如何计算 java 中 arraylist 中相同数字的出现次数？

Question

I have an arraylist in java where the content is added from input of the user. The user add name, number for the month and a number for the year. Like this: Name: Bob, month: 4, year: 11

My task is to find how many user have added the same month number to the arraylist. Count the occurence of the same month number and print it out.

I know I must iterate over the arraylist and to store the occurence somwhere until the iterator have finished to search trough the collection of the arraylist and then print out how many times the same month number has been added.

I am quiet stuck on this task. Even tough it is an easy task.

Thank for your help!

I have three classes

public class Person
{
    // The name of this user.
    private final String name;

    /**
     * Create a new user with the given name.
     * @param name The user's name.
     */
    public Person(String name)
    {
        this.name = name;
    }

    /**
     * @return The user's name.
     */
    public String getName()
    {
        return name;
    }
}

/////////////////////////////////

/**
 * Store details of a club membership.
 * 
 */
public class Membership
{
    // The name of the member.
    private String name;
    // The month in which the membership was taken out.
    private int month;
    // The year in which the membership was taken out.
    private int year;

    /**
     * Constructor for objects of class Membership.
     * @param name The name of the member.
     * @param month The month in which they joined. (1 ... 12)
     * @param year The year in which they joined.
     */
    public Membership(String name, int month, int year)
        throws IllegalArgumentException
    {
        if(month < 1 || month > 12) {
            throw new IllegalArgumentException(
                "Month " + month + " out of range. Must be in the range 1 ... 12");
        }
        this.name = name;
        this.month = month;
        this.year = year;
    }

    /**
     * @return The member's name.
     */
    public String getName()
    {
        return name;
    }

    /**
     * @return The month in which the member joined.
     * A value in the range 1 ... 12
     */
    public int getMonth()
    {
        return month;
    }

    /**
     * @return The year in which the member joined.
     */
    public int getYear()
    {
        return year;
    }

    /**
     * @return A string representation of this membership.
     */
    public String toString()
    {
        return "Name: " + name +
               " joined in month " +
               month + " of year " + year;
    }
}

////////////////////////////////////////////

    import java.util.ArrayList;
    import java.util.Iterator;
    /**
     * Store details of club memberships.
     * 
     */
    public class Club
    {
        // Define any necessary fields here ...
        private ArrayList club;
       // private String member = club;
        private int joined; 
        /**
         * Constructor for objects of class Club
         */
        public Club()
        {
            // Initialise any fields here ...
            club = new ArrayList();

        }

        /**
         * Add a new member to the club's list of members.
         * @param member The member object to be added.
         */
        public void join(Membership member)
        {
          club.add(member);
          //System.out.println(member);


        }

//////////////////////////////////////////////////////////    
//    public int getJoined()
//    {
//        return joined;
//    }
//////////////////////////////////////////////////////////    
    /**
     * @return The number of members (Membership objects) in
     * the club.
     */
     public int numberOfMembers()
    {
       return club.size();
    }
///////////////////////////////////////////////////////////////////////////////////    
    public int joinedInMonth(int month)
    {
         //int joined = month;
        if(month < 1 || month > 12)
        {
            System.out.println("Not a valid month");

        }


      else{

//     int countMonth(ArrayList<Person> person, int month)
  {
      int count = 0;
        for (Club club : people)
         if (person.getMonth() == month) count++;
           return count;
} 

        }
        // using traditional for loop
       //  int joined = month;
       // for(int i = 0; i < club.size(); i++)
        //                      {

        //   System.out.println(i+1 + ": " + club.get(i));
         //  }       
//              

//               for(Iterator i = club.iterator();i.hasNext();)
//                  {
//                     
//                    System.out.println(i.next());
//   
//
//          }
         return 0;   
      }

    }

/////////////////////////////////////////

This is what I have so far with the method to count occurence of the same number in the arraylist:

 public int joinedInMonth(int month)
    {
        int joined = month;
        if(joined < 1 || joined > 12){
            System.out.println("Not a valid month");
            }
      else{   
           int count = 0;
          Iterator it = club.iterator();
               while(it.hasNext()) {
          Membership membership = (Membership) it.next();
           if(joined == membership.getMonth());count++;


               System.out.println("Month " + membership.getMonth());
                    return count;

but I cant understand how I can store the value from count into a new arraylist? Any help?

Answer 1

Since you haven't posted for 2 hours, here's one way:

class Person {
    String name;
    int month;
    int year;
    // with getters()
}

int countMonth(List<Person> people, int month) {
    int count = 0;
    for (Person person : people)
        if (person.getMonth() == month) count++;
    return count;
}

Answer 2

For this kind of task a would use a Hashtable<Integer, List<Person>> where all person of a particular month would be index toghether in one List.

Hashtable<Integer, List<Person>> index = new Hashtable<Integer, List<Person>>();
for (Person person : persons) {
    if (index.get(person.month) == null) {
        index.put(person.month, new ArrayList<Person>());
    }
    index.get(person.month).add(person);
}

The you can retrive a list of person according to their month. This code will only print the month and the number of persons contained in list.

for (int i = 1; i <= 12; i++) {
    int count = 0;
    if (index.get(i) != null) {
        count = index.get(i).size();
    }
    System.out.println("%d: %d", i, count);
}

Hashtables are really nice for this kind of tasks were you have to regroup elements toghether or access an value using a key (here its the month as an Integer and the value is List<Person> ).