//Stream<Employee> stream2 = stream.limit(3);    此处错误，应该重新创建list的流，每个Stream流只能使用一次，否则会出现错误： java.lang.IllegalStateException: stream has already been operated upon or closed
        Stream<Employee> stream1 = list.stream();
        stream1.limit(3).forEach(System.out::println);
        System.out.println("~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~");

limit

        //skip(n) 跳过前n个元素
        Stream<Employee> stream2 = list.stream();
        stream2.skip(3).forEach(System.out::println);
        System.out.println("~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~");
  

distinct

        //distinct() 筛选，通过流所生成元素的hashCode() 和equals()去除重复元素
        List<Integer> list1 = Arrays.asList(1, 2, 2, 2, 5, 6, 7);
        list1.stream().distinct().forEach(System.out::println);

排序

    @Test
    public void test2() {
        //sorted()  自然排序
        List<Integer> list = Arrays.asList(1, 2, 3, 5, 6, -9, 0, 8, -99);
        String[] strArr = { "g","f","bc", "dee", "f" };
        list.stream().sorted().forEach(System.out::println);
        System.out.println("-----------");
        Arrays.stream(strArr).sorted().forEach(System.out::println);
        System.out.println("-----------");
        //sorted(Comparator com) 定制排序
        List<Employee> employees = EmployeeData.getEmployees();
        //按照salary排序
        //employees.stream().sorted(Comparator.comparingDouble(Employee::getSalary)).forEach(System.out::println);
        employees.stream().sorted((e1, e2) -> Double.compare(e1.getSalary(), e2.getSalary())).forEach(System.out::println);
        //按照age进行排序
        System.out.println("-----------");
        employees.stream().sorted((e1, e2) -> Integer.compare(e1.getAge(), e2.getAge())).forEach(System.out::println);
    }

三、终止操作

forEach

        List<Integer> list1 = Arrays.asList(1, 2, 3, 4);
        list1.forEach(System.out::println);

匹配与查找

  
   @Test
    public void test1() {
        //allMatch(Predicate p) 检查是否匹配所有元素
        List<Employee> employees = EmployeeData.getEmployees();
        boolean allMatch = employees.stream().allMatch(e -> e.getAge() > 18);
        //anyMatch(Predicate p) 检查是否至少匹配一个元素
        boolean anyMatch = employees.stream().anyMatch(e -> e.getSalary() > 1000);
        //noneMatch(Predicate p) 检查是否没有匹配所有元素(全部不满足则true,否则false)
        //检查是否有“马”开头的姓名，有则返回false 无则返回true
        boolean noneMatch = employees.stream().noneMatch(e -> e.getName().startsWith("马"));

        //findFirst() 返回第一个元素
        Optional<Employee> employee = employees.stream().findFirst();
        System.out.println(employee);

        //findAny() 返回当前流中的任意元素
        Optional<Employee> any = employees.stream().findAny();
        System.out.println(any);

        //count() 返回流中元素总数
        long count = employees.stream().filter(e -> e.getSalary() > 5000).count();
        System.out.println(count);

        //max(Comparator c) 返回流中最大值

        //获取工资最高的对象
        //Optional<Employee> max = list.stream().max(Comparator.comparingDouble(Employee::getSalary));
        //获取最高的工资
        Stream<Double> salaryStream = employees.stream().map(e -> e.getSalary());  //获取员工工资
        Optional<Double> max = salaryStream.max(Double::compare);  //得到最大的工资
        System.out.println(max);

        //min(Comparator c) 返回流中最小值
        //返回最低工资的员工
        Optional<Employee> employee1 = employees.stream().min((e1, e2) -> Double.compare(e1.getSalary(), e2.getSalary()));
        System.out.println(employee1);


        //forEach(Consumer c)   内部迭代
        employees.stream().forEach(System.out::println);
    }

归约

  @Test
    public void test2() {
        //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
        //计算1-10的自然数的和
        List<Integer> list = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
        Integer sum = list.stream().reduce(0, Integer::sum);//方法引用  类::静态方法
        System.out.println(sum);

        //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>

        //计算所有员工工资的总和
        List<Employee> employees = EmployeeData.getEmployees();
        Stream<Double> salaryStraem = employees.stream().map(Employee::getSalary); //获取工资流
        //Optional<Double> sum1 = salaryStraem.reduce(Double::sum);
        Optional<Double> sum1 = salaryStraem.reduce((s1, s2) -> s1 + s2);
        System.out.println(sum1);
    }

收集

 @Test
    public void test3() {
        //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
        //查找工资大于8000的员工，结果返回一个List或Set
        List<Employee> employees = EmployeeData.getEmployees();
        List<Employee> collect = employees.stream().filter(employee -> employee.getSalary() > 8000).collect(Collectors.toList());
        collect.forEach(System.out::println);

        Set<Employee> collect1 = employees.stream().filter(employee -> employee.getSalary() > 8000).collect(Collectors.toSet());
        collect1.forEach(System.out::println);
    }