Check out example codes for "java collectors mapping". It will help you in understanding the concepts better.

Code Example 1

static List<Employee> employeeList
      = Arrays.asList(new Employee("Tom Jones", 45, 15000.00),
      new Employee("Harry Andrews", 45, 7000.00),
      new Employee("Ethan Hardy", 65, 8000.00),
      new Employee("Nancy Smith", 22, 10000.00),
      new Employee("Deborah Sprightly", 29, 9000.00));
 
  public static void main(String[] args) {
    List<String> employeeNames = employeeList
        .stream()
        .collect(Collectors.mapping(Employee::getName, Collectors.toList()));

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