关于List、Map、Stream初始化方式

? ? ?初始化List
? ? ?List<string> list = new ArrayList<string><string>(); ?
? ? ?list.add("string1"); ?
? ? ?list.add("string2"); ?
? ? ?list.add("stringN"); ?
? ??
? ? ?初始化Map
? ? ?Map</String , Object> map = new HashMap</String , Object>(); ?
? ? ?map.put("key1", "value1"); ?
? ? ?map.put("key2", "value2"); ?
? ? ?map.put("keyN", "valueN"); ?

? ? ?初始化List ??
? ? ?List<string> list = new ArrayList<string>(){{ ?
? ? ?add("string1"); ?
? ? ?add("string2"); ?
? ? ?add("stringN"); ? ?}}; ?
? ? ?
? ? ?初始化Map ? ?
? ? ?Map</String , String> map = new HashMap</String , Object>(){{ ?
? ? ?put("keyOne", "valueOne"); ?
? ? ?put("keyTwo", "valueTwo"); ?
? ? ?put("keyThree", "valueThree"); ??
? ? ?}};

? ? ?Map map = Stream.of(1,2,3,4).collect(
? ? ?Collectors.toMap(x -> x, integer -> integer,(key, value) -> value, HashMap::new));
? ? ?System.out.println(JSONObject.toJSONString(map));
? ??
? ? ?List list = Stream.of(1,2,3,4).collect(Collectors.toList());
? ? ?System.out.println(JSONObject.toJSONString(list));

ArrayList<String> list = new ArrayList(Arrays.asList("banana", "milk", "bacon")); ?

public class Student {
?? ?private String name;
?? ?private Integer age;
?? ?private Integer math;
?? ?private Integer english;
?? ?//get set
?? ?public Student(String name, Integer age, Integer math, Integer english) {
?? ??? ?super();
?? ??? ?this.name = name;
?? ??? ?this.age = age;
?? ??? ?this.math = math;
?? ??? ?this.english = english;
?? ?}
?? ?@Override
?? ?public String toString() {
?? ??? ?return "Student [name=" + name + ", age=" + age + ", math=" + math + ", english=" + english + "]";
?? ?}
}

public class StreamDemo {
?? ?List<Student> list = null;
? ? //初始化数据
?? ?@Before
?? ?public void beforetest() {
?? ??? ?list = Arrays.asList(new Student("Tom", 18, 88, 90), new Student("Jerry", 20, 77, 89),
?? ??? ??? ??? ?new Student("Lily", 17, 98, 79), new Student("Lucy", 19, 70, 80), new Student("LiLei", 18, 88, 90),
?? ??? ??? ??? ?new Student("HanMeiMei", 21, 87, 79));
?? ?}?
?? ?
?? ?@Test
?? ?public void streamtest() {
?? ??? ?// filter 过滤器返回还是一个stream流对象
?? ??? ?//查询math成绩大于80的学生并遍历输出
?? ??? ?list.stream().filter(e->e.getMath()>80).forEach(System.out::println);//.forEach(e->System.out.println(e))
?? ??? ?//统计数量count
?? ??? ?System.out.println(list.stream().count());
?? ??? ?//如统计总分大于160的人数
?? ??? ?System.out.println(list.stream().filter(e->e.getEnglish()+e.getMath()>160).count());
?? ??? ?//limit ?取前n个值
?? ??? ?list.stream().limit(3).forEach(System.out::println);
?? ??? ?//skip 跳过前n个
?? ??? ?list.stream().skip(2).forEach(System.out::println);
?? ??? ?//distinct 去除重复数据
?? ??? ?list.stream().distinct().forEach(System.out::println);
?? ??? ?//map 映射元素可以对元素进行操作 ? 例如对每个学生年龄加1
?? ??? ?list.stream().map(e->{
?? ??? ??? ?e.setAge(e.getAge()+1);
?? ??? ??? ?return e;
?? ??? ?}).forEach(System.out::println);
?? ??? ?//sorted 排序?
?? ??? ?//升序
?? ??? ?list.stream().sorted((a,b)->{
?? ??? ??? ?return a.getEnglish().compareTo(b.getEnglish());
?? ??? ?});
?? ??? ?//降序
?? ??? ?list.stream().sorted((a,b)->{
?? ??? ??? ?return b.getEnglish().compareTo(a.getEnglish());
?? ??? ?});
?? ??? ?//返回第一个元素 ?
?? ??? ?Optional<Student> first = list.stream().findFirst();
?? ??? ?System.out.println(first.get());
?? ??? ?//返回任意一个元素
?? ??? ?System.out.println(list.stream().findAny().get());
?? ??? ?//anyMatch 是否匹配任意一元素 ?检查是否包含名字为Tom的
?? ??? ?System.out.println(list.stream().anyMatch(e->e.getName().equals("Tom")));
?? ??? ?//allMatch 是否匹配所有元素
?? ??? ?System.out.println(list.stream().allMatch(e->e.getName().equals("Tom")));
?? ??? ?//noneMatch ?是否未匹配所有元素
?? ??? ?System.out.println(list.stream().noneMatch(e->e.getName().equals("Tom")));
?? ??? ?//findFirst 返回元素中第一个值
?? ??? ?Student student = list.stream().findFirst().get();
?? ??? ?//findAny 返回元素中任意一个值
?? ??? ?Student student1 = list.stream().findAny().get();
?? ??? ?//max 返回最大值 查询英语成绩最高的学生
?? ??? ?Student student2 = list.stream().max((l1,l2)->l2.getEnglish().compareTo(l1.getEnglish())).get();
?? ??? ?//min 最小值 ?将上面l1，l2位置对调
?? ??? ?Student student3 = list.stream().max((l1,l2)->l2.getEnglish().compareTo(l1.getEnglish())).get();
?? ??? ?//将流对象转为list?
?? ??? ?list.stream().filter(e->e.getMath()>80).collect(Collectors.toList());
?? ??? ?//将流转未set
?? ??? ?list.stream().filter(e->e.getMath()>80).collect(Collectors.toSet());
?? ??? ?//对对象中的某项进行统计
?? ??? ?IntSummaryStatistics c = list.stream().collect(Collectors.summarizingInt(Student::getEnglish));
?? ??? ?System.out.println(c);//IntSummaryStatistics{count=6, sum=507, min=79, average=84.500000, max=90}
?? ?}? ?
}