I have the following problem to solve. I am using Java.
A restaurant recognizes 3 types of customers: “NEWBIES”, “REGULARS” and “VIPs”. When customers place their orders, all the orders join a queue. However the orders are always served in such a way that VIPs are served before regulars who are served before newbies.
I need a class which could be used to sort the customer orders. In case two customers are of the same type, the orderID should be used to sort them.
How can I sort by order priority based on the customer type using comparator?
Assuming I already have the following class Order
public class Order
{
public static int orderID;
private int tableNumber;
private String[] orderDetails;
private String customerType;
public Order(int tableNumber, String[] orderDetails, String customerType)
{
this.tableNumber = tableNumber;
this.orderDetails = orderDetails;
this.customerType = customerType;
orderID += 1;
}
// get and set methods declared
}
I have implemented the comparator as follows:
import java.util.Comparator;
public class OrderComparator implements Comparator<Order>
{
#Override
public int compare(Order o1, Order o2)
{
if(o1.getType().equals(o2.getType()))
return o1.getOrderID - o2.getOrderID;
else
// How does comparing the customer type text ensure that
// it will be printed in the right order?
return o1.getType().compareTo(o2.getType());
}
}
Not only do you want to sort on multiple fields, you also want a custom sort with one of those fields.
In the code below, I filled in the missing parts of both class Order and class OrderComparator. Notes after the code.
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;
public class Order {
public static final String NEWBIES = "NEWBIES";
public static final String REGULARS = "REGULARS";
public static final String VIP = "VIP";
private static int orderId;
private int orderID;
private int tableNumber;
private String[] orderDetails;
private String customerType;
public Order(int tableNumber, String[] orderDetails, String customerType) {
this.tableNumber = tableNumber;
this.orderDetails = orderDetails;
this.customerType = customerType;
orderID = ++orderId;
}
public int getOrderID() {
return orderID;
}
public int getTableNumber() {
return tableNumber;
}
public String[] getOrderDetails() {
return orderDetails;
}
public String getType() {
return customerType;
}
public String toString() {
return String.format("%d %s", orderID, customerType);
}
public static void main(String[] args) {
Order order1 = new Order(0, null, VIP);
Order order2 = new Order(0, null, REGULARS);
Order order3 = new Order(0, null, REGULARS);
List<Order> list = new ArrayList<>();
list.add(order3);
list.add(order2);
list.add(order1);
System.out.println("Unordered: " + list);
Collections.sort(list, new OrderComparator());
System.out.println("Ordered: " + list);
}
}
class OrderComparator implements Comparator<Order> {
#Override
public int compare(Order o1, Order o2) {
if (o1.getType().equals(o2.getType())) {
return o1.getOrderID() - o2.getOrderID();
}
else {
if (Order.VIP.equals(o1.getType())) {
return -1;
}
else if (Order.VIP.equals(o2.getType())) {
return 1;
}
else if (Order.REGULARS.equals(o1.getType())) {
return -1;
}
else if (Order.REGULARS.equals(o2.getType())) {
return 1;
}
else if (Order.NEWBIES.equals(o1.getType())) {
return -1;
}
else if (Order.NEWBIES.equals(o2.getType())) {
return 1;
}
throw new RuntimeException("Unexpected customer type.");
}
}
}
I added method main to class Order in order to test the code.
I added method toString to class Order so as to be able to check whether the code produces the expected results.
I understand that you want a kind of numerator for Order objects. Hence I made member orderID an instance member since every Order has its own ID and I added a new static member orderId (note that Java is case sensitive) which produces a new, unique order ID for each new Order object.
You want VIP orders to come before REGULARS orders and you want REGULARS orders to come before NEWBIES orders. By default, a Comparator sorts by ascending order, hence you want VIP to be lowest and NEWBIES to be highest (purely for sorting purposes). So in method compare (of class OrderComparator), if, for example, the type of o1 is VIP and the type of o2 is REGULARS then you want VIP to be lower that REGULAR. Hence in that situation, method compare returns -1 (minus one).
Running the above code produces the following output.
Unordered: [3 REGULARS, 2 REGULARS, 1 VIP]
Ordered: [1 VIP, 2 REGULARS, 3 REGULARS]
Note that since customerType (in class Order) is a String, there is a chance that an Order object will be created with an invalid customerType value. You could change the constructor of class Order and add a check for the supplied value (for customerType) and throw an Exception if the supplied value is invalid. Or you could use enum (also known as enumerated types). The below code uses enum instead of String for customerType - which also simplifies method compare in class OrderComparator.
