在java中，如何计算arraylist中相同数字的出现次数？
我有一个java的arraylist，其中的内容是通过用户的输入添加的。用户可以添加姓名、月份编号和年份编号。比如：姓名：鲍勃，月份：4，年份：11 我的任务是找出有多少用户在arraylist中添加了相同的月号。计算同一个月数的发生次数并打印出来 我知道我必须遍历arraylist并将发生的事件存储在何处，直到迭代器完成对arraylist集合的搜索，然后打印出添加了多少次相同的月数 我被这项任务困住了。即使困难，这也是一项容易的任务 谢谢你的帮助 我有三节课在java中，如何计算arraylist中相同数字的出现次数？,java,arraylist,Java,Arraylist,我有一个java的arraylist，其中的内容是通过用户的输入添加的。用户可以添加姓名、月份编号和年份编号。比如：姓名：鲍勃，月份：4，年份：11 我的任务是找出有多少用户在arraylist中添加了相同的月号。计算同一个月数的发生次数并打印出来 我知道我必须遍历arraylist并将发生的事件存储在何处，直到迭代器完成对arraylist集合的搜索，然后打印出添加了多少次相同的月数 我被这项任务困住了。即使困难，这也是一项容易的任务 谢谢你的帮助 我有三节课 public class Per
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;
}
}
/**
* 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;
}
}
/**
* 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;
}
}
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;
有什么帮助吗？因为你已经两个小时没有发帖了，这里有一个方法：
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;
}
对于这类任务，a将使用一个哈希表，其中特定月份的所有人都将在一个列表中索引到一起
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);
}
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);
}
哈希表非常适合此类任务，如果您必须将元素重新组合在一起或使用键访问值，则此处的月份为整数，值为List。您已经正确理解了这一点。。你困在哪里？听起来像是家庭作业。它是？到目前为止你试过什么？你得到了什么样的输出，与你期望的有什么不同？@M.Sameer几年前我在大学里上过java课程。我知道我正在努力更新对java的一些基本理解。我试图通过arraylist进行迭代，但我无法计算有多少人添加了第4个月作为他们的inputHello，感谢大家的回答。我尝试过几种方法，比如for循环和通过arraylist进行迭代，但我不知道如何正确编写代码。我是否应该制作第二个arraylist来存储我在arraylist中找到的月数，然后打印出添加了多少个月这是我到目前为止的代码。迭代器it=club.Iterator；whileit.hasNext{Membership Membership=membershit.next；ifjoined==Membership.getMonth count++；{ArrayList nmonth=new ArrayList；nmonth.addcount；int sum=0；forIterator i=nmonth.iterator；i.hasNext；{sum+=count；返回sum；}如何将计数添加到新的arraylist中，然后求和并打印出总数？@Lynch感谢您的回答。是否有一些方法可以不使用哈希表来完成此任务？@Kare是的，您可以使用列表列表：list PersonLists=new arraylist；PersonLists.addnew arraylist；。您必须在顶级列表中添加12个新列表。然后您可以使用以下命令填充列表中的元素：personsLists.getperson.month-1.addperson。@Lynch我到目前为止所拥有的。您能告诉我如何将指定的数字从ArrayList 1添加到ArrayList 2中，然后打印出来吗？public int joinedMontHint month{int joined=month；if joined<1 | | joined>12{System.out.println不是有效月份；}否则{int count=0；//int countMount Iterator it=club.Iterator；whileit.hasNext{Membership Membership=membershit.next；ifjoined==Membership.getMonth count++；@Kare您所做的似乎是正确的。您希望得到什么？所有计数的总和？还是只希望输出显示月份id，然后显示与之关联的计数？是的，所选月份的所有计数的总和。如何计算将计数值添加到新的arraylist或数组中，然后打印总和？