I have a class like this
ObjectA
id
type
where id is unique but the type can be duplicate so if I have a list of ObjectA like this
(id, type) = (1, "A") , (2, "B"), (3, "C"), (4, "A")
then I want Map<type, List<id>> where
map is
< "A",[1,4],
"B",[2],
"C",[3] >
It is working with Java 7 but can't find a way to do it in Java 8. In Java 8 I can create key, value pair but not able to create a list if a type is same.
yourTypes.stream()
.collect(Collectors.groupingBy(
ObjectA::getType,
Collectors.mapping(
ObjectA::getId, Collectors.toList()
)))
Employee list to map
List<Employee> list = new ArrayList<>();
list.add(new Employee(1, "John",80000));
list.add(new Employee(2, "Jack", 90000));
list.add(new Employee(3, "Ricky", 120000));
// key = id, value - name
Map<Integer, String> result1 = list.stream().collect(
Collectors.toMap(Employee::getId, Employee::getName));
Employee::getName?
Map<Integer, String> result1 = list.stream().collect( Collectors.toMap(Employee::getId, "constant value here like this")); @ArunGowdaThough you got some nice answer. I would like to share another way of doing same. It's when you have to tackle really complex conversions to map.
class Student {
private String name;
private Integer id;
//getters / setters / constructors
}
Map<String, List<Integer>> map = Arrays.asList(
new Student("Jack", 2),
new Student("Jack", 2),
new Student("Moira", 4)
)
.stream()
.collect(
Collectors.toMap(
Student::getName,
student -> {
List list = new ArrayList<Integer>();
list.add(student.getId());
return list;
},
(s, a) -> {
s.add(a.get(0));
return s;
}
)
);
Notice how complex you could play with value and return the new value accordingly.
toMap parameters are respectively keyMapper function, valueMapper function and merger respectively.
Cheers!
A way to do it without streams:
Map<String, List<Integer>> map = new HashMap<>();
list.forEach(object ->
map.computeIfAbsent(object.getType(), k -> new ArrayList<>())
.add(object.getId()));
This uses Iterable.forEach and Map.computeIfAbsent to iterate the list and group its elements (here named object) by its type attribute, creating a new ArrayList if there was no entry for that type in the map yet. Then, the id of the object is added to the list.
List<Employee> list = new ArrayList<>();
list.add(new Employee(1, "John",80000));
list.add(new Employee(2, "Jack", 90000));
list.add(new Employee(3, "Ricky", 120000));
// key = id, value - name
Map<Integer, String> result = list.stream().collect(
Collectors.toMap(Employee::getId, Employee::getName));
If you want to give a constant value instead of name:
// key = id, value - "ABC"
Map<Integer, String> result = list.stream().collect(
Collectors.toMap(Employee::getId, emp-> "ABC"));
List to Map object
employeeList.stream().collect(Collectors.groupingBy(Employee::getName, Collectors.toList())) .forEach((key,value)->System.out.println("key : "+key+" , value : "+value));
key: Name : value : List of employee Object