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;
public class Order {
private static int orderId;
private int orderID;
private int tableNumber;
private String[] orderDetails;
private CustomerType customerType;
public Order(int tableNumber, String[] orderDetails, CustomerType customerType) {
this.tableNumber = tableNumber;
this.orderDetails = orderDetails;
this.customerType = customerType;
orderID = ++orderId;
}
public int getOrderID() {
return orderID;
}
public int getTableNumber() {
return tableNumber;
}
public String[] getOrderDetails() {
return orderDetails;
}
public CustomerType getType() {
return customerType;
}
public String toString() {
return String.format("%d %s", orderID, customerType);
}
public static void main(String[] args) {
Order order1 = new Order(0, null, CustomerType.VIP);
Order order2 = new Order(0, null, CustomerType.REGULARS);
Order order3 = new Order(0, null, CustomerType.REGULARS);
List<Order> list = new ArrayList<>();
list.add(order3);
list.add(order2);
list.add(order1);
System.out.println("Unordered: " + list);
Collections.sort(list, new OrderComparator());
System.out.println("Ordered: " + list);
}
}
class OrderComparator implements Comparator<Order> {
#Override
public int compare(Order o1, Order o2) {
if (o1.getType().equals(o2.getType())) {
return o1.getOrderID() - o2.getOrderID();
}
else {
return o2.getType().ordinal() - o1.getType().ordinal();
}
}
}
enum CustomerType {
NEWBIES, REGULARS, VIP
}
You can read this question How to sort a collection by multiple fields. Especially the second answer, first option listed.
I have an application where a user provides me with the name of a field, e.g name or costInCents, and I have to sort by that field. I have ways of guaranteeing that the field name will be correct. This application causes the complication that I simply cannot make my class Comparable and implement a specific compareTo(), since with a custom implementation of compareTo() I need to know which fields / methods to use at implementation time.
So to achieve this goal, I am trying to use reflection in order to match the field to its accessor. Here's a MWE of what I want to do.
Class Product is a simple POJO class whose instances I want to pairwise compare:
public class Product
{
final String name;
final Integer quantity;
final Long costInCents;
public Product(final String name, final Integer quantity, final Long costInCents)
{
this.name = name;
this.quantity = quantity;
this.costInCents = costInCents;
}
public String getName()
{
return name;
}
public Integer getQuantity()
{
return quantity;
}
public Long getCostInCents()
{
return costInCents;
}
}
And my Main class, which is currently incomplete:
public class Main {
public static void main(String[] args) {
final Product[] productArray =
{
new Product("Clorox wipes", 50, 700L),
new Product("Desk chair", 10, 12000L),
new Product("TV", 5, 30000L),
new Product("Bookcase", 5, 12000L),
new Product("Water bottle", 20, 700L),
};
// The following void methods are supposed to sort in-place with something like Arrays.sort() or Collections.sort(),
// but I am also open to solutions involving stuff like Stream::sorted() or similar ones, which return a sorted array.
sortByField(productArray, "costInCents");
sortByField(productArray, "name");
}
private void sortByField(final Product[] productArray, final String sorterFieldName)
{
final Field sorterField = getSorterField(sorterFieldName, LiteProduct.class); // Gets the Field somehow
final Method sorterAccessor = getSorterAccessor(sorterField, LiteProduct.class); // Given the Field, this is easy
Arrays.sort((Product p1, Product p2)->((Comparable<?>)sorterAccessor.invoke(p1)).compareTo(sorterAccessor.invoke(p2)) > 0); // Capture of ? instead of Object
}
}
Unfortunately, the Arrays.sort() line results in a compile-time error with message Capture of ? instead of Object. I have tried casting the second argument to Comparable<?>, Comparable<? super sorterField.getType(), etc, with no luck. Ideas?
