Related
I read about sorting ArrayLists using a Comparator but in all of the examples people used compareTo which according to some research is a method for Strings.
I wanted to sort an ArrayList of custom objects by one of their properties: a Date object
(getStartDay()). Normally I compare them by item1.getStartDate().before(item2.getStartDate()) so I was wondering whether I could write something like:
public class CustomComparator {
public boolean compare(Object object1, Object object2) {
return object1.getStartDate().before(object2.getStartDate());
}
}
public class RandomName {
...
Collections.sort(Database.arrayList, new CustomComparator);
...
}
Since Date implements Comparable, it has a compareTo method just like String does.
So your custom Comparator could look like this:
public class CustomComparator implements Comparator<MyObject> {
#Override
public int compare(MyObject o1, MyObject o2) {
return o1.getStartDate().compareTo(o2.getStartDate());
}
}
The compare() method must return an int, so you couldn't directly return a boolean like you were planning to anyway.
Your sorting code would be just about like you wrote:
Collections.sort(Database.arrayList, new CustomComparator());
A slightly shorter way to write all this, if you don't need to reuse your comparator, is to write it as an inline anonymous class:
Collections.sort(Database.arrayList, new Comparator<MyObject>() {
#Override
public int compare(MyObject o1, MyObject o2) {
return o1.getStartDate().compareTo(o2.getStartDate());
}
});
Since java-8
You can now write the last example in a shorter form by using a lambda expression for the Comparator:
Collections.sort(Database.arrayList,
(o1, o2) -> o1.getStartDate().compareTo(o2.getStartDate()));
And List has a sort(Comparator) method, so you can shorten this even further:
Database.arrayList.sort((o1, o2) -> o1.getStartDate().compareTo(o2.getStartDate()));
This is such a common idiom that there's a built-in method to generate a Comparator for a class with a Comparable key:
Database.arrayList.sort(Comparator.comparing(MyObject::getStartDate));
All of these are equivalent forms.
Classes that has a natural sort order (a class Number, as an example) should implement the Comparable interface, whilst classes that has no natural sort order (a class Chair, as an example) should be provided with a Comparator (or an anonymous Comparator class).
Two examples:
public class Number implements Comparable<Number> {
private int value;
public Number(int value) { this.value = value; }
public int compareTo(Number anotherInstance) {
return this.value - anotherInstance.value;
}
}
public class Chair {
private int weight;
private int height;
public Chair(int weight, int height) {
this.weight = weight;
this.height = height;
}
/* Omitting getters and setters */
}
class ChairWeightComparator implements Comparator<Chair> {
public int compare(Chair chair1, Chair chair2) {
return chair1.getWeight() - chair2.getWeight();
}
}
class ChairHeightComparator implements Comparator<Chair> {
public int compare(Chair chair1, Chair chair2) {
return chair1.getHeight() - chair2.getHeight();
}
}
Usage:
List<Number> numbers = new ArrayList<Number>();
...
Collections.sort(numbers);
List<Chair> chairs = new ArrayList<Chair>();
// Sort by weight:
Collections.sort(chairs, new ChairWeightComparator());
// Sort by height:
Collections.sort(chairs, new ChairHeightComparator());
// You can also create anonymous comparators;
// Sort by color:
Collections.sort(chairs, new Comparator<Chair>() {
public int compare(Chair chair1, Chair chair2) {
...
}
});
For sorting an ArrayList you could use the following code snippet:
Collections.sort(studList, new Comparator<Student>(){
public int compare(Student s1, Student s2) {
return s1.getFirstName().compareToIgnoreCase(s2.getFirstName());
}
});
JAVA 8 lambda expression
Collections.sort(studList, (Student s1, Student s2) ->{
return s1.getFirstName().compareToIgnoreCase(s2.getFirstName());
});
OR
Comparator<Student> c = (s1, s2) -> s1.firstName.compareTo(s2.firstName);
studList.sort(c)
Yes, you can. There are two options with comparing items, the Comparable interface, and the Comparator interface.
Both of these interfaces allow for different behavior. Comparable allows you to make the object act like you just described Strings (in fact, String implements Comparable). The second, Comparator, allows you to do what you are asking to do. You would do it like this:
Collections.sort(myArrayList, new MyComparator());
That will cause the Collections.sort method to use your comparator for it's sorting mechanism. If the objects in the ArrayList implement comparable, you can instead do something like this:
Collections.sort(myArrayList);
The Collections class contains a number of these useful, common tools.
With Java 8 you can use a method reference for your comparator:
import static java.util.Comparator.comparing;
Collections.sort(list, comparing(MyObject::getStartDate));
Since technologies appear everyday, the answer will change in the time. I took a look at LambdaJ and seems very interesting.
You can try solving these tasks with LambdaJ. You can find it here: http://code.google.com/p/lambdaj/
Here you have an example:
Sort Iterative
List<Person> sortedByAgePersons = new ArrayList<Person>(persons);
Collections.sort(sortedByAgePersons, new Comparator<Person>() {
public int compare(Person p1, Person p2) {
return Integer.valueOf(p1.getAge()).compareTo(p2.getAge());
}
});
Sort with lambda
List<Person> sortedByAgePersons = sort(persons, on(Person.class).getAge());
Of course, having this kind of beauty impacts in the performance (an average of 2 times), but can you find a more readable code?
