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How to sort first name also in this same compare method [duplicate]

I have array of objects person (int age; String name;).
How can I sort this array alphabetically by name and then by age?
Which algorithm would you use for this ?

You can use Collections.sort as follows:
private static void order(List<Person> persons) {
Collections.sort(persons, new Comparator() {
public int compare(Object o1, Object o2) {
String x1 = ((Person) o1).getName();
String x2 = ((Person) o2).getName();
int sComp = x1.compareTo(x2);
if (sComp != 0) {
return sComp;
}
Integer x1 = ((Person) o1).getAge();
Integer x2 = ((Person) o2).getAge();
return x1.compareTo(x2);
}});
}
List<Persons> is now sorted by name, then by age.
String.compareTo "Compares two strings lexicographically" - from the docs.
Collections.sort is a static method in the native Collections library. It does the actual sorting, you just need to provide a Comparator which defines how two elements in your list should be compared: this is achieved by providing your own implementation of the compare method.

For those able to use the Java 8 streaming API, there is a neater approach that is well documented here:
Lambdas and sorting
I was looking for the equivalent of the C# LINQ:
.ThenBy(...)
I found the mechanism in Java 8 on the Comparator:
.thenComparing(...)
So here is the snippet that demonstrates the algorithm.
Comparator<Person> comparator = Comparator.comparing(person -> person.name);
comparator = comparator.thenComparing(Comparator.comparing(person -> person.age));
Check out the link above for a neater way and an explanation about how Java's type inference makes it a bit more clunky to define compared to LINQ.
Here is the full unit test for reference:
#Test
public void testChainedSorting()
{
// Create the collection of people:
ArrayList<Person> people = new ArrayList<>();
people.add(new Person("Dan", 4));
people.add(new Person("Andi", 2));
people.add(new Person("Bob", 42));
people.add(new Person("Debby", 3));
people.add(new Person("Bob", 72));
people.add(new Person("Barry", 20));
people.add(new Person("Cathy", 40));
people.add(new Person("Bob", 40));
people.add(new Person("Barry", 50));
// Define chained comparators:
// Great article explaining this and how to make it even neater:
// http://blog.jooq.org/2014/01/31/java-8-friday-goodies-lambdas-and-sorting/
Comparator<Person> comparator = Comparator.comparing(person -> person.name);
comparator = comparator.thenComparing(Comparator.comparing(person -> person.age));
// Sort the stream:
Stream<Person> personStream = people.stream().sorted(comparator);
// Make sure that the output is as expected:
List<Person> sortedPeople = personStream.collect(Collectors.toList());
Assert.assertEquals("Andi", sortedPeople.get(0).name); Assert.assertEquals(2, sortedPeople.get(0).age);
Assert.assertEquals("Barry", sortedPeople.get(1).name); Assert.assertEquals(20, sortedPeople.get(1).age);
Assert.assertEquals("Barry", sortedPeople.get(2).name); Assert.assertEquals(50, sortedPeople.get(2).age);
Assert.assertEquals("Bob", sortedPeople.get(3).name); Assert.assertEquals(40, sortedPeople.get(3).age);
Assert.assertEquals("Bob", sortedPeople.get(4).name); Assert.assertEquals(42, sortedPeople.get(4).age);
Assert.assertEquals("Bob", sortedPeople.get(5).name); Assert.assertEquals(72, sortedPeople.get(5).age);
Assert.assertEquals("Cathy", sortedPeople.get(6).name); Assert.assertEquals(40, sortedPeople.get(6).age);
Assert.assertEquals("Dan", sortedPeople.get(7).name); Assert.assertEquals(4, sortedPeople.get(7).age);
Assert.assertEquals("Debby", sortedPeople.get(8).name); Assert.assertEquals(3, sortedPeople.get(8).age);
// Andi : 2
// Barry : 20
// Barry : 50
// Bob : 40
// Bob : 42
// Bob : 72
// Cathy : 40
// Dan : 4
// Debby : 3
}
/**
* A person in our system.
*/
public static class Person
{
/**
* Creates a new person.
* #param name The name of the person.
* #param age The age of the person.
*/
public Person(String name, int age)
{
this.age = age;
this.name = name;
}
/**
* The name of the person.
*/
public String name;
/**
* The age of the person.
*/
public int age;
#Override
public String toString()
{
if (name == null) return super.toString();
else return String.format("%s : %d", this.name, this.age);
}
}