Possibly the best way - with sorting strategies. No need for reflection, compatible with more complex sorting logic:
Map<String, Comparator<Product>> sortingStrategies = new HashMap<>(){
{
put("costInCents", Comparator.comparingLong(p->p.costInCents));
put("quantity", Comparator.comparingLong(p->p.quantity));
put("name", Comparator.comparing(p->p.name));
}
};
private void sortByField(final Product[] productArray, final String sorterFieldName)
{
Arrays.sort(productArray, sortingStrategies.get(sorterFieldName));
}
You could write a Comparator for each field and use it by name via a Map:
public class Product
{
private final static Map<String,Comparator<Product>> COMPARATORS;
static {
COMPARATORS = new HashMap<>();
COMPARATORS.put("name", new NameComparator());
COMPARATORS.put("costInCents", new CostInCentsComparator());
}
final String name;
final Integer quantity;
final Long costInCents;
public Product(final String name, final Integer quantity, final Long costInCents)
{
this.name = name;
this.quantity = quantity;
this.costInCents = costInCents;
}
public String getName()
{
return name;
}
public Integer getQuantity()
{
return quantity;
}
public Long getCostInCents()
{
return costInCents;
}
static class NameComparator implements Comparator<Product> {
#Override
public int compare(Product o1, Product o2) {
return o1.getName().compareTo(o2.getName());
}
}
static class CostInCentsComparator implements Comparator<Product> {
#Override
public int compare(Product o1, Product o2) {
return o1.getCostInCents().compareTo(o2.getCostInCents());
}
}
static Comparator<Product> getComparator(String name) {
return COMPARATORS.get(name);
}
}
and use that in the main class
public class Main {
public static void main(String[] args) {
final Product[] productArray =
{
new Product("Clorox wipes", 50, 700L),
new Product("Desk chair", 10, 12000L),
new Product("TV", 5, 30000L),
new Product("Bookcase", 5, 12000L),
new Product("Water bottle", 20, 700L),
};
// The following void methods are supposed to sort in-place with something like Arrays.sort() or Collections.sort(),
// but I am also open to solutions involving stuff like Stream::sorted() or similar ones, which return a sorted array.
sortByField(productArray, "costInCents");
sortByField(productArray, "name");
}
private static void sortByField(final Product[] productArray, final String sorterFieldName)
{
Arrays.sort(productArray, Product.getComparator(sorterFieldName));
}
}
You may have to do minor changes like makeing it null safe or something
you can try this:
public class Test{
public static void main(String arg[]){
final Product[] productArray =
{
new Product("Clorox wipes", 50, 700L),
new Product("Desk chair", 10, 12000L),
new Product("TV", 5, 30000L),
new Product("Bookcase", 5, 12000L),
new Product("Water bottle", 20, 700L),
};
Arrays.sort(productArray,(p1, p2) -> p1.getName().compareTo(p2.getName()));
for(Product p: productArray){
System.out.println(p.getName());
}
}
}
in this case Comparator is a functional interface so I used lambad expression. but you can do this too
Is it possible to use Lambda expression or better way to write down the for-loop?
public TaskDTO convertToDTO(Task task) {
for (int i = 0; i < task.getPrecedingTasks().size(); i++)
this.precedingTasks.add(task.getPrecedingTasks().get(i).getName());
}
Your solution is good as:
task.getPrecedingTasks().stream().map(Task::getName).forEach(this.precedingTasks::add);
But since you are just retrieving the part of the Task, map and then collect as a list as:
this.precedingTasks = task.getPrecedingTasks().stream().map(Task::getName).collect(Collectors.toList());
Isn't it more straightforward and easier to understand? Since stream here is to do the mapping/converting and then collecting.
And also in this way, you don't need to do the initialisation for the this.precedingTasks as
this.precedingTasks = new ArrayList<>(); // to ensure it's not null;
Anyway, just personal preference here.
This is a complete example, where I put System.out.println ... you should use a this.precedingTasks.addAll( ...
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;
public class Main {
public static void main(String[] args) {
Task t1 = new Task("myTask", Arrays.asList(new Task("innerTask1"), new Task("innerTask2")));
System.out.println(t1.precedingTasks.stream().map(Task::getName).collect(Collectors.toList()));
}
static class Task {
private String name;
private List<Task> precedingTasks = new ArrayList<>();
public Task(String name) {
this.name = name;
}
public Task(String name, List<Task> precedingTasks) {
this.name = name;
this.precedingTasks = precedingTasks;
}
public String getName() {
return name;
}
public List<Task> getPrecedingTasks() {
return precedingTasks;
}
}
}
The output is
[innerTask1, innerTask2]
I found the correct solution in my case:
task.getPrecedingTasks().stream().map(Task::getName).forEach(this.precedingTasks::add);
Thanks, for tips : )
I have simple class
public class ActiveAlarm {
public long timeStarted;
public long timeEnded;
private String name = "";
private String description = "";
private String event;
private boolean live = false;
}
and List<ActiveAlarm> con. How to sort in ascending order by timeStarted, then by timeEnded? Can anybody help? I know in C++ with generic algorithm and overload operator <, but I am new to Java.