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.Date;
public class test {
public static class Person {
public String name;
public int id;
public Date hireDate;
public Person(String iname, int iid, Date ihireDate) {
name = iname;
id = iid;
hireDate = ihireDate;
}
public String toString() {
return name + " " + id + " " + hireDate.toString();
}
// Comparator
public static class CompId implements Comparator<Person> {
#Override
public int compare(Person arg0, Person arg1) {
return arg0.id - arg1.id;
}
}
public static class CompDate implements Comparator<Person> {
private int mod = 1;
public CompDate(boolean desc) {
if (desc) mod =-1;
}
#Override
public int compare(Person arg0, Person arg1) {
return mod*arg0.hireDate.compareTo(arg1.hireDate);
}
}
}
public static void main(String[] args) {
// TODO Auto-generated method stub
SimpleDateFormat df = new SimpleDateFormat("mm-dd-yyyy");
ArrayList<Person> people;
people = new ArrayList<Person>();
try {
people.add(new Person("Joe", 92422, df.parse("12-12-2010")));
people.add(new Person("Joef", 24122, df.parse("1-12-2010")));
people.add(new Person("Joee", 24922, df.parse("12-2-2010")));
} catch (ParseException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
Collections.sort(people, new Person.CompId());
System.out.println("BY ID");
for (Person p : people) {
System.out.println(p.toString());
}
Collections.sort(people, new Person.CompDate(false));
System.out.println("BY Date asc");
for (Person p : people) {
System.out.println(p.toString());
}
Collections.sort(people, new Person.CompDate(true));
System.out.println("BY Date desc");
for (Person p : people) {
System.out.println(p.toString());
}
}
}
Function & method reference
The Collections.sort method can sort a List using a Comparator you pass. That Comparator can be implemented using the Comparator.comparing method where you can pass a method reference as the necessary Function. Fortunately, the actual code is much simpler and shorter than this description.
For Java 8:
Collections.sort(list, comparing(ClassName::getName));
or
Collections.sort(list, comparing(ClassName::getName).reversed());
Another way is
Collections.sort(list, comparing(ClassName::getName, Comparator.nullsLast(Comparator.naturalOrder())));
Best easy way with JAVA 8 is for English Alphabetic sort
Class Implementation
public class NewspaperClass implements Comparable<NewspaperClass>{
public String name;
#Override
public int compareTo(NewspaperClass another) {
return name.compareTo(another.name);
}
}
Sort
Collections.sort(Your List);
If you want to sort for alphabet that contains non English characters you can use Locale... Below code use Turkish character sort...
Class Implementation
public class NewspaperClass implements Comparator<NewspaperClass> {
public String name;
public Boolean isUserNewspaper=false;
private Collator trCollator = Collator.getInstance(new Locale("tr_TR"));
#Override
public int compare(NewspaperClass lhs, NewspaperClass rhs) {
trCollator.setStrength(Collator.PRIMARY);
return trCollator.compare(lhs.name,rhs.name);
}
}
Sort
Collections.sort(your array list,new NewspaperClass());
You can Sort using java 8
yourList.sort(Comparator.comparing(Classname::getName));
or
yourList.stream().forEach(a -> a.getBObjects().sort(Comparator.comparing(Classname::getValue)));
From Java 8 and onward we don't have to use Collections.sort() directly. List interface has a default sort() method:
List<User> users = Arrays.asList(user1,user2,user3);
users.sort( (u1, u2) -> {
return u1.getFirstName.compareTo(u2.getFirstName());});
See http://visvv.blogspot.in/2016/01/sorting-objects-in-java-8.html.
Java 8 Lambda shortens the sort.
Collections.sort(stdList, (o1, o2) -> o1.getName().compareTo(o2.getName()));
You can use the Bean Comparator to sort on any property in your custom class.
Yes, that's possible for instance in this answer I sort by the property v of the class IndexValue
// Sorting by property v using a custom comparator.
Arrays.sort( array, new Comparator<IndexValue>(){
public int compare( IndexValue a, IndexValue b ){
return a.v - b.v;
}
});
If you notice here I'm creating a anonymous inner class ( which is the Java for closures ) and passing it directly to the sort method of the class Arrays
Your object may also implement Comparable ( that's what String and most of the core libraries in Java does ) but that would define the "natural sort order" of the class it self, and doesn't let you plug new ones.
I found most if not all of these answers rely on the underlying class (Object) to implement comparable or to have a helper comparable interface.
Not with my solution! The following code lets you compare object's field by knowing their string name. You could easily modify it not to use the name, but then you need to expose it or construct one of the Objects you want to compare against.
Collections.sort(anArrayListOfSomeObjectPerhapsUsersOrSomething, new ReflectiveComparator(). new ListComparator("name"));
public class ReflectiveComparator {
public class FieldComparator implements Comparator<Object> {
private String fieldName;
public FieldComparator(String fieldName){
this.fieldName = fieldName;
}
#SuppressWarnings({ "unchecked", "rawtypes" })
#Override
public int compare(Object object1, Object object2) {
try {
Field field = object1.getClass().getDeclaredField(fieldName);
field.setAccessible(true);
Comparable object1FieldValue = (Comparable) field.get(object1);
Comparable object2FieldValue = (Comparable) field.get(object2);
return object1FieldValue.compareTo(object2FieldValue);
}catch (Exception e){}
return 0;
}
}
public class ListComparator implements Comparator<Object> {
private String fieldName;
public ListComparator(String fieldName) {
this.fieldName = fieldName;
}
#SuppressWarnings({ "unchecked", "rawtypes" })
#Override
public int compare(Object object1, Object object2) {
try {
Field field = object1.getClass().getDeclaredField(fieldName);
field.setAccessible(true);
Comparable o1FieldValue = (Comparable) field.get(object1);
Comparable o2FieldValue = (Comparable) field.get(object2);
if (o1FieldValue == null){ return -1;}
if (o2FieldValue == null){ return 1;}
return o1FieldValue.compareTo(o2FieldValue);
} catch (NoSuchFieldException e) {
throw new IllegalStateException("Field doesn't exist", e);
} catch (IllegalAccessException e) {
throw new IllegalStateException("Field inaccessible", e);
}
}
}
}
You can try Guava Ordering:
Function<Item, Date> getStartDate = new Function<Item, Date>() {
public Date apply(Item item) {
return item.getStartDate();
}
};
List<Item> orderedItems = Ordering.natural().onResultOf(getStartDate).
sortedCopy(items);
Well if you using Java 8 or older version Here is the Best solution.
