在Java8中将arraylist嵌套到hashmap

Question

I have below Objects -

Company.java

private String compName;
private List<Department> departments;

Department.java

private String deptId;
private String deptName;
private List<Employee> employees;

Employee.java

private String empId;
private String legalStatement;

I want to return Map of each employee legal statement -

Map<String, String> legalStatement = new HashMap<>();

For that existing logic is -

for(Department department : departments){
   if(department.getEmployees() != null && 
      department.getEmployees().size() > 0){
      for(Employee employee : department.getEmployees()){
        legalStatement.put(employee.getEmpId(), 
        employee.getLegalStatement());
      } 
   }
}

How I can write same thing in Java 8 Stream API.

Answer 1

You can use stream's filter for if , flatMap for internal employee list and collect to collect as a Map :

Map<String, String> legalStatement = departments.stream()
        .filter(department -> department.getEmployees() != null && !department.getEmployees().isEmpty())
        .flatMap(department -> department.getEmployees().stream())
        .collect(Collectors.toMap(Employee::getEmpId, Employee::getLegalStatement));

Answer 2

By using flatMap and ternary operator

Map<String,String> result = departments.stream()
 .flatMap(d->(Objects.nonNull(d.getEmployees()) && !d.getEmployees().isEmpty()) ? d.getEmployees().stream() : Stream.empty())
 .collect(Collectors.toMap(Employee::getEmpId, Employee::getLegalStatement));

As @nullpointer suggestion if there is a chance for duplicate employee id use mergeFunction

public static <T,K,U> Collector<T,?,Map<K,U>> toMap(Function<? super T,? extends K> keyMapper,
                                                Function<? super T,? extends U> valueMapper,
                                                BinaryOperator<U> mergeFunction)

If the mapped keys contains duplicates (according to Object.equals(Object)), the value mapping function is applied to each equal element, and the results are merged using the provided merging function.