ArrayList are also called as dynamic arrays that can grow as needed. In Java ArrayList class extends AbstractList and implements List interface. In Java ArrayList’s are created with an initial size. The size of the collection is changes dynamically as the items are exceeded the size or deleted. This example will explain sorting of ArrayList using Comparator, Comparable and Collectiolns.sort() method.

[box style=”1″]Note: Typically standard Array in java is fixed sized in nature. Once created, they cannot grow or shrink, which means that you must know in advance how many elements an array will hold.[/box]

At times, you may need to sort the ArrayList to make it alphabetically order. In this below example, it shows the use of Collections.sort and comparator to sort an ArrayList.

1. Sorting using Collections.sort method:

public class SortArrayList {

	public static void main(String[] args) {
		ArrayList unsortList = new ArrayList();
		unsortList.add("1011");
		unsortList.add("5460");
		unsortList.add("1000");
		unsortList.add("Abdul");
		unsortList.add("1012");
		unsortList.add("Adam");

		System.out.println("********** Value before sorting **************");
		for (String str : unsortList) {
			System.out.println(str);
		}

		Collections.sort(unsortList);

		System.out.println("********** Value after sorting **************");
		for (String str : unsortList) {
			System.out.println(str);
		}

	}
}

Here in this above example the sorting happens according to ASCII.

Output:

 2. Sorting objects in an ArrayList using Comparable interface

In my example I want to sort the employees list as per their salary in descending order. My Employee class implements Comparable interface and overridden compareTo method. In Compare to method add your logic, weather to sort ascending or descending order.

public class Employee implements Comparable {
	private String name;
	private String id;
	private double salary;

	public Employee(String id, String name, double salary) {
		this.id = id;
		this.name = name;
		this.salary = salary;
	}

	public String getName() {
		return name;
	}

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

	public String getId() {
		return id;
	}

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

	public double getSalary() {
		return salary;
	}

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

	@Override
	public int compareTo(Employee employee) {
		double compareSalary = ((Employee) employee).getSalary();

		// ascending order
		// return (int) (this.salary - compareSalary);

		// descending order
		return (int) (compareSalary - this.salary);
	}

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

}

3. Sorting Objects in an ArrayList using Comparator

Check out the below code snippet, I am sorting Employees list using Anonymous Comparator

//Sorting using Anonymous inner class type
		Collections.sort(emplyoees, new Comparator() {
			@Override
			public int compare(Employee e1, Employee e2) {
				String id1 = ((Employee) e1).getId();
				String id2 = ((Employee) e2).getId();

				// ascending order
				 return id1.compareTo(id2);

				// descending order
				//return id2.compareTo(id1);
			}
		});

Complete Code

public class SortArrayListObjects {

	public static void main(String[] args) {

		ArrayList employees = new ArrayList();
		employees.add(new Employee("1001", "Adam", 5000));
		employees.add(new Employee("1006", "Rajan", 8400));
		employees.add(new Employee("1040", "Keay", 2500));
		employees.add(new Employee("1056", "Ashok", 12000));	

		System.out.println("**** Value before sorting ***");
		for(Employee employee: employees){
			System.out.println(employee);
		}

		Collections.sort(employees);

		System.out.println("**** After sorting salary descending ***");
		for(Employee employee: employees){
			System.out.println(employee);
		}

		//Sorting using Anonymous inner class type
		Collections.sort(employees, new Comparator() {
			@Override
			public int compare(Employee e1, Employee e2) {
				String id1 = ((Employee) e1).getId();
				String id2 = ((Employee) e2).getId();

				// ascending order
				 return id1.compareTo(id2);

				// descending order
				//return id2.compareTo(id1);
			}
		});

		System.out.println("**** After sorting id ascending ***");
		for(Employee employee: employees){
			System.out.println(employee);
		}

	}
}

Output