Using the Java 8 Streams approach, with method references on the getters...
// Create a stream...
var sortedList = persons.stream()
// sort it (does not sort the original list)...
.sorted(Comparator.comparing(Person::getName)
.thenComparing(Person::getAge));
// and collect to a new list
.collect(Collectors.toList());
Collection to an array ist also possible:
persons.stream()
.sorted(Comparator.comparing(Person::getName)
.thenComparing(Person::getAge));
.toArray(String[]::new);
And the Java 8 Lambda approach...
//Sorts the original list Lambda style
persons.sort((p1, p2) -> {
if (p1.getName().compareTo(p2.getName()) == 0) {
return p1.getAge().compareTo(p2.getAge());
} else {
return p1.getName().compareTo(p2.getName());
}
});
Lastly...
// This syntax is similar to the Streams example above, but sorts the original list!!!
persons.sort(Comparator.comparing(Person::getName).thenComparing(Person::getAge));

You need to implement your own Comparator, and then use it: for example
Arrays.sort(persons, new PersonComparator());
Your Comparator could look a bit like this:
public class PersonComparator implements Comparator<? extends Person> {
public int compare(Person p1, Person p2) {
int nameCompare = p1.name.compareToIgnoreCase(p2.name);
if (nameCompare != 0) {
return nameCompare;
} else {
return Integer.valueOf(p1.age).compareTo(Integer.valueOf(p2.age));
}
}
}
The comparator first compares the names, if they are not equals it returns the result from comparing them, else it returns the compare result when comparing the ages of both persons.
This code is only a draft: because the class is immutable you could think of building an singleton of it, instead creating a new instance for each sorting.

Have your person class implement Comparable<Person> and then implement the compareTo method, for instance:
public int compareTo(Person o) {
int result = name.compareToIgnoreCase(o.name);
if(result==0) {
return Integer.valueOf(age).compareTo(o.age);
}
else {
return result;
}
}
That will sort first by name (case insensitively) and then by age. You can then run Arrays.sort() or Collections.sort() on the collection or array of Person objects.

Guava's ComparisonChain provides a clean way of doing it. Refer to this link.
A utility for performing a chained comparison statement. For example:
public int compareTo(Foo that) {
return ComparisonChain.start()
.compare(this.aString, that.aString)
.compare(this.anInt, that.anInt)
.compare(this.anEnum, that.anEnum, Ordering.natural().nullsLast())
.result();
}

You can do like this:
List<User> users = Lists.newArrayList(
new User("Pedro", 12),
new User("Maria", 10),
new User("Rafael",12)
);
users.sort(
Comparator.comparing(User::getName).thenComparing(User::getAge)
);

I would be careful when using Guava's ComparisonChain because it creates an instance of it per element been compared so you would be looking at a creation of N x Log N comparison chains just to compare if you are sorting, or N instances if you are iterating and checking for equality.
I would instead create a static Comparator using the newest Java 8 API if possible or Guava's Ordering API which allows you to do that, here is an example with Java 8:
import java.util.Comparator;
import static java.util.Comparator.naturalOrder;
import static java.util.Comparator.nullsLast;
private static final Comparator<Person> COMPARATOR = Comparator
.comparing(Person::getName, nullsLast(naturalOrder()))
.thenComparingInt(Person::getAge);
#Override
public int compareTo(#NotNull Person other) {
return COMPARATOR.compare(this, other);
}
Here is how to use the Guava's Ordering API: https://github.com/google/guava/wiki/OrderingExplained

Create as many comparators as necessary. After, call the method "thenComparing" for each order category. It's a way of doing by Streams. See:
//Sort by first and last name
System.out.println("\n2.Sort list of person objects by firstName then "
+ "by lastName then by age");
Comparator<Person> sortByFirstName
= (p, o) -> p.firstName.compareToIgnoreCase(o.firstName);
Comparator<Person> sortByLastName
= (p, o) -> p.lastName.compareToIgnoreCase(o.lastName);
Comparator<Person> sortByAge
= (p, o) -> Integer.compare(p.age,o.age);
//Sort by first Name then Sort by last name then sort by age
personList.stream().sorted(
sortByFirstName
.thenComparing(sortByLastName)
.thenComparing(sortByAge)
).forEach(person->
System.out.println(person));
Look: Sort user defined object on multiple fields – Comparator (lambda stream)