Using Comparator
For Example:
class Score {
private String name;
private List<Integer> scores;
// +accessor methods
}
Collections.sort(scores, new Comparator<Score>() {
public int compare(Score o1, Score o2) {
// compare two instance of `Score` and return `int` as result.
return o2.getScores().get(0).compareTo(o1.getScores().get(0));
}
});
With Java 8 onwards, you can simply use lambda expression to represent Comparator instance.
Collections.sort(scores, (s1, s2) -> { /* compute and return int */ });
Either make ActiveAlarm implement Comparable<ActiveAlarm> or implement Comparator<ActiveAlarm> in a separate class. Then call:
Collections.sort(list);
or
Collections.sort(list, comparator);
In general, it's a good idea to implement Comparable<T> if there's a single "natural" sort order... otherwise (if you happen to want to sort in a particular order, but might equally easily want a different one) it's better to implement Comparator<T>. This particular situation could go either way, to be honest... but I'd probably stick with the more flexible Comparator<T> option.
EDIT: Sample implementation:
public class AlarmByTimesComparer implements Comparator<ActiveAlarm> {
#Override
public int compare(ActiveAlarm x, ActiveAlarm y) {
// TODO: Handle null x or y values
int startComparison = compare(x.timeStarted, y.timeStarted);
return startComparison != 0 ? startComparison
: compare(x.timeEnded, y.timeEnded);
}
// I don't know why this isn't in Long...
private static int compare(long a, long b) {
return a < b ? -1
: a > b ? 1
: 0;
}
}
JAVA 8 and Above Answer (Using Lambda Expressions)
In Java 8, Lambda expressions were introduced to make this even easier! Instead of creating a Comparator() object with all of it's scaffolding, you can simplify it as follows: (Using your object as an example)
Collections.sort(list, (ActiveAlarm a1, ActiveAlarm a2) -> a1.timeStarted-a2.timeStarted);
or even shorter:
Collections.sort(list, Comparator.comparingInt(ActiveAlarm ::getterMethod));
That one statement is equivalent to the following:
Collections.sort(list, new Comparator<ActiveAlarm>() {
#Override
public int compare(ActiveAlarm a1, ActiveAlarm a2) {
return a1.timeStarted - a2.timeStarted;
}
});
Think of Lambda expressions as only requiring you to put in the relevant parts of the code: the method signature and what gets returned.
Another part of your question was how to compare against multiple fields. To do that with Lambda expressions, you can use the .thenComparing() function to effectively combine two comparisons into one:
Collections.sort(list, (ActiveAlarm a1, ActiveAlarm a2) -> a1.timeStarted-a2.timeStarted
.thenComparing ((ActiveAlarm a1, ActiveAlarm a2) -> a1.timeEnded-a2.timeEnded)
);
The above code will sort the list first by timeStarted, and then by timeEnded (for those records that have the same timeStarted).
One last note: It is easy to compare 'long' or 'int' primitives, you can just subtract one from the other. If you are comparing objects ('Long' or 'String'), I suggest you use their built-in comparison. Example:
Collections.sort(list, (ActiveAlarm a1, ActiveAlarm a2) -> a1.name.compareTo(a2.name) );
EDIT: Thanks to Lukas Eder for pointing me to .thenComparing() function.
We can sort the list in one of two ways:
1. Using Comparator : When required to use the sort logic in multiple places
If you want to use the sorting logic in a single place, then you can write an anonymous inner class as follows, or else extract the comparator and use it in multiple places
Collections.sort(arrayList, new Comparator<ActiveAlarm>() {
public int compare(ActiveAlarm o1, ActiveAlarm o2) {
//Sorts by 'TimeStarted' property
return o1.getTimeStarted()<o2.getTimeStarted()?-1:o1.getTimeStarted()>o2.getTimeStarted()?1:doSecodaryOrderSort(o1,o2);
}
//If 'TimeStarted' property is equal sorts by 'TimeEnded' property
public int doSecodaryOrderSort(ActiveAlarm o1,ActiveAlarm o2) {
return o1.getTimeEnded()<o2.getTimeEnded()?-1:o1.getTimeEnded()>o2.getTimeEnded()?1:0;
}
});
We can have null check for the properties, if we could have used 'Long' instead of 'long'.
