Napredno tips

dzocesrce

Jun 8th, 2025

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Java 4.33 KB | None | 0 0

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1. GROUPING BY EXAMPLE
public Map<Integer, Set<File>> byYear() {
return files.stream()
.collect(Collectors.groupingBy(
file -> file.getCreateAt().getYear(),
Collectors.toSet()
));
}
2. GROUPING BY EXAMPLE
Map<String, Set<Employee>> employeesByLevels = employees.stream()
.filter(i -> levels.contains(i.getLevel()))
.collect(Collectors.groupingBy(
Employee::getLevel,
TreeMap::new, // map sorted by key
Collectors.toCollection(() -> new TreeSet<>(Comparator.comparing(Employee::getSalary,Comparator.reverseOrder()))) // values sorted too
));
return employeesByLevels;
3. SORTING IN A MAP
Map<String, List<Shape>> shapesByUser = shapes.stream()
.collect(Collectors.groupingBy(
Shape::getId,
Collectors.toList()
))
.entrySet()
.stream()
.sorted(Comparator
.<Map.Entry<String, List<Shape>>>comparingInt(e -> e.getValue().size())
.reversed()
.thenComparingDouble(e -> e.getValue().stream().mapToDouble(Shape::getArea).sum())
.reversed()
)
.collect(Collectors.toMap(
Map.Entry::getKey,
e -> e.getValue().stream()
.sorted(Comparator.comparingDouble(Shape::getPerimeter).reversed())
.collect(Collectors.toList()),
(e1, e2) -> e1,
LinkedHashMap::new
));
4. ATOMIC INTEGER
AtomicInteger counter = new AtomicInteger(1);
paymentList.stream().filter(i->i.index.equals(id))
.sorted(Comparator.comparing(Payment::getPrice,Comparator.reverseOrder()))
.forEach(i->printStream.println(String.format("%d. %s %d",
counter.getAndIncrement(),i.title,i.price)));
5. FLATMAP
studentsByBranch = studentsByBranch.entrySet().stream().sorted(
Comparator.comparingLong(e->e.getValue().stream().flatMap(i->i.getGrades()
.stream()).filter(i->i==10).count())
).collect(Collectors.toMap(
Map.Entry::getKey,
Map.Entry::getValue,
(e1,e2)->e1,
LinkedHashMap::new
));
List<String> branches = studentsByBranch.keySet().stream().collect(Collectors.toList()).reversed();
for(String branch : branches){
Map<Integer,Long> numberOfStudentsPerGrade = studentsByBranch.get(branch).stream()
.flatMap(s -> s.getGrades().stream())
.collect(Collectors.groupingBy(
e->e.intValue(),
TreeMap::new,
Collectors.counting()
));
printStream.println(branch);
numberOfStudentsPerGrade.entrySet().stream().forEach(
i-> printStream.println(String.format("%d | (%d)",i.getKey(),i.getValue()))
);
}
6. COLLECTORS JOINING
String[] allCourses = studentsByIndex.values().stream()
.map(i->i.getCoursesByTerm().values().stream().flatMap(c->c.stream()).collect(Collectors.joining(",")))
.collect(Collectors.joining(",")).split(",");
7. MORE MAP SORTING
gamesByTeam.entrySet().stream()
.sorted(Comparator.comparing(
(Map.Entry<String, Team> e) -> e.getValue().getPoints(),
Comparator.reverseOrder())
.thenComparing(e -> e.getValue().getGoalDifference(),
Comparator.reverseOrder())
.thenComparing(e->e.getKey()))
8. MAP GROUPING BY 0 PROBLEM
return IntStream.range(5, 11)
.boxed()
.collect(Collectors.toMap(
grade -> grade,
grade -> studentsByGrade.getOrDefault(grade, 0L),
(a, b) -> a,
TreeMap::new
));

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