Use Comparator and then put objects into Collection, then Collections.sort();
class Person {
String fname;
String lname;
int age;
public Person() {
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
public String getFname() {
return fname;
}
public void setFname(String fname) {
this.fname = fname;
}
public String getLname() {
return lname;
}
public void setLname(String lname) {
this.lname = lname;
}
public Person(String fname, String lname, int age) {
this.fname = fname;
this.lname = lname;
this.age = age;
}
#Override
public String toString() {
return fname + "," + lname + "," + age;
}
}
public class Main{
public static void main(String[] args) {
List<Person> persons = new java.util.ArrayList<Person>();
persons.add(new Person("abc3", "def3", 10));
persons.add(new Person("abc2", "def2", 32));
persons.add(new Person("abc1", "def1", 65));
persons.add(new Person("abc4", "def4", 10));
System.out.println(persons);
Collections.sort(persons, new Comparator<Person>() {
#Override
public int compare(Person t, Person t1) {
return t.getAge() - t1.getAge();
}
});
System.out.println(persons);
}
}

Or you can exploit the fact that Collections.sort() (or Arrays.sort()) is stable (it doesn't reorder elements that are equal) and use a Comparator to sort by age first and then another one to sort by name.
In this specific case this isn't a very good idea but if you have to be able to change the sort order in runtime, it might be useful.

You can use generic serial Comparator to sort collections by multiple fields.
import org.apache.commons.lang3.reflect.FieldUtils;
import java.util.Arrays;
import java.util.Comparator;
import java.util.List;
/**
* #author MaheshRPM
*/
public class SerialComparator<T> implements Comparator<T> {
List<String> sortingFields;
public SerialComparator(List<String> sortingFields) {
this.sortingFields = sortingFields;
}
public SerialComparator(String... sortingFields) {
this.sortingFields = Arrays.asList(sortingFields);
}
#Override
public int compare(T o1, T o2) {
int result = 0;
try {
for (String sortingField : sortingFields) {
if (result == 0) {
Object value1 = FieldUtils.readField(o1, sortingField, true);
Object value2 = FieldUtils.readField(o2, sortingField, true);
if (value1 instanceof Comparable && value2 instanceof Comparable) {
Comparable comparable1 = (Comparable) value1;
Comparable comparable2 = (Comparable) value2;
result = comparable1.compareTo(comparable2);
} else {
throw new RuntimeException("Cannot compare non Comparable fields. " + value1.getClass()
.getName() + " must implement Comparable<" + value1.getClass().getName() + ">");
}
} else {
break;
}
}
} catch (IllegalAccessException e) {
throw new RuntimeException(e);
}
return result;
}
}

Arrays.sort(persons, new PersonComparator());
import java.util.Comparator;
public class PersonComparator implements Comparator<? extends Person> {
#Override
public int compare(Person o1, Person o2) {
if(null == o1 || null == o2 || null == o1.getName() || null== o2.getName() ){
throw new NullPointerException();
}else{
int nameComparisonResult = o1.getName().compareTo(o2.getName());
if(0 == nameComparisonResult){
return o1.getAge()-o2.getAge();
}else{
return nameComparisonResult;
}
}
}
}
class Person{
int age; String name;
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}
Updated version:
public class PersonComparator implements Comparator<? extends Person> {
#Override
public int compare(Person o1, Person o2) {
int nameComparisonResult = o1.getName().compareToIgnoreCase(o2.getName());
return 0 == nameComparisonResult?o1.getAge()-o2.getAge():nameComparisonResult;
}
}