Collections.sort(studentList, Comparator.comparing(Student::getCgpa).reversed().thenComparing(Student:: getFname).thenComparing(Student::getId));
In this case, it will first sort with 'getCgpa' first and for the second part it will sort with getFname and getId. Which is field into the pojo class.
your customComparator class must implement java.util.Comparator in order to be used.
it must also overide compare() AND equals()
compare() must answer the question: Is object 1 less than, equal to or greater than object 2?
full docs: http://java.sun.com/j2se/1.5.0/docs/api/java/util/Comparator.html
This code snippets might be useful. If you want to sort an Object
in my case I want to sort by VolumeName:
public List<Volume> getSortedVolumes() throws SystemException {
List<Volume> volumes = VolumeLocalServiceUtil.getAllVolumes();
Collections.sort(volumes, new Comparator<Volume>() {
public int compare(Volume o1, Volume o2) {
Volume p1 = (Volume) o1;
Volume p2 = (Volume) o2;
return p1.getVolumeName().compareToIgnoreCase(
p2.getVolumeName());
}
});
return volumes;
}
This works. I use it in my jsp.
With this library here you can sort the list of custom objects on multiple columns. The library uses version 8.0 features. Sample is also available there. Here is a sample to do
SortKeys sortKeys = new SortKeys();
sortKeys.addField("firstName")
.addField("age", true); // This (true) will sort the age descending
// Other ways to specify a property to the sorter are
// .addField("lastName", String.class);
// .addField("dob", Date.class, true);
// Instantiate a ListSorter
ListSorter listSorter = new ListSorter();
// Pass the data to sort (listToSort) and the "by keys" to sort (sortKeys)
List sortedList = (List<Person>) listSorter.sortList(listToSort, sortKeys);
You can have a look into this presentation hold at the Java Forum in Stuttgart Germany in 2016.
Only a few slides use German language, 99% of the content is "English based" Java source code; like
someCollection.sort(
OurCustomComparator
.comparing(Person::getName)
.thenComparing(Person::getId)
);
where OurCustomComparator is using default methods (and other interesting ideas). As shown, leading to very concise code to pick some getter method for sorting; and super simple chaining (or reversing) of sort criteria.
If you are into java8, you find a lot of material there to get you started.
New since 1.8 is a List.sort() method instead of using the Collection.sort()
so you directly call mylistcontainer.sort()
Here is a code snippet which demonstrates the List.sort() feature:
List<Fruit> fruits = new ArrayList<Fruit>();
fruits.add(new Fruit("Kiwi","green",40));
fruits.add(new Fruit("Banana","yellow",100));
fruits.add(new Fruit("Apple","mixed green,red",120));
fruits.add(new Fruit("Cherry","red",10));
// a) using an existing compareto() method
fruits.sort((Fruit f1,Fruit f2) -> f1.getFruitName().compareTo(f2.getFruitName()));
System.out.println("Using String.compareTo(): " + fruits);
//Using String.compareTo(): [Apple is: mixed green,red, Banana is: yellow, Cherry is: red, Kiwi is: green]
// b) Using a comparable class
fruits.sort((Fruit f1,Fruit f2) -> f1.compareTo(f2));
System.out.println("Using a Comparable Fruit class (sort by color): " + fruits);
// Using a Comparable Fruit class (sort by color): [Kiwi is green, Apple is: mixed green,red, Cherry is: red, Banana is: yellow]
The Fruit class is:
public class Fruit implements Comparable<Fruit>
{
private String name;
private String color;
private int quantity;
public Fruit(String name,String color,int quantity)
{ this.name = name; this.color = color; this.quantity = quantity; }
public String getFruitName() { return name; }
public String getColor() { return color; }
public int getQuantity() { return quantity; }
#Override public final int compareTo(Fruit f) // sorting the color
{
return this.color.compareTo(f.color);
}
#Override public String toString()
{
return (name + " is: " + color);
}
} // end of Fruit class
I prefer this process:
public class SortUtil
{
public static <T> List<T> sort(List<T> list, String sortByProperty)
{
Collections.sort(list, new BeanComparator(sortByProperty));
return list;
}
}
List<T> sortedList = SortUtil<T>.sort(unsortedList, "startDate");
If you list of objects has a property called startDate, you call use this over and over. You can even chain them startDate.time.
This requires your object to be Comparable which means you need a compareTo, equals, and hashCode implementation.
Yes, it could be faster... But now you don't have to make a new Comparator for each type of sort. If you can save on dev time and give up on runtime, you might go with this one.
using the java-8 stream api you can sort an ArrayList by:
Comparator<Person> birthdayComparator = Comparator.comparing(Person::getBirthday);
List<Person> sortedList = list.stream().sorted(birthdayComparator).collect(toList());
Using Java 8 use can define the Comparator in one line using Comparator.comparing()
Use any of the following way:
Option 1:
listToBeSorted.sort(Comparator.comparing(CustomObject::getStartDate));
Option 2:
Collections.sort(listToBeSorted, Comparator.comparing(CustomObject::getStartDate));
Your custom class can implement the "Comparable" interface, which requires an implementation of the CompareTo method. In the CompareTo method, you can then define what it means that an object is less than or more than the other object. So in your example, it can look something like this:
public class MyCustomClass implements Comparable<MyCustomClass>{
..........
