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Java 8 - Streams - Collectors - Grouping By


  • groupingBy operation on Collectors provides a way to aggregate the result of a stream operation into a map, where the map contains items whose keys are the groupings.
  • There are two implementations, the first implementation performs on-stage grouping, the second performs two-stage grouping (grouping and reduction)

stream of employees
Stream<Employee> employees = Stream.of( new Employee("Joe", "Blogs", "Sales", 100.0), new Employee("Richardo", "Banks", "Engineering", 50.0), new Employee("Hewlet", "Packer", "Warehouse", 40.0), new Employee("Shelly", "Cooper", "Warehouse", 40.0) );

another stream of employees
Stream<Employee> employees_2 = Stream.of( new Employee("Joe", "Blogs", "Sales", 100.0), new Employee("Richardo", "Banks", "Engineering", 50.0), new Employee("Hewlet", "Packer", "Sales", 140.0) );



USAGE

 collecting stream in a Map using one-stage grouping

Map<String, List<Employee>> employeeGroup = employees .collect(Collectors.groupingBy(employee -> employee.getDepartment()));
 
collecting stream in a Map using two-stage grouping and reduction

 Map<String, Double> employeeGroup_reduced = employees_2.collect( Collectors.groupingBy(employee -> employee.getDepartment(), Collectors.summingDouble(employee -> employee.getSalary()) ) );



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