HashMaps list sorting based on hashMap values ​​[not keys]

Question

Here is what I have -

How can I match multiple keys and their values? I am using employeeId right now, but I would like to include departmentId and others in my comparison for sorting ...

import java.util.ArrayList; import java.util.Collections; import java.util.Comparator; import java.util.Date; import java.util.HashMap; import java.util.List; public class Tester { boolean flag = false ; public static void main(String args[]) { Tester tester = new Tester() ; tester.printValues() ; } public void printValues () { List<HashMap<String,Object>> list = new ArrayList<HashMap<String,Object>>() ; HashMap<String,Object> map = new HashMap<String,Object>(); map = new HashMap<String,Object>(); map.put("employeeId", new Integer(1234)) ; map.put("departmentId", new Integer(110)) ; map.put("someFlag", "B") ; map.put("eventTypeId", new Integer(11)) ; map.put("startDate", new Date() ) ; map.put("endDate", new Date() ) ; list.add(map); map = new HashMap<String,Object>(); map.put("employeeId", new Integer(456)) ; map.put("departmentId", new Integer(100)) ; map.put("someFlag", "B") ; map.put("eventTypeId", new Integer(11)) ; map.put("startDate", new Date() ) ; map.put("endDate", new Date() ) ; list.add(map); map = new HashMap<String,Object>(); map.put("employeeId", new Integer(1234)) ; map.put("departmentId", new Integer(10)) ; map.put("someFlag", "B") ; map.put("eventTypeId", new Integer(17)) ; map.put("startDate", new Date() ) ; map.put("endDate", new Date() ) ; list.add(map); map = new HashMap<String,Object>(); map.put("employeeId", new Integer(1234)) ; map.put("departmentId", new Integer(99)) ; map.put("someFlag", "B") ; map.put("eventTypeId", new Integer(11)) ; map.put("startDate", new Date() ) ; map.put("endDate", new Date() ) ; list.add(map); map = new HashMap<String,Object>(); map.put("employeeId", new Integer(1234)) ; map.put("departmentId", new Integer(100)) ; map.put("someFlag", "B") ; map.put("eventTypeId", new Integer(11)) ; map.put("startDate", new Date() ) ; map.put("endDate", new Date() ) ; list.add(map); map = new HashMap<String,Object>(); map.put("employeeId", new Integer(567)) ; map.put("departmentId", new Integer(200)) ; map.put("someFlag", "P") ; map.put("eventTypeId", new Integer(12)) ; map.put("startDate", new Date() ) ; map.put("endDate", new Date() ) ; list.add(map); Collections.sort ( list , new HashMapComparator2 () ) ; for( int i = 0 ; i < list.size() ; i ++ ) { System.out.println(list.get(i)); } System.out.println("======================================"); flag = true ; // desc Collections.sort ( list , new HashMapComparator2 () ) ; for( int i = 0 ; i < list.size() ; i ++ ) { System.out.println(list.get(i)); } } public class HashMapComparator2 implements Comparator { public int compare ( Object object1 , Object object2 ) { if ( flag == false ) { Integer obj1Value = ( Integer ) ( ( HashMap ) object1 ).get ( "employeeId" ) ; Integer obj2Value = ( Integer ) ( ( HashMap ) object2 ).get ( "employeeId" ) ; return obj1Value.compareTo ( obj2Value ) ; } else { Integer obj1Value = ( Integer ) ( ( HashMap ) object1 ).get ( "employeeId" ) ; Integer obj2Value = ( Integer ) ( ( HashMap ) object2 ).get ( "employeeId" ) ; return obj2Value.compareTo ( obj1Value ) ; } } } } 

Answer 1

The easiest is to use CompareToBuilder from commons- lang . Your example would look like this:

 Map<String, Object> map1 = (Map<String, Object>) object1; Map<String, Object> map2 = (Map<String, Object>) object2; if ( flag == false ) { return new CompareToBuilder() .append(map1.get("employeeId"), map2.get("employeeId")) .append(map1.get("departmentId"), map2.get("departmentId")) .toComparison(); } else { return new CompareToBuilder() .append(map2.get("employeeId"), map1.get("employeeId")) .append(map2.get("departmentId"), map1.get("departmentId")) .toComparison(); } 

Or something like that. In any case, I would definitely recommend using Genrics in your comparators, as Daniel suggested.

Answer 2

First I will create a class for storing data instead of using the HashMaps list. Then make this class implement the Comparable interface, which allows you to define a fine-grained comparison algorithm.

If you absolutely must use HashMap, then I would create a class that extends HashMap and implements Comparable. But I do not recommend this approach.

 public class Foo extends HashMap implements Comparable { private boolean ascending = true; public int compareTo(Object bar) { int result; if (bar == null || !(bar instanceof Foo)) { result = -1; } Foo _rhs = (Foo)bar; result = new CompareToBuilder().append(get("employeeId"),_rhs.get("employeeId")) .append(get("departmentId"),_rhs.get("departmentId")).toComparison(); return (ascending ? result : -result); } public void setAscending(boolean asc) { ascending = asc; } } 

There is no guarantee that this code will compile or return the correct results. I really like CompareToBuilder

Answer 3

Martinatime's answer is correct. Create a class to store your data. Then you put it in a map that supports key sorting, like TreeMap:

 new TreeMap<Integer, YouNewClass>(new Comparator<YourNewClass>() { public int compare(YourNewClass o1, YourNewClass o2) { implement the method here as per your logic. } }); 

Enjoy: http://java.sun.com/j2se/1.4.2/docs/api/java/util/TreeMap.html