For a class Book like this:
package books;
public class Book {
private Integer id;
private Integer number;
private String name;
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public Integer getNumber() {
return number;
}
public void setNumber(Integer number) {
this.number = number;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
#Override
public String toString() {
return "book{" +
"id=" + id +
", number=" + number +
", name='" + name + '\'' + '\n' +
'}';
}
}
sorting main class with mock objects
package books;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;
public class Main {
public static void main(String[] args) {
System.out.println("Hello World!");
Book b = new Book();
Book c = new Book();
Book d = new Book();
Book e = new Book();
Book f = new Book();
Book g = new Book();
Book g1 = new Book();
Book g2 = new Book();
Book g3 = new Book();
Book g4 = new Book();
b.setId(1);
b.setNumber(12);
b.setName("gk");
c.setId(2);
c.setNumber(12);
c.setName("gk");
d.setId(2);
d.setNumber(13);
d.setName("maths");
e.setId(3);
e.setNumber(3);
e.setName("geometry");
f.setId(3);
f.setNumber(34);
b.setName("gk");
g.setId(3);
g.setNumber(11);
g.setName("gk");
g1.setId(3);
g1.setNumber(88);
g1.setName("gk");
g2.setId(3);
g2.setNumber(91);
g2.setName("gk");
g3.setId(3);
g3.setNumber(101);
g3.setName("gk");
g4.setId(3);
g4.setNumber(4);
g4.setName("gk");
List<Book> allBooks = new ArrayList<Book>();
allBooks.add(b);
allBooks.add(c);
allBooks.add(d);
allBooks.add(e);
allBooks.add(f);
allBooks.add(g);
allBooks.add(g1);
allBooks.add(g2);
allBooks.add(g3);
allBooks.add(g4);
System.out.println(allBooks.size());
Collections.sort(allBooks, new Comparator<Book>() {
#Override
public int compare(Book t, Book t1) {
int a = t.getId()- t1.getId();
if(a == 0){
int a1 = t.getNumber() - t1.getNumber();
return a1;
}
else
return a;
}
});
System.out.println(allBooks);
}
}

I'm not sure if it's ugly to write the compartor inside the Person class in this case. Did it like this:
public class Person implements Comparable <Person> {
private String lastName;
private String firstName;
private int age;
public Person(String firstName, String lastName, int BirthDay) {
this.firstName = firstName;
this.lastName = lastName;
this.age = BirthDay;
}
public int getAge() {
return age;
}
public String getFirstName() {
return firstName;
}
public String getLastName() {
return lastName;
}
#Override
public int compareTo(Person o) {
// default compareTo
}
#Override
public String toString() {
return firstName + " " + lastName + " " + age + "";
}
public static class firstNameComperator implements Comparator<Person> {
#Override
public int compare(Person o1, Person o2) {
return o1.firstName.compareTo(o2.firstName);
}
}
public static class lastNameComperator implements Comparator<Person> {
#Override
public int compare(Person o1, Person o2) {
return o1.lastName.compareTo(o2.lastName);
}
}
public static class ageComperator implements Comparator<Person> {
#Override
public int compare(Person o1, Person o2) {
return o1.age - o2.age;
}
}
}
public class Test {
private static void print() {
ArrayList<Person> list = new ArrayList();
list.add(new Person("Diana", "Agron", 31));
list.add(new Person("Kay", "Panabaker", 27));
list.add(new Person("Lucy", "Hale", 28));
list.add(new Person("Ashley", "Benson", 28));
list.add(new Person("Megan", "Park", 31));
list.add(new Person("Lucas", "Till", 27));
list.add(new Person("Nicholas", "Hoult", 28));
list.add(new Person("Aly", "Michalka", 28));
list.add(new Person("Adam", "Brody", 38));
list.add(new Person("Chris", "Pine", 37));
Collections.sort(list, new Person.lastNameComperator());
Iterator<Person> it = list.iterator();
while(it.hasNext())
System.out.println(it.next().toString());
}
}

Sorting in Collections Comparator/Comparable

I am trying to sort the Person reverse alphabetically by name and then sorting in ascending order by age
I could sort in reverse order of name, but unable to sort again in ascending order of age.
PerId Name Age
--------------
7 Simpson 8
3 Michel 10
9 Mark 35
2 Mark 30
8 Lee 40
1 Jorge 19
5 Frank 28
6 Bill 34
4 Bill 16
My Code
class Util {
public static List<Person> getPersons() {
List<Person> col = new ArrayList<Person>();
col.add(new Person(5, "Frank", 28));
col.add(new Person(1, "Jorge", 19));
col.add(new Person(6, "Bill", 34));
col.add(new Person(3, "Michel", 10));
col.add(new Person(7, "Simpson", 8));
col.add(new Person(4, "Bill",16 ));
col.add(new Person(8, "Lee", 40));
col.add(new Person(9, "Mark", 35));
col.add(new Person(2, "Mark", 30));
return col;
}
}
class Person implements Comparator<Person> {
private int perId;
private String name;
private int age;
public Person(){}
public Person(int perId, String name, int age) {
this.perId = perId;
this.name = name;
this.age = age;
}
//.... getters/setters
#Override
public int compare(Person e1, Person e2) {
return e2.getName().compareTo(e1.getName());
}
}
class PersonId implements Comparable<Person>{
private Integer perId;
#Override
public int compareTo(Person o) {
return this.perId.compareTo(o.getPerId());
}
//...getter
}
public class TestPersonSort {
public static void main(String[] args) {
List<Person> coll = Util.getPersons();
Collections.sort(coll, new Person());
// sort method
printList(coll);
//problem here ***********************************************
//Collections.sort(coll);
}
private static void printList(List<Person> list) {
System.out.println("PerId\tName\tAge");
for (Person e: list) {
System.out.println(e.getPerId() + "\t" + e.getName() + "\t" + e.getAge());
}
}
}