2. Using Comparable(natural ordering): If sort algorithm always stick to one property:
write a class that implements 'Comparable' and override 'compareTo' method as defined below
class ActiveAlarm implements Comparable<ActiveAlarm>{
public long timeStarted;
public long timeEnded;
private String name = "";
private String description = "";
private String event;
private boolean live = false;
public ActiveAlarm(long timeStarted,long timeEnded) {
this.timeStarted=timeStarted;
this.timeEnded=timeEnded;
}
public long getTimeStarted() {
return timeStarted;
}
public long getTimeEnded() {
return timeEnded;
}
public int compareTo(ActiveAlarm o) {
return timeStarted<o.getTimeStarted()?-1:timeStarted>o.getTimeStarted()?1:doSecodaryOrderSort(o);
}
public int doSecodaryOrderSort(ActiveAlarm o) {
return timeEnded<o.getTimeEnded()?-1:timeEnded>o.getTimeEnded()?1:0;
}
}
call sort method to sort based on natural ordering
Collections.sort(list);
In java8+ this can be written in single line as follows:
collectionObjec.sort(comparator_lamda) or comparator.comparing(CollectionType::getterOfProperty)
code:
ListOfActiveAlarmObj.sort((a,b->a.getTimeStarted().compareTo(b.getTimeStarted())))
or
ListOfActiveAlarmObj.sort(Comparator.comparing(ActiveAlarm::getTimeStarted))
public class ActiveAlarm implements Comparable<ActiveAlarm> {
public long timeStarted;
public long timeEnded;
private String name = "";
private String description = "";
private String event;
private boolean live = false;
public int compareTo(ActiveAlarm a) {
if ( this.timeStarted > a.timeStarted )
return 1;
else if ( this.timeStarted < a.timeStarted )
return -1;
else {
if ( this.timeEnded > a.timeEnded )
return 1;
else
return -1;
}
}
That should give you a rough idea. Once that's done, you can call Collections.sort() on the list.
Since Java8 this can be done even cleaner using a combination of Comparator and Lambda expressions
For Example:
class Student{
private String name;
private List<Score> scores;
// +accessor methods
}
class Score {
private int grade;
// +accessor methods
}
Collections.sort(student.getScores(), Comparator.comparing(Score::getGrade);
Java-8 solution using Stream API:
A. When timeStarted and timeEnded are public (as mentioned in the requirement) and therefore do not (need to) have public getter methods:
List<ActiveAlarm> sorted =
list.stream()
.sorted(Comparator.comparingLong((ActiveAlarm alarm) -> alarm.timeStarted)
.thenComparingLong((ActiveAlarm alarm) -> alarm.timeEnded))
.collect(Collectors.toList());
B. When timeStarted and timeEnded have public getter methods:
List<ActiveAlarm> sorted =
list.stream()
.sorted(Comparator.comparingLong(ActiveAlarm::getTimeStarted)
.thenComparingLong(ActiveAlarm::getTimeEnded))
.collect(Collectors.toList());
If you want to sort the original list itself:
A. When timeStarted and timeEnded are public (as mentioned in the requirement) and therefore do not (need to) have public getter methods:
list.sort(Comparator.comparingLong((ActiveAlarm alarm) -> alarm.timeStarted)
.thenComparingLong((ActiveAlarm alarm) -> alarm.timeEnded));
B. When timeStarted and timeEnded have public getter methods:
list.sort(Comparator.comparingLong(ActiveAlarm::getTimeStarted)
.thenComparingLong(ActiveAlarm::getTimeEnded));
Guava's ComparisonChain:
Collections.sort(list, new Comparator<ActiveAlarm>(){
#Override
public int compare(ActiveAlarm a1, ActiveAlarm a2) {
return ComparisonChain.start()
.compare(a1.timestarted, a2.timestarted)
//...
