java stream分組排序統計求和

java stream多條件分組

其中Student是學生，將學生依次以grade(年級) -> class(班級) -> teacher(任課老師) 分組

Map<String,Map<String, Map<String, List<Student>>>> result = students.stream().collect(
                    Collectors.groupingBy(Student::getGrade,
                        Collectors.groupingBy(Student::getClass,
                            Collectors.groupingBy(Student::getTeacher)))
);

多條件去重

students.stream().collect(Collectors.collectingAndThen(
                Collectors.toCollection(() -> new TreeSet<>(
                        Comparator.comparing(o -> o.getAge() + ";" + o.getName()))), ArrayList::new)).forEach(s -> println(s));

filter過濾

students.stream().filter(s -> s.getAge() == 10).forEach(s -> println(s));

sorted排序

students.stream().sorted(Comparator.comparing(s-> s.getAge())).forEach(s -> println(s));

1.自然序排序

students.stream().sorted();

2.自然序逆序元素，使用Comparator 提供的reverseOrder() 方法

students.stream().sorted(Comparator.reverseOrder());

3.按照年齡倒序排序

students.stream().sorted(Comparator.comparing(Student::getAge).reversed());

4.不借助stream排序

分數正序排序

chineseScores.sort(Comparator.comparing(Integer::intValue));

分數倒序排序

chineseScores.sort(Comparator.comparing(Integer::intValue).reversed());

年齡正序排序

students.sort(Comparator.comparing(Student::getAge));

年齡倒序排序

students.sort(Comparator.comparing(Student::getAge).reversed());

limit方法限制最多返回多少元素

students.stream().limit(2).forEach(s -> println(s));

不要前多n個元素，n大於滿足條件的元素個數就返回空的流

students.stream().skip(2).forEach(s -> println(s));

最大值 最小值

Optional<User> min = students.stream().min(Comparator.comparing(Student::getAge));
println(min);
Optional<User> max = students.stream().max(Comparator.comparing(Student::getAge));
println(max);

轉單數組遍歷

students.stream().map(Student::getName).forEach(name -> println(name));
students.stream().mapToInt(Student::getAge).forEach(age -> println(age));
students.stream().mapToDouble(Student::getScoreOfChinese).forEach(scoreOfChinese -> println(scoreOfChinese));
students.stream().mapToLong(Student::getAge).forEach(getAge -> println(getAge));

轉單數組求和

students.stream().mapToDouble(Student::getScoreOfChinese).sum();

查找匹配指定數據是否存在

allMatch方法與anyMatch差不多，表示所有的元素都滿足才返回true。noneMatch方法表示沒有元素滿足

boolean anyMatch = students.stream().anyMatch(s -> s.getAge() == 100);
boolean allMatch = students.stream().allMatch(s -> s.getName() == 'hello word');
boolean noneMatch = students.stream().noneMatch(s -> s.getStudentId() == '10010');

簡化操作 最大值，最小值，求和

Optional<Integer> sum = list.stream().map(User::getAge).reduce(Integer::sum);
Optional<Integer> max = list.stream().map(User::getAge).reduce(Integer::max);
Optional<Integer> min = list.stream().map(User::getAge).reduce(Integer::min);
println(sum);
println(max);
println(min);

統計

IntSummaryStatistics statistics = students.stream().collect(Collectors.summarizingInt(User::getAge));
double average = statistics.getAverage();
long count = statistics.getCount();
int max = statistics.getMax();
int min = statistics.getMin();
long sum = statistics.getSum();

轉set

Set<User> collect = students.stream().collect(Collectors.toSet());
Iterator<User> iterator = collect.iterator();
while(iterator.hasNext()) {
    System.out.println(iterator.next().getUserId());
}

轉map

Map<String, User> collect = students.stream().collect(Collectors.toMap(Student::getName, s -> s));
for (String name : collect.keySet()) {
    //得到每個key對應多個value的值
    Student s = collect.get(name);
    println(s);
}