#Override
public int compareTo(MyCustomClass a) {
if(this.getStartDate().before(a.getStartDate())){
return -1;
}else if(a.getStartDate().before(this.getStartDate())){
return 1;
}else {
return 0;
}
}
A negative number indicates that this is smaller than the object being compared to. A positive number indicates that this is larger than the compared to object and a Zero means that the objects are equal.
You can then use the collections.sort(myList) to sort your list without having to feed in a comparator. This method also has the advantage of having things sorted automatically if you use a sorted collection data structures like a TreeSet or a TreeMap.
You can check this article if you would like to read more about the Comparable interface (disclosure: I am the author ;) )
https://nullbeans.com/the-java-comparable-interface-automatic-sort-of-collections/
You could also use Springs PropertyComparator if you have just a String property path to the (nested) property you want to sort:
List<SomeObject> list = ...;
PropertyComparator<HitWithInfo> propertyComparator = new PropertyComparator<>(
"property.nested.myProperty", false, true);
list.sort(propertyComparator);
The drawback is, that this comparator silently ignores properties which does not exist or are not accessible and handles this as null value for comparison. This means, you should carefully test such a comparator or validate the existence of the property path somehow.
I have tried lots of different solutions available on internet but solution which works for me is available at below link.
https://www.java67.com/2017/07/how-to-sort-arraylist-of-objects-using.html
I use this code to sort a collection having the key as String type:
class Myclass {
String Key;
other variables
}
The Collection is defined like this:
ArrayList<Myclass> MyCollection = new ArrayList<MyClass>();
I add some items to MyColletion, and finally sort the collection:
Collections.sort(MyCollection, new Comparator<MyClass>() {
#Override public int compare(MyClass c1, MyClass c2) {
return c1.Key.compareToIgnoreCase(c2.Key);
}
});
This code works and sorts the collection if the key is a string. But I want to change my key to be a long, so the class will look like this:
class Myclass {
long Key;
other variables
}
I am not able to find the code to sort a collection having the key as long type. It should have to work also with older Android apis than 19.
You can use Long.compare like this:
return Long.compare(c1.Key,c2.Key);
If the above is not supported by older Android apis than 19 then you can do:
Collections.sort(MyCollection, new Comparator<MyClass>() {
#Override public int compare(MyClass c1, MyClass c2) {
if(c1.Key < c2.Key) return -1;
if(c1.Key > c2.Key) return 1;
return 0;
}
});
Note:this comparator will not work for primitive datatypes like int ,long,double ,yet i am giving this solution if wrapper class is used this can be useful
For the generic way of sorting your list values you can use this comparator
public class Genericcomparator<T extends Myclass> implements Comparator<T> {
#Override
public int compare(T o1, T o2) {
// TODO Auto-generated method stub
return o1.key.compareTo(o2.key);
}
}
and use it in your class as
Collections.sort(list, new Genericcomparator<Myclass>());
so when ever you change the datatype there will be no issue in comparing it
This question already has answers here:
Sorting an ArrayList of objects using a custom sorting order
(12 answers)
Closed 6 years ago.
If I have a simple list of Strings:
List<String> stringList = new ArrayList<String>();
I can sort it with:
Collections.sort(stringList);
But suppose I have a Person class:
public class Person
{
private String name;
private Integer age;
private String country;
}
And a list of it:
List<Person> personList = new ArrayList<Person>();
And I want to sort it sometimes by name, sometimes by age, sometimes by country.
What is the easiest way to accomplish that?
I know that I can implement the Comparable interface, but that seems to limit me to sort it by one specific property.
Collections.sort can be called with a custom comparator. And that comparator can be implemented to allow sorting in different sort orders. Here's an example (for your Person model - with age as an Integer):
public class FlexiblePersonComparator implements Comparator<Person> {
public enum Order {Name, Age, Country}
private Order sortingBy = Name;
#Override
public int compare(Person person1, Person person2) {
switch(sortingBy) {
case Name: return person1.name.compareTo(person2.name);
case Age: return person1.age.compareTo(person2.age);
case Country: return person1.country.compareTo(person2.country);
}
throw new RuntimeException("Practically unreachable code, can't be thrown");
}
public void setSortingBy(Order sortBy) {
this.sortingBy = sortingBy;
}
}
And you use it like that (assuming persons is a field):
public void sortPersonsBy(FlexiblePersonComparator.Order sortingBy) {
List<Person> persons = this.persons; // useless line, just for clarification
FlexiblePersonComparator comparator = new FlexiblePersonComparator();
comparator.setSortingBy(sortingBy);
Collections.sort(persons, comparator); // now we have a sorted list
}
Implement the Comparator interface (once for each different sort order) and use the Collections.sort() method that takes a Comparator as additional parameter.
Thanks to the responders. For the benefit of others, I'd like to include a complete example.
The solution is the create the following additional classes:
public class NameComparator implements Comparator<Person>
{
public int compare(Person o1, Person o2)
{
return o1.getName().compareTo(o2.getName());
}
}
public class AgeComparator implements Comparator<Person>
{
public int compare(Person o1, Person o2)
{
return o1.getAge().compareTo(o2.getAge());
}
}
public class CountryComparator implements Comparator<Person>
{
public int compare(Person o1, Person o2)
{
return o1.getCountry().compareTo(o2.getCountry());
}
}
The list can then be sorted like this:
Collections.sort(personList, new NameComparator());
Collections.sort(personList, new AgeComparator());
Collections.sort(personList, new CountryComparator());
The Java 8 way of doing this is to use List.sort as follows:
personList.sort(Comparator.comparing(Person::getName));
To quote Stuart Marks in his answer over here.
