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5 ways to sort an array of custom objects by property in java with examples


It is a short tutorial on multiple ways to sort an array of objects in java.

Let’s define an Employee.javaclass.

public class Employee {
    private Integer id;
    private String name;
    private Integer salary;
    public Employee(Integer id, String name,Integer salary) {
        this.id = id;
        this.name = name;
        this.salary=salary;
    }

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Integer getSalary() {
        return salary;
    }

    public void setSalary(Integer salary) {
        this.salary = salary;
    }

    @Override
    public String toString() {
        return "{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", salary=" + salary +
                '}';
    }
}

Employee class fields - id, name and salary

Let’s add some data and display the ArrayList

import java.util.*;

public class Main {
    public static void main(String[] args) {
        ArrayList<Employee> list=new ArrayList<>();
        Employee e1=new Employee(1,"Franc",5000);
        Employee e2=new Employee(11,"John",8000);
        Employee e3=new Employee(12,"Eric",4000);
        Employee e4=new Employee(14,"Andrew",7000);

        list.add(e1);
        list.add(e2);
        list.add(e3);
        list.add(e4);

        System.out.println(list);
    }
}

Output:

[{id=1, name='Franc', salary=5000},
{id=11, name='John', salary=8000},
{id=12, name='Eric', salary=4000}, 
{id=14, name='Andrew', salary=7000}]

In the above example,

  • Created List of Employee objects
  • Populate ArrayList with employee data
  • Finally, Print a list of employee objects
  • List objects are displayed in insert order by default.

sort ArrayList of objects by property java

There are multiple ways we can sort objects by property We will see multiple ways to sort employee objects with salary in ascending or descending order.

Using java.util.Comparator

Collections class provides a sort method that accepts two arguments.

  • list argument is the source array list to sort
  • Comparator object which has compared method
        Collections.sort(list, new Comparator<Employee>(){
            public int compare(Employee e1, Employee e2){
                return e1.getSalary().compareTo(e2.getSalary());
            }
        });
        System.out.println(list);

Output:

[{id=12, name='Eric', salary=4000},
{id=1, name='Franc', salary=5000},
{id=14, name='Andrew', salary=7000},
{id=11, name='John', salary=8000}]

Sort list of Employee Object in salary field descending order

Collections.sort(list, new Comparator<Employee>(){
            public int compare(Employee e1, Employee e2){
                return e2.getSalary().compareTo(e1.getSalary());
            }
        });
    System.out.println(list);

Output

[{id=11, name='John', salary=8000}, 
{id=14, name='Andrew', salary=7000}, 
{id=1, name='Franc', salary=5000}, 
{id=12, name='Eric', salary=4000}]

The following logic returns the list in descending order

                return e1.getSalary().compareTo(e2.getSalary());

for ascending order

                return e2.getSalary().compareTo(e1.getSalary());

Java 8 sort objects list by field

java 8 extended comparator behavior and added comparing static method.

Syntax for Comparting:

static <T,U extends Comparable<? super U>> Comparator<T> comparing(
   Function<? super T,? extends U> keyExtractor)

   static <T,U> Comparator<T> comparing(
  Function<? super T,? extends U> keyExtractor,
    Comparator<? super U> keyComparator)

Comparing method has the below arguments.

keyExtractor : key function to sort keyComparator: Optional to add custom logic comparison.

This comparator can be applied to List as well as Collections classes. Here is an example sort list by fields ascending with List and Collections.

list.sort(Comparator.comparing(Employee::getSalary)); (or)
Collections.sort(list,Comparator.comparing(Employee::getSalary));

To sort descending

list.sort(Comparator.comparing(Employee::getSalary,(emp1, emp2) -> {
                return emp2.compareTo(emp1);
           }));
        
        Collections.sort(list,Comparator.comparing(Employee::getSalary,(emp1, emp2) -> {
            return emp2.compareTo(emp1);
        }));

Custom object with Comparable class

It is an example of sorting object lists with property alphabetically.

Let’s implement Comparable in Employee object override compareTo with custom logic, Here Need sorting based on employee name, so getName field logic added to this method.

public class Employee implements Comparable<Employee>{
    private Integer id;
    private String name;
    private Integer salary;
    public Employee(Integer id, String name,Integer salary) {
        this.id = id;
        this.name = name;
        this.salary=salary;
    }

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Integer getSalary() {
        return salary;
    }

    public void setSalary(Integer salary) {
        this.salary = salary;
    }

    @Override
    public String toString() {
        return "{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", salary=" + salary +
                '}';
    }

    @Override
    public int compareTo(Employee u) {
        if (getName() == null || u.getName() == null) {
            return 0;
        }
        return getName().compareTo(u.getName());
    }
}

Object with name are sorted alphabetically i.e ascending order

Collections.sort(list);

Descending order

Collections.sort(list);
Collections.reverse(list);

Sort using Java8 stream sorted

This approach returns a new array list.

  • iterate ArrayList using stream()

  • use the sorted method to return the element in sorted

  • finally returns the collection to the list

        List<Employee> result = (ArrayList<Employee>) list
                .stream().sorted(Comparator.comparing(Employee::getName))
                .collect(Collectors.toList());

Sort ArrayList of Objects by multiple properties

Here is a sequence of steps

  • First, create a Comparator using the Comparator. comparing method
  • In this comparator, Sorting first id, next name and salary.
  • Pass this comparator to List for soring using List.
  • pass this comparator to List for sorting using Collections.
  • Comparator returns a list in ascending order
  • Comparator.reverse returns list in descending order
  Comparator<Employee> employeeComparator = Comparator.comparing(Employee::getId)
                .thenComparing(Employee::getName)
                .thenComparing(Employee::getSalary);
        Collections.sort(list, employeeComparator);

Example for Sorting multiple fields with list and collections in ascending order

Collections.sort(list, employeeComparator); // ascending
        list.sort(employeeComparator);

Example for Sorting multiple fields with list and collections in descending order

Collections.sort(list, employeeComparator.reversed());
list.sort(employeeComparator.reversed());
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