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How to write list filtering and displaying lambda to list

This is the source:

List<Role> managedRoles = new ArrayList<>();
for (Role role : roles) {
    if (role.getManagedRole() != null) { // a list of Role.
        managedRoles.addAll(role.getManagedRole());
    }
}

This is what I want:

managedRoles = roles.stream().filter(r -> r.getManagedRole() != null).map(role -> role.getManagedRole()).collect(); // how to addAll ?

But role.getManagedRole() is a List<Role>

I think we need some function like addAll

. So how do you do this in Lambda?

+3


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2 answers


You need to use flatMap

instead map

to flatten everything List<Role>

returned role.getManagedRole()

by one Stream<Role>

.



List<Role> managedRoles = 
    roles.stream()
         .filter(r -> r.getManagedRole() != null)
         .flatMap(role -> role.getManagedRole().stream())
         .collect(Collectors.toList());
+5


source 


managedRoles = roles.stream()
      .filter(r -> r.getManagedRole() != null)
      .flatMap(role -> role.getManagedRole())
      .collect(Collectors.toList());

to create as a list or try this example to add elementrs to an existing list



List<String> destList = Collections.synchronizedList(
                 new ArrayList<>(Arrays.asList("foo")));
List<String> newList = Arrays.asList("0", "1", "2", "3", "4", "5");
newList.stream()
       .collect(Collectors.toCollection(() -> destList));
System.out.println(destList);
0


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