How to loop to find maximum value in a Arraylist

Question

I have this project that i'm making that you import your birthday, name, birthyear, and birthmonth. I'm trying to make a method that finds the most popular birthday for example, "the most popular date is the 16th with blank birthdays. I know I would have to loop around birthDayStats but how would I write that? here's my code, and the method i'm having trouble with is on the bottom.

import java.util.ArrayList;
import java.util.Collections;

public class Analyzer {
    // instance variables - replace the example below with your own
    private final static int DAYS_PER_MONTH = 31;
    private final static int MONTHS_PER_YEAR = 12;
    private int[] birthDayStats;
    private int[] birthMonthStats;
    private ArrayList<Person> people;

    /**
     * Constructor for objects of class Analyzer
     */
    public Analyzer() {
        this.people = new ArrayList<Person>();
        this.birthDayStats = new int[Analyzer.DAYS_PER_MONTH];
        this.birthMonthStats = new int[Analyzer.MONTHS_PER_YEAR];
    }

    public void addPerson(String name, int birthDay, int birthMonth, int birthYear) {
        Person person = new Person(name, birthDay, birthMonth, birthYear);
        if (person.getBirthDay() != -1 || person.getBirthMonth() != -1) {
            people.add(person);
            birthMonthStats[birthMonth - 1]++;
            birthDayStats[birthDay - 1]++;
        } else {
            System.out.println("Your current Birthday is " + birthDay + " or "
                    + birthMonth + " which is not a correct number 1-31 or 1-12 please put in a correct number ");
        }
    }

    public void printPeople() { //prints all people in form:   “  Name: Tom   Month: 5   Day: 2  Year: 1965”
        int index = 0;
        while (index < people.size()) {
            Person person = (Person) people.get(index);
            System.out.println(person);
            index++;
        }
    }

    public void printMonthList() { //prints the number of people born in each month Sample output to the right with days being similar
        int index = 0;
        while (index < birthMonthStats.length) {
            System.out.println("Month number " + (index + 1) + " has " + birthMonthStats[index] + " people");
            index++;
        }
    }

    public void mostPopularDay() { //finds the most popular Day of the year
        Object obj = Collections.mostPopularDay(birthDayStats);
        System.out.println(obj);
    }
}

What is Object obj = Collections.mostPopularDay(birthDayStats); supposed to do? That looks like all kinds of invalid. What have you tried already to solve this? — Carcigenicate
– Carcigenicate, Commented Sep 25, 2017 at 13:31
Collections.mostPopularDay isn't a thing... What did you expect that to do? — OneCricketeer
– OneCricketeer, Commented Sep 25, 2017 at 13:32
if(person.getBirthDay()!=-1|| person.getBirthMonth() != -1) should be if(person.getBirthDay()>0 && person.getBirthMonth() > 0) — Yahya
– Yahya, Commented Sep 25, 2017 at 13:35
Possible duplicate of How can I locate and print the index of a max value in an array? — PM 77-1
– PM 77-1, Commented Sep 25, 2017 at 13:41

Tobias Geiselmann · Accepted Answer · 2017-09-25 13:39:56Z

1

You have to iterate over your array which holds the days, find the highest count and return the index of that value.

public int mostPopularDay() { //finds the most popular Day of the year
    int popularDay = 0;
    int max = -1;
    for(int i = 0; i < birthDayStats.length; i++) {
        if(birthDayStats[i] > max) {
            max = birthDayStats[i];
            popularDay = i;            
        }
    }
    return popularDay + 1;  //Adding +1 since there is no 0th day of the month
}

Keep in mind, that this does only return the most popular day for birthdays, not the most popular date. But I assume that this is what you wanted.

edited Sep 25, 2017 at 13:39

answered Sep 25, 2017 at 13:36

Tobias Geiselmann

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5 Comments

Stevo4586 Over a year ago

i get the error incompatible types: unexpected return value under

Stevo4586 Over a year ago

return popularDay;

Tobias Geiselmann Over a year ago

I updated it, should work now. The method signature stated void where it should have been int for the return type.

Stevo4586 Over a year ago

Ok cool and yes im just trying to find the most popular birth day for birthdays and then ill find the make a method for most popular birth months for birthdays

Tobias Geiselmann Over a year ago

That's perfectly alright. But keep in mind, that those two results (day and month) don't necessarily correspond. Let's say you get day = 16 and month = 5. That doesn't mean, that the 16th of May is the most popular birth date ;)

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