java 根据其属性而不是值对 HashMap 对象进行排序
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Sorting HashMap objects on their properties rather values
提问by Aahil
This is not my real code I have just simulated in order to understand what to do next.
I have class Person with properties age, height weight.
Now In my class Group
I create two four objects
这不是我为了了解下一步要做什么而模拟的真实代码。
我有一个具有年龄、身高体重属性的 Person 类。
现在在我的班级 Group
我创建了两个四个对象
Person programmer, student, clerk, tech;
I have HashMap rollCall
我有 HashMap rollCall
Map<Person, Integer> rollCall = new HashMap<Person, Integer>();
to add all these using Person and number of Persons as type Integer
使用 Person 和 Number of Persons 作为整数类型添加所有这些
rollCall.put(programmer, 1);
rollCall.put(clerk, 2);
rollCall.put(student, 1);
rollCall.put(tech, 3);
I have seen alot of people sorting HashMap using TreeMap on value I want to sort on a property of Person rather on value. I want to sort all these people on their age (i.e. programmer.getAge();). I am not sure if I will use comprator which works only on collection not map. . Please help ... .
我见过很多人使用 TreeMap 根据值对 HashMap 进行排序，我想根据 Person 的属性而不是值进行排序。我想按年龄对所有这些人进行排序（即程序员.getAge();）。我不确定我是否会使用仅适用于集合而非地图的比较器。. 请帮忙 ... 。
回答by maerics
You can get a Map<Person,Integer>which iterates by age increasing or decreasing order by using a custom comparator:
您可以Map<Person,Integer>使用自定义比较器获得按年龄递增或递减顺序迭代的 a ：
Map<Person, Integer> rollCall = new TreeMap<Person, Integer>(
new Comparator<Person>() {
@Override public int compare(Person p1, Person p2) {
return p1.getAge() - p2.getAge(); // Acending.
// or p2.getAge() - p1.getAge(); // Descending.
}
}
);
When you add Persons to the collection they will be inserted in order by age.
当您将 Person 添加到集合中时，它们将按年龄顺序插入。
回答by Ernest Friedman-Hill
First of all, TreeMapsorts on keys, not values. So that's already working in your favor. Any object you use as a key in a TreeMapmustimplement Comparable, or you must provide a Comparatoras a constructor argument. All you need to do is have your compareTo()method (from Comparable) or compare()method (from Comparator) compare based on your getAge()property.
首先，TreeMap按键排序，而不是按值排序。所以这已经对你有利了。您在 a 中用作键的任何对象都TreeMap必须实现Comparable，或者您必须提供 aComparator作为构造函数参数。您需要做的就是根据您的财产比较您的compareTo()方法 (from Comparable) 或compare()方法 (from Comparator) getAge()。
The TreeMapconstructor that takes a Comparatoris described here.The Comparatorwill be used to sort the keys in the map.
回答by Pa?lo Ebermann
You need to be able to compare your Person objects. If there is a canonical way to compare them, let them implement Comparable<Person>(i.e. give them a compareTo(Person)method.
您需要能够比较您的 Person 对象。如果有比较它们的规范方法，让它们实现Comparable<Person>（即给它们一个compareTo(Person)方法。
If this is done, you can use the persons as keys for a SortedMap (like TreeMap).
如果这样做，您可以使用人作为 SortedMap（如 TreeMap）的键。
If there are multiple ways two persons could be compared, implement a Comparator<Person>as a separate object.
如果有多种方法可以比较两个人，请将 a 实现Comparator<Person>为单独的对象。
Then give this comparator to the SortedMap on construction.
然后在构造时将此比较器提供给 SortedMap。
This will not sort your HashMap (a HashMap has always a seemingly random order), but give you another sorted datastructure instead.
这不会对您的 HashMap 进行排序（HashMap 总是看似随机的顺序），而是为您提供另一个已排序的数据结构。
回答by Saurabh Kachhia
import java.util.Comparator;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.TreeMap;
public class PersonSort {
private MySort sort = new MySort();
private Map<Person, String> map = new HashMap<Person, String> ();
private Map<Person, String> treeMap = new TreeMap<Person, String>(sort);
Person e1 = new Person(500, "Saurabh");
Person e2 = new Person(400, "Kishan");
Person e3 = new Person(900, "Ashwini");
public void myMap() {
map.put(e3, "Ash");
map.put(e2, "Krish");
map.put(e1, "Sau");
Iterator it = map.keySet().iterator();
System.out.println("UnSorted Map");
while(it.hasNext()) {
System.out.println(map.get(it.next()));
}
treeMap.putAll(map);
System.out.println("SortedMap");
Iterator it1 = treeMap.keySet().iterator();
while(it1.hasNext()) {
System.out.println(treeMap.get(it1.next()));
}
}
public static void main(String[] args) {
PersonSort es = new PersonSort();
es.myMap();
}
}
class Person {
Person(int id, String name) {
this.id = id;
this.name = name;
}
private int id;
private String name;
//Getters and Setters
}
class MySort implements Comparator<Object> {
public int compare(Object o1, Object o2) {
return ((Person) o1).getId() - ((Person)o2).getId();
}
}
回答by utpal kanta kar
import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.Comparator;
import java.util.HashMap;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;
/*
* Sort HashMap that contains Student object
*/
public class SortHashMap implements Comparator<Student>
{
public static void main(String[] args)
{
Map map = new HashMap();
map.put("s1", new Student(5,"utpal"));
map.put("s2", new Student(4,"ramesh"));
map.put("s3", new Student(10,"tushar"));
map.put("s4", new Student(2,"anindya"));
Collection<Student> students = map.values();
List list = new ArrayList(students);
Collections.sort(list,new SortHashMap());
for (Iterator it = list.iterator(); it.hasNext();)
{
Student stdn = (Student)it.next();
System.out.println("Student id : "+stdn.id);
System.out.println("Student Name : "+stdn.name);
}
}
@Override
public int compare(Student s1, Student s2)
{
return s1.name.compareTo(s2.name);
}
}
class Student
{
int id;
String name;
Student(int id,String name)
{
this.id = id;
this.name = name;
}
}