This is the big advantage of the List.sort(cmp) extension method over Collections.sort(list, cmp). It might seem that this is merely a small syntactic advantage being able to write myList.sort(cmp) instead of Collections.sort(myList, cmp). The difference is that myList.sort(cmp), being an interface extension method, can be overridden by the specific List implementation. For example, ArrayList.sort(cmp) sorts the list in-place using Arrays.sort() whereas the default implementation implements the old copyout-sort-copyback technique.
You could also use the BeanComparator from apache commons beanutils, like this:
Collections.sort(personList, new BeanComparator("name"));
Implement 3 different types of Comparator.
you can add the comparator to the sort command. The comparator you define, will sort the elements by name, age, or what ever.
Collections.sort(list, new Comparator() {
public int compare(Object arg0, Object arg1) {
if (!(arg0 instanceof Person)) {
return -1;
}
if (!(arg1 instanceof Person)) {
return -1;
}
Person pers0 = (Person)arg0;
Person pers1 = (Person)arg1;
// COMPARE NOW WHAT YOU WANT
// Thanks to Steve Kuo for your comment!
return pers0.getAge() - pers1.getAge();
}
});
The Collections.sort method can be invoked with a second argument which is the comparator to use.
Create 3 comparators and use the one you want when appropriate.
Collections.sort(list , new Comparator() {
public int compare(Object o1, Object o2) {
...
}
});
Using lambdaj ( http://code.google.com/p/lambdaj/ ) you can achieve what you're asking in the following way:
sort(personList, on(Person.class).getName());
sort(personList, on(Person.class).getAge());
sort(personList, on(Person.class).getCountry());
I asked a very similar question (about searching rather than sorting), perhaps there is some useful information (I ended up using an enum that implements Comparator so I pass the enum value as a comparator selector).
I read about sorting ArrayLists using a Comparator but in all of the examples people used compareTo which according to some research is a method for Strings.
I wanted to sort an ArrayList of custom objects by one of their properties: a Date object
(getStartDay()). Normally I compare them by item1.getStartDate().before(item2.getStartDate()) so I was wondering whether I could write something like:
public class CustomComparator {
public boolean compare(Object object1, Object object2) {
return object1.getStartDate().before(object2.getStartDate());
}
}
public class RandomName {
...
Collections.sort(Database.arrayList, new CustomComparator);
...
}
Since Date implements Comparable, it has a compareTo method just like String does.
So your custom Comparator could look like this:
public class CustomComparator implements Comparator<MyObject> {
#Override
public int compare(MyObject o1, MyObject o2) {
return o1.getStartDate().compareTo(o2.getStartDate());
}
}
The compare() method must return an int, so you couldn't directly return a boolean like you were planning to anyway.
Your sorting code would be just about like you wrote:
Collections.sort(Database.arrayList, new CustomComparator());
A slightly shorter way to write all this, if you don't need to reuse your comparator, is to write it as an inline anonymous class:
Collections.sort(Database.arrayList, new Comparator<MyObject>() {
#Override
public int compare(MyObject o1, MyObject o2) {
return o1.getStartDate().compareTo(o2.getStartDate());
}
});
Since java-8
You can now write the last example in a shorter form by using a lambda expression for the Comparator:
Collections.sort(Database.arrayList,
(o1, o2) -> o1.getStartDate().compareTo(o2.getStartDate()));
And List has a sort(Comparator) method, so you can shorten this even further:
Database.arrayList.sort((o1, o2) -> o1.getStartDate().compareTo(o2.getStartDate()));
This is such a common idiom that there's a built-in method to generate a Comparator for a class with a Comparable key:
Database.arrayList.sort(Comparator.comparing(MyObject::getStartDate));
All of these are equivalent forms.
Classes that has a natural sort order (a class Number, as an example) should implement the Comparable interface, whilst classes that has no natural sort order (a class Chair, as an example) should be provided with a Comparator (or an anonymous Comparator class).
Two examples:
public class Number implements Comparable<Number> {
private int value;
public Number(int value) { this.value = value; }
public int compareTo(Number anotherInstance) {
return this.value - anotherInstance.value;
}
}
public class Chair {
private int weight;
private int height;
public Chair(int weight, int height) {
this.weight = weight;
this.height = height;
}
/* Omitting getters and setters */
}
class ChairWeightComparator implements Comparator<Chair> {
public int compare(Chair chair1, Chair chair2) {
return chair1.getWeight() - chair2.getWeight();
}
}
class ChairHeightComparator implements Comparator<Chair> {
public int compare(Chair chair1, Chair chair2) {
return chair1.getHeight() - chair2.getHeight();
}
}
Usage:
List<Number> numbers = new ArrayList<Number>();
...
Collections.sort(numbers);
List<Chair> chairs = new ArrayList<Chair>();
// Sort by weight:
Collections.sort(chairs, new ChairWeightComparator());
// Sort by height:
Collections.sort(chairs, new ChairHeightComparator());
// You can also create anonymous comparators;
// Sort by color:
Collections.sort(chairs, new Comparator<Chair>() {
public int compare(Chair chair1, Chair chair2) {
...
}
});
For sorting an ArrayList you could use the following code snippet:
Collections.sort(studList, new Comparator<Student>(){
public int compare(Student s1, Student s2) {
return s1.getFirstName().compareToIgnoreCase(s2.getFirstName());
}
});
JAVA 8 lambda expression
Collections.sort(studList, (Student s1, Student s2) ->{
return s1.getFirstName().compareToIgnoreCase(s2.getFirstName());
});
OR
Comparator<Student> c = (s1, s2) -> s1.firstName.compareTo(s2.firstName);
studList.sort(c)
Yes, you can. There are two options with comparing items, the Comparable interface, and the Comparator interface.