Have a Comparator class first compare the person's name. If the comparison is not 0, return it. If it is 0, then move on to compare the person's age.
public int compare(Person e1, Person e2) {
int comp = e2.getName().compareTo(e1.getName()); // desc
if (comp != 0) return comp;
return e1.getAge() - e2.getAge(); // asc
}
Another option, a bit more complicated, but more flexible, is to have a Comparator that uses other Comparators. Then you can supply Comparator<Person>s that compare one attribute each, in a List, to combine their effects.
public class FlexiblePersonComparator implements Comparator<Person>
{
private List<Comparator<Person>> comparators;
public FlexiblePersonComparator(List<Comparator<Person>> comparators)
{
this.comparators = comparators;
}
#Override
public int compare(Person p1, Person p2)
{
int comp;
for (Comparator<Person> comparator : comparators)
{
comp = comparator.compare(p1, p2);
if (comp != 0)
return comp;
}
return 0;
}
}

An easy way is to have Person implement Comparable<Person>. main() can then use Collections.sort(coll);
class Person implements Comparable<Person> {
private int perId;
private String name;
private int age;
public Person(){}
public Person(int perId, String name, int age) {
this.perId = perId;
this.name = name;
this.age = age;
}
//.... getters/setters
#Override
public int compareTo(Person other) {
int result;
if (other == null)
return(0);
if (other == this)
return(0);
result = name.compareTo(other.name);
if (result != 0)
return(-result); // Sort names in reverse
return(age - other.age); // Sort age ascending
}
}

How to use Java comparator properly?

If I have the following class:
public class Employee {
private int empId;
private String name;
private int age;
public Employee(int empId, String name, int age) {
// set values on attributes
}
// getters & setters
}
How can I use comparator that compares by name, then age, then id?

You need to implement it so that it orders by preferred elements. That is, you need to compare by name, then if that comparison is equal, compare by age, etc. An example is listed below:
public class EmployeeComparator implements Comparator<Employee> {
#Override
public int compare(Employee e1, Employee e2) {
int nameDiff = e1.getName().compareTo(e2.getName());
if(nameDiff != 0) {
return nameDiff;
}
int ageDiff = e1.getAge() - e2.getAge();
if(ageDiff != 0) {
return ageDiff;
}
int idDiff = e1.getEmpId() - e2.getEmpId();
return idDiff;
}
}

Update
Came across this a moment ago: How to compare objects by multiple fields One of the answers linked to ComparatorChain which will invoke multiple comparators in sequence until a non-zero result is received from a comparator or all comparators are invoked. This should probably be your preferred solution.
Perhaps this (untested) implementation of Comparator#compare() will do the trick.
int compare(Employee e, Employee f)
{
int val = e.name.compareTo(f.name);
if(val == 0)
{
val = e.age - f.age;
if(val == 0)
{
val = e.empId - f.empId;
}
}
return val;
}

You can also implement the Comparable Interface in your class.
for example, something like this:
public class Employee implements Comparable<Employee>{
private int empId;
private String name;
private int age;
public Employee(int empId, String name, int age) {
// set values on attributes
}
// getters & setters
public int compareTo(Employee o) {
int ret = this.name.compareTo(o.name);
if(ret == 0)
ret = this.age - o.age;
if(ret == 0)
ret = this.empId - o.empId;
return ret;
}
}
so you don't have to implement a extra class to compare your Employees.