.compare(a1.timeEnded, a1.timeEnded).result();
}});
We can use the Comparator.comparing() method to sort a list based on an object's property.
class SortTest{
public static void main(String[] args) {
ArrayList<ActiveAlarm> activeAlarms = new ArrayList<>(){{
add(new ActiveAlarm("Alarm 1", 5, 10));
add(new ActiveAlarm("Alarm 2", 2, 12));
add(new ActiveAlarm("Alarm 3", 0, 8));
}};
/* I sort the arraylist here using the getter methods */
activeAlarms.sort(Comparator.comparing(ActiveAlarm::getTimeStarted)
.thenComparing(ActiveAlarm::getTimeEnded));
System.out.println(activeAlarms);
}
}
Note that before doing it, you'll have to define at least the getter methods of the properties you want to base your sort on.
public class ActiveAlarm {
public long timeStarted;
public long timeEnded;
private String name = "";
private String description = "";
private String event;
private boolean live = false;
public ActiveAlarm(String name, long timeStarted, long timeEnded) {
this.name = name;
this.timeStarted = timeStarted;
this.timeEnded = timeEnded;
}
public long getTimeStarted() {
return timeStarted;
}
public long getTimeEnded() {
return timeEnded;
}
#Override
public String toString() {
return name;
}
}
Output:
[Alarm 3, Alarm 2, Alarm 1]
Employee POJO Class
package in.ac.adit.oop.sort;
public class Employee {
private int id;
private String name;
private String department;
public int getId() {
return id;
}
public Employee() {
super();
}
public Employee(int id, String name, String department) {
super();
this.id = id;
this.name = name;
this.department = department;
}
#Override
public String toString() {
return "Employee [id=" + id + ", name=" + name + ", department=" + department + "]";
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getDepartment() {
return department;
}
public void setDepartment(String department) {
this.department = department;
}
}
Employee Class To Manage Employee
package in.ac.adit.oop.sort;
import java.util.ArrayList;
import java.util.List;
public class Example {
public static void main(String[] args) {
/*
* Create 10 Employee Object
*/
Employee emp1 = new Employee(1, "Nayan", "IT");
Employee emp2 = new Employee(2, "Siddarth", "CP");
Employee emp3 = new Employee(3, "Samarth", "AE");
Employee emp4 = new Employee(4, "Bhavesh", "CV");
Employee emp5 = new Employee(5, "Sam", "FT");
Employee emp6 = new Employee(6, "Keyur", "IT");
Employee emp7 = new Employee(7, "Bala", "ME");
Employee emp8 = new Employee(8, "Mitul", "ME");
Employee emp9 = new Employee(9, "Kamlesh", "EE");
Employee emp10 = new Employee(10, "Piyush", "EE");
/*
* List of Employee Object
*/
List<Employee> employeeList = new ArrayList<Employee>();
employeeList.add(emp1);
employeeList.add(emp2);
employeeList.add(emp3);
employeeList.add(emp4);
employeeList.add(emp5);
employeeList.add(emp6);
employeeList.add(emp7);
employeeList.add(emp8);
employeeList.add(emp9);
employeeList.add(emp10);
CustomObjectSort customObjectSort = new CustomObjectSort();
List<Employee> sortByDepartment = customObjectSort.sortByDepartment(employeeList);
/*
* Sorted By Department
*/
for (Employee employee : sortByDepartment) {
System.out.println(employee);
}
/*
* Sorted By Name
*/
List<Employee> sortByName = customObjectSort.sortByName(employeeList);
for (Employee employee : sortByName) {
System.out.println(employee);
}
/*
* Sorted By Id
*/
List<Employee> sortById = customObjectSort.sortById(employeeList);
for (Employee employee : sortById) {
System.out.println(employee);
}
}
}
Custom Sorting
package in.ac.adit.oop.sort;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;
public class CustomObjectSort {
public List<Employee> sortByName(List<Employee> employeeList) {
Collections.sort(employeeList, new Comparator<Employee>() {
#Override
public int compare(Employee employee1, Employee employee2) {
return employee1.getName().compareTo(employee2.getName());
}
});
return employeeList;
}
public List<Employee> sortByDepartment(List<Employee> employeeList) {
Collections.sort(employeeList, new Comparator<Employee>() {
#Override
public int compare(Employee employee1, Employee employee2) {
return employee1.getDepartment().compareTo(employee2.getDepartment());
}
});
return employeeList;
}
public List<Employee> sortById(List<Employee> employeeList) {
Collections.sort(employeeList, new Comparator<Employee>() {
#Override
public int compare(Employee employee1, Employee employee2) {
return employee1.getId() - employee2.getId();
}
});
return employeeList;
}
}
You can use Collections.sort and pass your own Comparator<ActiveAlarm>
In java you need to use the static Collections.sort method. Here is an example for a list of CompanyRole objects, sorted first by begin and then by end. You can easily adapt for your own object.