Both of these interfaces allow for different behavior. Comparable allows you to make the object act like you just described Strings (in fact, String implements Comparable). The second, Comparator, allows you to do what you are asking to do. You would do it like this:
Collections.sort(myArrayList, new MyComparator());
That will cause the Collections.sort method to use your comparator for it's sorting mechanism. If the objects in the ArrayList implement comparable, you can instead do something like this:
Collections.sort(myArrayList);
The Collections class contains a number of these useful, common tools.
With Java 8 you can use a method reference for your comparator:
import static java.util.Comparator.comparing;
Collections.sort(list, comparing(MyObject::getStartDate));
Since technologies appear everyday, the answer will change in the time. I took a look at LambdaJ and seems very interesting.
You can try solving these tasks with LambdaJ. You can find it here: http://code.google.com/p/lambdaj/
Here you have an example:
Sort Iterative
List<Person> sortedByAgePersons = new ArrayList<Person>(persons);
Collections.sort(sortedByAgePersons, new Comparator<Person>() {
public int compare(Person p1, Person p2) {
return Integer.valueOf(p1.getAge()).compareTo(p2.getAge());
}
});
Sort with lambda
List<Person> sortedByAgePersons = sort(persons, on(Person.class).getAge());
Of course, having this kind of beauty impacts in the performance (an average of 2 times), but can you find a more readable code?
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.Date;
public class test {
public static class Person {
public String name;
public int id;
public Date hireDate;
public Person(String iname, int iid, Date ihireDate) {
name = iname;
id = iid;
hireDate = ihireDate;
}
public String toString() {
return name + " " + id + " " + hireDate.toString();
}
// Comparator
public static class CompId implements Comparator<Person> {
#Override
public int compare(Person arg0, Person arg1) {
return arg0.id - arg1.id;
}
}
public static class CompDate implements Comparator<Person> {
private int mod = 1;
public CompDate(boolean desc) {
if (desc) mod =-1;
}
#Override
public int compare(Person arg0, Person arg1) {
return mod*arg0.hireDate.compareTo(arg1.hireDate);
}
}
}
public static void main(String[] args) {
// TODO Auto-generated method stub
SimpleDateFormat df = new SimpleDateFormat("mm-dd-yyyy");
ArrayList<Person> people;
people = new ArrayList<Person>();
try {
people.add(new Person("Joe", 92422, df.parse("12-12-2010")));
people.add(new Person("Joef", 24122, df.parse("1-12-2010")));
people.add(new Person("Joee", 24922, df.parse("12-2-2010")));
} catch (ParseException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
Collections.sort(people, new Person.CompId());
System.out.println("BY ID");
for (Person p : people) {
System.out.println(p.toString());
}
Collections.sort(people, new Person.CompDate(false));
System.out.println("BY Date asc");
for (Person p : people) {
System.out.println(p.toString());
}
Collections.sort(people, new Person.CompDate(true));
System.out.println("BY Date desc");
for (Person p : people) {
System.out.println(p.toString());
}
}
}
Function & method reference
The Collections.sort method can sort a List using a Comparator you pass. That Comparator can be implemented using the Comparator.comparing method where you can pass a method reference as the necessary Function. Fortunately, the actual code is much simpler and shorter than this description.
For Java 8:
Collections.sort(list, comparing(ClassName::getName));
or
Collections.sort(list, comparing(ClassName::getName).reversed());
Another way is
Collections.sort(list, comparing(ClassName::getName, Comparator.nullsLast(Comparator.naturalOrder())));
Best easy way with JAVA 8 is for English Alphabetic sort
Class Implementation
public class NewspaperClass implements Comparable<NewspaperClass>{
public String name;
#Override
public int compareTo(NewspaperClass another) {
return name.compareTo(another.name);
}
}
Sort
Collections.sort(Your List);
If you want to sort for alphabet that contains non English characters you can use Locale... Below code use Turkish character sort...
Class Implementation
public class NewspaperClass implements Comparator<NewspaperClass> {
public String name;
public Boolean isUserNewspaper=false;
private Collator trCollator = Collator.getInstance(new Locale("tr_TR"));
#Override
public int compare(NewspaperClass lhs, NewspaperClass rhs) {
trCollator.setStrength(Collator.PRIMARY);
return trCollator.compare(lhs.name,rhs.name);
}
}
Sort
Collections.sort(your array list,new NewspaperClass());
You can Sort using java 8
yourList.sort(Comparator.comparing(Classname::getName));
or
yourList.stream().forEach(a -> a.getBObjects().sort(Comparator.comparing(Classname::getValue)));
From Java 8 and onward we don't have to use Collections.sort() directly. List interface has a default sort() method:
List<User> users = Arrays.asList(user1,user2,user3);
users.sort( (u1, u2) -> {
return u1.getFirstName.compareTo(u2.getFirstName());});
See http://visvv.blogspot.in/2016/01/sorting-objects-in-java-8.html.
Java 8 Lambda shortens the sort.
Collections.sort(stdList, (o1, o2) -> o1.getName().compareTo(o2.getName()));
You can use the Bean Comparator to sort on any property in your custom class.
Yes, that's possible for instance in this answer I sort by the property v of the class IndexValue
// Sorting by property v using a custom comparator.
Arrays.sort( array, new Comparator<IndexValue>(){
public int compare( IndexValue a, IndexValue b ){
return a.v - b.v;
}
});
If you notice here I'm creating a anonymous inner class ( which is the Java for closures ) and passing it directly to the sort method of the class Arrays
Your object may also implement Comparable ( that's what String and most of the core libraries in Java does ) but that would define the "natural sort order" of the class it self, and doesn't let you plug new ones.
I found most if not all of these answers rely on the underlying class (Object) to implement comparable or to have a helper comparable interface.
Not with my solution! The following code lets you compare object's field by knowing their string name. You could easily modify it not to use the name, but then you need to expose it or construct one of the Objects you want to compare against.