Implement it
public class Employee {
private int empId;
private String name;
private int age;
/**
* #param empId
* #param name
* #param age
*/
public Employee(int empId, String name, int age) {
super();
this.empId = empId;
this.name = name;
this.age = age;
}
/**
*
*/
public Employee() {
super();
// TODO Auto-generated constructor stub
}
public int getEmpId() {
return empId;
}
public void setEmpId(int empId) {
this.empId = empId;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
//Compare by name, age and then id
public static Comparator<Employee> COMPARE_EMPLOYEE = new Comparator<Employee>() {
public int compare(Employee one, Employee other) {
//Compare Name
if (one.getName().compareToIgnoreCase(other.getName()) == 0) {
//Compare age
if((one.getAge() - other.getAge()) == 0) {
// Now check with id is useless
// So directly return result of compare by id
return one.getEmpId() - other.getEmpId();
} else { //If age Not equal
return one.getAge() - other.getAge();
}
} else { //If name not equal
return one.getName().compareToIgnoreCase(other.getName());
}
}
};
}
Use :
List<Employee> contacts = new ArrayList<Employee>();
//Fill it.
//Sort by address.
Collections.sort(contacts, Employee.COMPARE_EMPLOYEE);
Read Sorting an ArrayList of Contacts , this must help you and you will get more ideas and different different types of use of Comparator.

guava ComparisonChain:
List<Employee> list = new ArrayList<Employee>();
//...
Collections.sort(list, new Comparator<Employee>(){
#Override
public int compare(Employee e1, Employee e2) {
return ComparisonChain.start()
.compare(e1.empId, e2.empId)
.compare(e1.name, e2.name)
.compare(e1.age, e2.age).result();
}});

Use this:
public class Test
{
public static void main(String[] args)
{
Employee emp1 = new Employee(2, "Tom", 20);
Employee emp2 = new Employee(1, "Tom", 20);
Employee emp3 = new Employee(3, "Hank", 21);
List<Employee> list = new ArrayList<>();
list.add(emp1);
list.add(emp2);
list.add(emp3);
Collections.sort(list, new Employee().new MyComparator());
System.out.println(list);
}
}
class Employee
{
private int empId;
private String name;
private int age;
public Employee()
{}
public Employee(int empId, String name, int age)
{
this.empId = empId;
this.name = name;
this.age = age;
}
class MyComparator implements Comparator<Employee>
{
#Override
public int compare(Employee e1, Employee e2)
{
if(e1.name.compareTo(e2.name) == 0)
{
if(((Integer)e1.age).compareTo(e2.age) == 0)
{
return ((Integer)e1.empId).compareTo(e2.empId);
}
else
{
return ((Integer)e1.age).compareTo(e2.age);
}
}
return e1.name.compareTo(e2.name);
}
}
#Override
public String toString()
{
return "Employee [empId=" + empId + ", name=" + name + ", age=" + age + "]";
}
}

The Comparator interface defines two methods: compare() and equals().
The compare() method, compares two elements for order:
int compare(Object obj1, Object obj2)
obj1 and obj2 are the objects to be compared. This method returns zero if the objects are equal. It returns a positive value if obj1 is greater than obj2. Otherwise, a negative value is returned.
By overriding compare(), you can alter the way that objects are ordered. For example, to sort in a reverse order, you can create a comparator that reverses the outcome of a comparison.
The equals() method, tests whether an object equals the invoking comparator: boolean equals(Object obj)
obj is the object to be tested for equality. The method returns true if obj and the invoking object are both Comparator objects and use the same ordering. Otherwise, it returns false.
Example:
import java.util.*;
class Dog implements Comparator<Dog>, Comparable<Dog> {
private String name;
private int age;
Dog() {
}
Dog(String n, int a) {
name = n;
age = a;
}
public String getDogName() {
return name;
}
public int getDogAge() {
return age;
}
// Overriding the compareTo method
public int compareTo(Dog d) {
return (this.name).compareTo(d.name);
}
// Overriding the compare method to sort the age
public int compare(Dog d, Dog d1) {
return d.age - d1.age;
}
}
public class Example {
public static void main(String args[]) {
// Takes a list o Dog objects
List<Dog> list = new ArrayList<Dog>();
list.add(new Dog("Shaggy", 3));
list.add(new Dog("Lacy", 2));
list.add(new Dog("Roger", 10));
list.add(new Dog("Tommy", 4));
list.add(new Dog("Tammy", 1));
Collections.sort(list); // Sorts the array list
for(Dog a: list) // printing the sorted list of names
System.out.print(a.getDogName() + ", ");
// Sorts the array list using comparator
Collections.sort(list, new Dog());
System.out.println(" ");
for(Dog a: list) // printing the sorted list of ages
System.out.print(a.getDogName() +" : "+ a.getDogAge() + ", ");
}
}
Check it out for more Java Comparator examples.

