Display the names of people's birthdays under the months

Question

I need to display the names of people whose birthday occur in a certain month. The name of the month must still appear, even if there was no birthday that month. (All info is extracted from a text file)

Eg.

January:
Sam
Kate

February:
//no birthday in Feb

March:
Fred
etc

Method:

private Employee[] arr = new Employee[8];//Employee class

public String birthdays() throws ParseException{
        String list = "";
        for(int i = 0; i < arr.length; i++){
            String month = arr[i].getDob().substring(3, 5);
            if(month.equals("01")){
                System.out.println("Jan" + "\n" + arr[i].getName() + " " + arr[i].getSurname());
            }else if(month.equals("02")){
                System.out.println("Feb" + "\n" + arr[i].getName() + " " + arr[i].getSurname());
            }//if
        }//for
        
        return list;
    }//birthdays

The problem is that it seems like that repeating the "if statement" is not really good coding and that if a birthday does not occur in a certain month, the name of the month will not appear.

How would I solve this?

You can create regex for each month and list down matches accordingly. — semicolon, Aug 21 '21 at 17:15
Can you include the definition for `arr` and the type of its elements? — kingkupps, Aug 21 '21 at 17:20
Instead of a series of if else statements, you could use a switch block to define all the conditions, and prepare a Hashmap with keys as month. — vaibhavsahu, Aug 21 '21 at 17:23
Is your `dob` (date of birth) a `String`? That’s bad. Unconditionally use a `LocalDate` for it. — Ole V.V., Aug 21 '21 at 19:38

score 2 · Answer 1 · answered Aug 21 '21 at 18:21

java.time

I recommend you use java.time, the modern Date-Time API to avoid the repetition of if-else if conditions. With this, you will reduce the existing code inside your for loop to just this much:

int month = Integer.parseInt(arr[i].getDob().substring(3, 5));
System.out.println(Month.of(month).getDisplayName(TextStyle.SHORT, Locale.ENGLISH) + "\n" + arr[i].getName() + " " + arr[i].getSurname());

Demo:

import java.time.Month;
import java.time.format.TextStyle;
import java.util.Locale;

public class Main {
    public static void main(String[] args) {
        System.out.println(Month.of(1).getDisplayName(TextStyle.SHORT, Locale.ENGLISH));
    }
}

Output:

Jan

ONLINE DEMO

Learn more about the modern Date-Time API^* from Trail: Date Time.

^{* For any reason, if you have to stick to Java 6 or Java 7, you can use ThreeTen-Backport which backports most of the java.time functionality to Java 6 & 7. If you are working for an Android project and your Android API level is still not compliant with Java-8, check Java 8+ APIs available through desugaring and How to use ThreeTenABP in Android Project.}

Thanks. That does remove the repetition of the "if statement", but I need to display all the months, and then all the names of the employees whose birthday falls in that month. — user14229161, Aug 21 '21 at 18:44

score 2 · Answer 2 · edited Aug 21 '21 at 23:34

java.time

Like Arvind Kumar Avinash I strongly recommend that you use java.time, the modern Java date and time API, for your date work. It starts in the Employee class, where dob is a LocalDate (not a string; you don’t keep numbers and Boolean values in strings either do you? I hope not).

public class Employee {

    String name;
    LocalDate dob;
    
    /** Convenience constructor accepting dob as string, for example 1986-05-17 */
    public Employee(String name, String dob) {
        this.name = name;
        this.dob = LocalDate.parse(dob);
    }

    public String getName() {
        return name;
    }

    public Month getBirthMonth() {
        return dob.getMonth();
    }
}

In Java 16 and later, write that class more briefly as a record, with or without the constructor seen above.

record Employee ( String name , LocalDate dob ) {}

Now we can do for example:

    Employee[] arr = {
            new Employee("James", "2008-04-15"),
            new Employee("Jennifer", "1984-10-08"),
            new Employee("John", "2003-05-22"),
            new Employee("Linda", "1965-11-29"),
            new Employee("Mary", "1982-06-15"),
            new Employee("Michael", "1942-12-07"),
            new Employee("Patricia", "2004-03-11"),
            new Employee("Robert", "1951-11-09")
    };
    
    Map<Month, List<Employee>> dobPerMonth = Arrays.stream(arr)
            .collect(Collectors.groupingBy(Employee::getBirthMonth));
    
    for (Month m : Month.values()) {
        System.out.println(m.getDisplayName(TextStyle.FULL_STANDALONE, Locale.ENGLISH));
        List<Employee> empsThisMonth = dobPerMonth.get(m);
        if (empsThisMonth == null) {
            System.out.println("//no birthday in " + m.getDisplayName(TextStyle.SHORT_STANDALONE, Locale.ENGLISH));
        } else {
            for (Employee employee : empsThisMonth) {
                System.out.println(employee.getName());
            } 
        }
        System.out.println();
    }

I am giving you the full output:

January
//no birthday in Jan

February
//no birthday in Feb

March
Patricia

April
James

May
John

June
Mary

July
//no birthday in Jul

August
//no birthday in Aug

September
//no birthday in Sep

October
Jennifer

November
Linda
Robert

December
Michael

Link

Oracle tutorial: Date Time explaining how to use java.time.

score 0 · Answer 3 · answered Aug 21 '21 at 17:31

Use Hashmap:

    HashMap<String, ArrayList<Employee>> filterEmpBirthdays = new HashMap<>();
    for (int i = 0; i < arr.length; i++) {
        String month = arr[i].getDob().substring(3, 5);
        if (filterEmpBirthdays.containsKey(month)) {
            ArrayList<Employee> empBirthday = filterEmpBirthdays.get(month);
            empBirthday.add(arr[i]);
            filterEmpBirthdays.put(month, empBirthday);
        } else {
            ArrayList<Employee> empBirthday = new ArrayList<>();
            empBirthday.add(arr[i]);
            filterEmpBirthdays.put(month, empBirthday);
        }
    }

After that iterate the loop on HashMap

score 0 · Answer 4 · answered Aug 22 '21 at 01:36

Assuming the dob is in dd-mm-yyyy format, try this:

package wonderings;

import java.text.DateFormatSymbols;
import java.util.Arrays;
import java.util.Comparator;
import java.util.List;
import java.util.Map;
import java.util.function.Function;
import java.util.stream.Collectors;
import java.util.stream.IntStream;

import org.junit.Test;

public class CollectionsTest {

    @Test
    public void test1() {
        // Get an array of Month names
        final String[] months = new DateFormatSymbols().getMonths();
        // Create a function to convert dob to month of birth
        final Function<Person, String> toBirthMonth = person -> months[Integer.valueOf(person.getDob().substring(3, 5))
                - 1];
        // Synthesize test data
        final List<Person> people = Arrays.asList(new Person("Jane", "20-03-2000"), new Person("Tom", "22-04-1999"),
                new Person("Heroku", "01-01-1987"), new Person("Ivan", "30-04-1978"));
        // Group the Person instances by month of birth
        final Map<String, List<Person>> groupedBy = people.stream().collect(Collectors.groupingBy(toBirthMonth));
        // Now stream the results, printing a header and the either "none" or the list of Persons in name order
        IntStream.range(0, months.length - 1).forEach(month -> {
            System.out.println(months[month]);
            if (groupedBy.containsKey(months[month])) {
                groupedBy.get(months[month]).stream().sorted(Comparator.comparing(Person::getName))
                        .map(person -> "  " + person.getName()).forEach(System.out::println);
            } else {
                System.out.println("none");
            }
        });

    }

    private static class Person {
        private final String name;
        private final String dob;

        public Person(final String name, final String dob) {
            super();
            this.name = name;
            this.dob = dob;
        }

        public String getName() {
            return name;
        }

        public String getDob() {
            return dob;
        }

    }
}

An all Java 8 solution.

*An all Java 8 solution.* Not completely. `DateFormatSymbols` is outdated since Java 8. Use the `Month` enum and its `getDisplayName` method (as in my answer for example). And Java 8 or not, using substring and `Integer.parseInt()` for finding the month number is way to manual for my taste and also skips the date validation that I would have wanted to have. — Ole V.V., Aug 22 '21 at 05:56
Nice catch on the DateFormatSymbols. As for the date, given the example of a string the solution was correct. I agree with others that string dates are the last thing you want to do. — Earnie Dyke, Aug 22 '21 at 13:41

score 0 · Answer 5 · answered Aug 22 '21 at 14:31

You could use a Map to define your months, declare it as a data member of your class:

protected Map<String, String> months = null;

Inside your constructor, instantiate it and fill it:

this.months = new HashMap<String, String>();
this.months.put("01", "Jan");
this.months.put("02", "Feb");
this.months.put("03", "Mar");
this.months.put("04", "Apr");
this.months.put("05", "May");
this.months.put("06", "Jun");
this.months.put("07", "Jul");
this.months.put("08", "Aug");
this.months.put("09", "Sep");
this.months.put("10", "Oct");
this.months.put("11", "Nov");
this.months.put("12", "Dec");

Don't forget to import Map and HashMap. Now, let's refactor your method:

public String birthdays() throws ParseException{
        Map<String, List<String>> birthdays = new HashMap<String, String[]>();
        for (String key : months.keySet()) {
            birthdays.put(key, new ArrayList<String>());
        }
        String list = "";
        for(int i = 0; i < arr.length; i++){
            String month = arr[i].getDob().substring(3, 5);
            birthdays.get(month).push(arr[i].getName() + " " + arr[i].getSurname());
        }

        for (String secondKey : month.keySet()) {
            list += months.get(secondKey) + ":\n";
            for (String line : birthday.get(secondKey)) {
                list += "\t" + line + "\n";
            }
            list += "\n";
        }
        
        return list;
    }//birthdays

Explanation:

we first find out which month corresponds to which number
then we create an empty map for birthdays, with the keys being the months
we get through the names, putting each into the corresponding months
we get through the months, display them independently of whether there is any birthday associated to it
we display the names related to the month (if any) below the month

You’re reinventing the wheel. I recommend that instead you use [java.time, the modern Java date and time API](https://docs.oracle.com/javase/tutorial/datetime/). — Ole V.V., Aug 22 '21 at 17:17
@OleV.V. it can be used, that's certainly true. I prefer to answer questions in a way that's instructive for the askers and if they are beginners, then I aim to explain these things in a way that can be understood. If they just call some methods, from an API, then what do they learn? Yet, you are right, there's nothing wrong in using an API. — Lajos Arpad, Aug 22 '21 at 17:25
IMHO one of the most important thing for a programmer to learn is exactly using an API. Well, opinions differ, so thanks for presenting yours. PS Isn’t `Map` API too? Just teasing. :-) — Ole V.V., Aug 22 '21 at 20:18
@OleV.V. you are correct, LOL. I think we understand each-other's point though. — Lajos Arpad, Aug 24 '21 at 08:03

Display the names of people's birthdays under the months

5 Answers5

java.time

java.time

Link