Collections.sort(anArrayListOfSomeObjectPerhapsUsersOrSomething, new ReflectiveComparator(). new ListComparator("name"));
public class ReflectiveComparator {
public class FieldComparator implements Comparator<Object> {
private String fieldName;
public FieldComparator(String fieldName){
this.fieldName = fieldName;
}
#SuppressWarnings({ "unchecked", "rawtypes" })
#Override
public int compare(Object object1, Object object2) {
try {
Field field = object1.getClass().getDeclaredField(fieldName);
field.setAccessible(true);
Comparable object1FieldValue = (Comparable) field.get(object1);
Comparable object2FieldValue = (Comparable) field.get(object2);
return object1FieldValue.compareTo(object2FieldValue);
}catch (Exception e){}
return 0;
}
}
public class ListComparator implements Comparator<Object> {
private String fieldName;
public ListComparator(String fieldName) {
this.fieldName = fieldName;
}
#SuppressWarnings({ "unchecked", "rawtypes" })
#Override
public int compare(Object object1, Object object2) {
try {
Field field = object1.getClass().getDeclaredField(fieldName);
field.setAccessible(true);
Comparable o1FieldValue = (Comparable) field.get(object1);
Comparable o2FieldValue = (Comparable) field.get(object2);
if (o1FieldValue == null){ return -1;}
if (o2FieldValue == null){ return 1;}
return o1FieldValue.compareTo(o2FieldValue);
} catch (NoSuchFieldException e) {
throw new IllegalStateException("Field doesn't exist", e);
} catch (IllegalAccessException e) {
throw new IllegalStateException("Field inaccessible", e);
}
}
}
}
You can try Guava Ordering:
Function<Item, Date> getStartDate = new Function<Item, Date>() {
public Date apply(Item item) {
return item.getStartDate();
}
};
List<Item> orderedItems = Ordering.natural().onResultOf(getStartDate).
sortedCopy(items);
Well if you using Java 8 or older version Here is the Best solution.
Collections.sort(studentList, Comparator.comparing(Student::getCgpa).reversed().thenComparing(Student:: getFname).thenComparing(Student::getId));
In this case, it will first sort with 'getCgpa' first and for the second part it will sort with getFname and getId. Which is field into the pojo class.
your customComparator class must implement java.util.Comparator in order to be used.
it must also overide compare() AND equals()
compare() must answer the question: Is object 1 less than, equal to or greater than object 2?
full docs: http://java.sun.com/j2se/1.5.0/docs/api/java/util/Comparator.html
This code snippets might be useful. If you want to sort an Object
in my case I want to sort by VolumeName:
public List<Volume> getSortedVolumes() throws SystemException {
List<Volume> volumes = VolumeLocalServiceUtil.getAllVolumes();
Collections.sort(volumes, new Comparator<Volume>() {
public int compare(Volume o1, Volume o2) {
Volume p1 = (Volume) o1;
Volume p2 = (Volume) o2;
return p1.getVolumeName().compareToIgnoreCase(
p2.getVolumeName());
}
});
return volumes;
}
This works. I use it in my jsp.
With this library here you can sort the list of custom objects on multiple columns. The library uses version 8.0 features. Sample is also available there. Here is a sample to do
SortKeys sortKeys = new SortKeys();
sortKeys.addField("firstName")
.addField("age", true); // This (true) will sort the age descending
// Other ways to specify a property to the sorter are
// .addField("lastName", String.class);
// .addField("dob", Date.class, true);
// Instantiate a ListSorter
ListSorter listSorter = new ListSorter();
// Pass the data to sort (listToSort) and the "by keys" to sort (sortKeys)
List sortedList = (List<Person>) listSorter.sortList(listToSort, sortKeys);
You can have a look into this presentation hold at the Java Forum in Stuttgart Germany in 2016.
Only a few slides use German language, 99% of the content is "English based" Java source code; like
someCollection.sort(
OurCustomComparator
.comparing(Person::getName)
.thenComparing(Person::getId)
);
where OurCustomComparator is using default methods (and other interesting ideas). As shown, leading to very concise code to pick some getter method for sorting; and super simple chaining (or reversing) of sort criteria.
If you are into java8, you find a lot of material there to get you started.
New since 1.8 is a List.sort() method instead of using the Collection.sort()
so you directly call mylistcontainer.sort()
Here is a code snippet which demonstrates the List.sort() feature:
List<Fruit> fruits = new ArrayList<Fruit>();
fruits.add(new Fruit("Kiwi","green",40));
fruits.add(new Fruit("Banana","yellow",100));
fruits.add(new Fruit("Apple","mixed green,red",120));
fruits.add(new Fruit("Cherry","red",10));
// a) using an existing compareto() method
fruits.sort((Fruit f1,Fruit f2) -> f1.getFruitName().compareTo(f2.getFruitName()));
System.out.println("Using String.compareTo(): " + fruits);
//Using String.compareTo(): [Apple is: mixed green,red, Banana is: yellow, Cherry is: red, Kiwi is: green]
// b) Using a comparable class
fruits.sort((Fruit f1,Fruit f2) -> f1.compareTo(f2));
System.out.println("Using a Comparable Fruit class (sort by color): " + fruits);
// Using a Comparable Fruit class (sort by color): [Kiwi is green, Apple is: mixed green,red, Cherry is: red, Banana is: yellow]
The Fruit class is:
public class Fruit implements Comparable<Fruit>
{
private String name;
private String color;
private int quantity;
public Fruit(String name,String color,int quantity)
{ this.name = name; this.color = color; this.quantity = quantity; }
public String getFruitName() { return name; }
public String getColor() { return color; }
public int getQuantity() { return quantity; }
#Override public final int compareTo(Fruit f) // sorting the color
{
return this.color.compareTo(f.color);
}
#Override public String toString()
{
return (name + " is: " + color);
}
} // end of Fruit class
I prefer this process:
public class SortUtil
{
public static <T> List<T> sort(List<T> list, String sortByProperty)
{
Collections.sort(list, new BeanComparator(sortByProperty));
return list;
}
}
List<T> sortedList = SortUtil<T>.sort(unsortedList, "startDate");
If you list of objects has a property called startDate, you call use this over and over. You can even chain them startDate.time.