Sort a list of object w.r.t to a particular member/state in descending order

I have a list of java object that have 4 members.
int id;
String name;
String age;
int order;
I am using his code to sort object fields in ascending order according to if condition.
Collections.sort(list, new Comparator<com.MyObject>() {
public int compare(com.MyObject object1, com.MyObject object2) {
if(myString.equalsIgnoreCase("name")){
return object1.getName().compareTo(object2.getName());
}else if(myString.equalsIgnoreCase("age")){
return object1.getAge().compareTo(object2.getAge());
}else{
return object1.getOrder() - object2.getOrder();
}
}
});
I want to add another condition if(myString.equalsIgnoreCase("nameReverse")) If this will be the case the list should be sorted in descending order.

The Collections class has a reverseOrder method which returns a reverse Comparator for a given comparator. IMO, it would be more cleaner to not modify the Comparator if all you want to do is get a reverse Comparator.
A bit of sample code:
public class Person {
public static void testComparator() {
Person p1 = new Person("c");
Person p2 = new Person("a");
Person p3 = new Person("b");
List<Person> persons = Arrays.asList(p1, p2, p3);
Collections.sort(persons, PersonComparator.INSTANCE);
System.out.println("After normal sorting: " + persons);
Collections.sort(persons, Collections.reverseOrder(PersonComparator.INSTANCE));
System.out.println("After reverse sorting: " + persons);
}
public Person(String name) { this.name = name; }
public String name;
#Override
public String toString() {
StringBuilder builder = new StringBuilder();
builder.append("Person [name=").append(name).append("]");
return builder.toString();
}
}
class PersonComparator implements Comparator<Person> {
public static final PersonComparator INSTANCE = new PersonComparator();
#Override
public int compare(Person o1, Person o2) {
return o1.name.compareTo(o2.name);
}
}

Can I use a Comparator without implementing Comparable?

Is it possible to use a Comparator without implementing the Comparable class? For instance, if I had the following:
MyClass {
Comparator comp;
OrderedListInheritance(Comparator c) {
this.comp = c;
}
}
Could I then use comp to compare two objects? If so, how would I go about doing that?
Thanks...

You don't use Comparable. You use Comparator.
Comparable is an interface implemented by objects to specify their sort order with other objects of the same type.
Comparator is a generic interface that simply takes two objects and tells you their sort order. So you can do:
public class Student {
private final int id;
private final String name;
private final int age;
public Student(int id, String name, int age) {
this.id = id;
this.name = name;
this.age = age;
}
public int getId() { return id; }
public String getName() { return name; }
public int getAge() { return age; }
}
with:
public class AgeComparator implements Comparator<Student> {
public int compare(Student s1, Student s2) {
if (s1.getAge() == s2.getAge()) {
return 0;
} else {
return s1.getAge() < s2.getAge() ? -1 : 1;
}
}
and:
List<Student> students = new ArrayList<Student>();
students.add(new Student(1, "bob", 15));
students.add(new Student(2, "Jane", 14));
students.add(new Student(3, "Gary", 16));
SortedSet<Student> set1 = new TreeSet<Student>(new AgeComparator());
set1.addAll(students);
for (Student student : set1) {
// age order
}

Comparator<T> has public int compare(T lhs, T rhs). So use that method to compare objects.
Also, the sorted collections will accept a Comparator as an argument so you can (for example) say:
Comparator<Integer> comparator = new Comparator<Integer>() {
#Override public int compare(Integer lhs, Integer rhs) {
if (rhs.intValue() < lhs.intValue())
return -1;
else if (rhs.intValue() > lhs.intValue())
return 1;
else
return 0;
}
};
new TreeMap<Integer, Integer>(comparator);
To create a tree map where the sort order is (int this case) reversed.

Yes.
Comparator and Comparable are two separate and independent entities, only their purpose is similar.
In your code simply do: comp.compare(obj1, obj2)