private static void order(List<TextComponent> roles) {
Collections.sort(roles, new Comparator() {
#Override
public int compare(Object o1, Object o2) {
int x1 = ((CompanyRole) o1).getBegin();
int x2 = ((CompanyRole) o2).getBegin();
if (x1 != x2) {
return x1 - x2;
} else {
int y1 = ((CompanyRole) o1).getEnd();
int y2 = ((CompanyRole) o2).getEnd();
return y2 - y1;
}
}
});
}
You can call Collections.sort() and pass in a Comparator which you need to write to compare different properties of the object.
As mentioned you can sort by:
Making your object implement Comparable
Or pass a Comparator to Collections.sort
If you do both, the Comparable will be ignored and Comparator will be used. This helps that the value objects has their own logical Comparable which is most reasonable sort for your value object, while each individual use case has its own implementation.
public class ActiveAlarm {
public long timeStarted;
public long timeEnded;
private String name = "";
private String description = "";
private String event;
private boolean live = false;
}
List<ActiveAlarm> con = new ArrayList<ActiveAlarm>();
Collections.sort(con , (a1, a2) -> a1.timeStarted.compareTo(a2.timeStarted));
Collections.sort(con , (a1, a2) -> a1.timeEnded.compareTo(a2.timeEnded));
Here's what did the trick for me.
Was much shorter and easier than everything else I found:
Collections.sort(listName, Comparator.comparing(Object::getProperty).reversed());
The ".reversed()" part at the end was a requirement for my specific project but I'm sharing it too, as it took a while to find it
The best and the easiest way to sort any list of objects in Java (Java 8 and above).
Lets sort a basket of fruits based on the property "fruitName"
Fruit POJO:
class Fruit
{
int price;
String fruitName;
public Fruit(int price, String fruitName) {
super();
this.price = price;
this.fruitName = fruitName;
}
public int getPrice() {
return price;
}
public void setPrice(int price) {
this.price = price;
}
public String getFruitName() {
return fruitName;
}
public void setFruitName(String fruitName) {
this.fruitName = fruitName;
}
#Override
public String toString() {
return "Fruits [price=" + price + ", fruitName=" + fruitName + "]";
}
}
Now lets add fruits into a list and then sort it
List<Fruit> basketOfFruits = new ArrayList<>();
basketOfFruits.add(new Fruit(123, "oranges"));
basketOfFruits.add(new Fruit(45, "nectarine"));
basketOfFruits.add(new Fruit(369, "blueberries"));
basketOfFruits.add(new Fruit(248, "apple"));
basketOfFruits.add(new Fruit(968, "peaches"));
basketOfFruits.add(new Fruit(436, "grapes"));
basketOfFruits.add(new Fruit(596, "figs"));
//sorting by the property fruitName
Collections.sort(basketOfFruits, (f1, f2)->{return f1.getFruitName().compareTo(f2.getFruitName());});
You can now print the list (i.e basketOfFruits) and the fruits in the list would be sorted in ASCENDING order (lexicographically).
The output would look like this:
[Fruits [price=248, fruitName=apple], Fruits [price=369, fruitName=blueberries], Fruits [price=596, fruitName=figs], Fruits [price=436, fruitName=grapes], Fruits [price=45, fruitName=nectarine], Fruits [price=123, fruitName=oranges], Fruits [price=968, fruitName=peaches]]
Instead of Collections.sort(), Java streams can also be used (Java 8 and above). The following is the code using Java streams
List<Fruit> sortedFruits = basketOfFruits.stream().sorted( (f1, f2)->{return f1.getFruitName().compareTo(f2.getFruitName());}).collect(Collectors.toList());
here the list is sorted in the same manner as Collections.sort(), but the sorted items would be stored/collected in another list "sortedFruits". So, if we want to print the sorted items of the list, we need to print "sortedFruits" instead of "basketOfFruits" in this case