This requires your object to be Comparable which means you need a compareTo, equals, and hashCode implementation.
Yes, it could be faster... But now you don't have to make a new Comparator for each type of sort. If you can save on dev time and give up on runtime, you might go with this one.
using the java-8 stream api you can sort an ArrayList by:
Comparator<Person> birthdayComparator = Comparator.comparing(Person::getBirthday);
List<Person> sortedList = list.stream().sorted(birthdayComparator).collect(toList());
Using Java 8 use can define the Comparator in one line using Comparator.comparing()
Use any of the following way:
Option 1:
listToBeSorted.sort(Comparator.comparing(CustomObject::getStartDate));
Option 2:
Collections.sort(listToBeSorted, Comparator.comparing(CustomObject::getStartDate));
Your custom class can implement the "Comparable" interface, which requires an implementation of the CompareTo method. In the CompareTo method, you can then define what it means that an object is less than or more than the other object. So in your example, it can look something like this:
public class MyCustomClass implements Comparable<MyCustomClass>{
..........
#Override
public int compareTo(MyCustomClass a) {
if(this.getStartDate().before(a.getStartDate())){
return -1;
}else if(a.getStartDate().before(this.getStartDate())){
return 1;
}else {
return 0;
}
}
A negative number indicates that this is smaller than the object being compared to. A positive number indicates that this is larger than the compared to object and a Zero means that the objects are equal.
You can then use the collections.sort(myList) to sort your list without having to feed in a comparator. This method also has the advantage of having things sorted automatically if you use a sorted collection data structures like a TreeSet or a TreeMap.
You can check this article if you would like to read more about the Comparable interface (disclosure: I am the author ;) )
https://nullbeans.com/the-java-comparable-interface-automatic-sort-of-collections/
You could also use Springs PropertyComparator if you have just a String property path to the (nested) property you want to sort:
List<SomeObject> list = ...;
PropertyComparator<HitWithInfo> propertyComparator = new PropertyComparator<>(
"property.nested.myProperty", false, true);
list.sort(propertyComparator);
The drawback is, that this comparator silently ignores properties which does not exist or are not accessible and handles this as null value for comparison. This means, you should carefully test such a comparator or validate the existence of the property path somehow.
I have tried lots of different solutions available on internet but solution which works for me is available at below link.
https://www.java67.com/2017/07/how-to-sort-arraylist-of-objects-using.html
I'm learning Java and I saw the code below on Youtube. I was just wondering how this part of the code works.
static final Comparator<Employee> SENIORITY_ORDER =
new Comparator<Employee>() {
public int compare(Employee e1, Employee e2) {
return e2.hireDate().compareTo(e1.hireDate());
}
};
Could someoen please explain it to me? Thanks in advance for any help!
import java.util.*;
public class EmpSort {
static final Comparator<Employee> SENIORITY_ORDER =
new Comparator<Employee>() {
public int compare(Employee e1, Employee e2) {
return e2.hireDate().compareTo(e1.hireDate());
}
};
// Employee database
static final Collection<Employee> employees = ... ;
public static void main(String[] args) {
List<Employee> e = new ArrayList<Employee>(employees);
Collections.sort(e, SENIORITY_ORDER);
System.out.println(e);
}
}
The SENIORITY_ORDER Comparator (used for comparing Employees in the sort) is an Anonymous Class. The linked Java Tutorial reads (in part)
Anonymous classes enable you to make your code more concise. They enable you to declare and instantiate a class at the same time. They are like local classes except that they do not have a name. Use them if you need to use a local class only once.
Well as we all konw . the Comparator is the rule that how to compare tow Object.
you can come into this Method : Collections.sort(e, SENIORITY_ORDER);
and you will see the answer you want
public static <T> void sort(List<T> list, Comparator<? super T> c) {
Object[] a = list.toArray();
Arrays.sort(a, (Comparator)c); //this is your rule to compare
ListIterator i = list.listIterator();
for (int j=0; j<a.length; j++) {
i.next();
i.set(a[j]);
}
}
if your Object not implements comparable so you must have a comparetor
or it's will be a wrong. The Collections.sort will to call the compare() Method.
static final Comparator<Employee> SENIORITY_ORDER =
new Comparator<Employee>() {
public int compare(Employee e1, Employee e2) {
return e2.hireDate().compareTo(e1.hireDate());
}
};
The statement static final Comparator<Employee> SENIORITY_ORDER = new Comparator<Employee>(){}
is creating a reference of the Comparator<Employee> interface that points to an instance of an Anonymous Inner Class which implements the Comparator<Employee> interface. Hence, you are overrding the compare() method of Comparator Interface inside your anonymous class.
Inside the compare() method you are comparing the hireDate attribute of two Employee objects using the compareTo() method of the Comparable interface in java. This method, compareTo(), compares the two attributes i.e, e1.hireDate and e2.hireDate, lexicographically and returns either a positive integer, a negative integer or zero, depending on whether e2.hireDate is greater than, less than or equal to e1.hireDate.
(HOPE YOU ARE ALREADY AWARE WITH THE CONCEPTS OF COMPARATOR INTERFACE AND NESTED CLASSES IN JAVA)