Organizing an ArrayList of User-defined objects by a specific attribute is a frequent task in Java development, particularly when handling extensive datasets. At times, you may wish to arrange an ArrayList according to one or multiple attributes of the objects. Java provides numerous methods to accomplish this, which you will explore in this article. Let’s begin with the fundamentals.
Java ArrayList is a component of the collections framework, and it belongs to the java.util package. It offers us dynamic-sized arrays in Java.
An ArrayList is a resizable array that can store a collection of objects. Java provides several mechanisms to realize this, the most common being the use of the Comparator interface or lambda expressions.
Syntax for creating an empty ArrayList:
ArrayList<E> list = new ArrayList<>();
Various Methods to Organize an ArrayList of Objects by Attribute
There are primarily two approaches to organizing an ArrayList of user-defined objects. These are:
Using the Comparator Interface
Using the Comparable Interface
Let’s delve into both of these in depth.
1. Utilizing the Comparator Interface
The Comparator interface is applied when you need to establish custom sorting logic that is distinct from that of the object’s class. It offers flexibility because you can design various comparators for different sorting criteria (for example, by age, by name, etc.) and provide them to the sorting methods.
Example:
Java
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var isMobile = window.innerWidth people = new ArrayList();
people.add(new Person(“Alok”, 30));
people.add(new Person(“babu”, 25));
people.add(new Person(“Chandu”, 35));
people.add(new Person(“David”, 20));
// Sort by age using Comparator
Collections.sort(people, new Comparator() {
@Override
public int compare(Person p1, Person p2) {
return Integer.compare(p1.getAge(), p2.getAge()); // Sorting by age
}
});
// Print the sorted list
for (Person person : people) {
System.out.println(person);
}
}
}
class Person {
String name;
int age;
// Constructor
public Person(String name, int age) {
this.name = name;
this.age = age;
}
// Getter methods
public String getName() {
return name;
}
public int getAge() {
return age;
}
// toString() for easier printing
@Override
public String toString() {
return “Person{name='” + name + “‘, age=” + age + “}”;
}
}`);
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Result:
Clarification: The code above utilizes the Comparator interface to order the Person objects according to age. A Comparator is provided to Collections.sort() with a comparison method that evaluates the ages. This enables sorting the ArrayList based on age.
2. Implementing the Comparable Interface
The Comparable interface is applied when the inherent order of the object is specified within its class. This implies you can define the compareTo method inside the class of the object, indicating how instances of that class should be arranged by default.
Illustration:
Java
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// alert(“Code copied to clipboard!”);
function closeOutput27214() {
var code = editor27214.getSession().getValue();
jQuery(".mainEditor27214 .code-editor-output").hide();
}
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document.getElementById("copyBtn27214").addEventListener("click", copyCodeToClipboard27214);
document.getElementById("runBtn27214").addEventListener("click", executeCode27214);
document.getElementById("closeOutputBtn27214").addEventListener("click", closeOutput27214);
Result:
Clarification: The preceding code employs the Person class that implements the Comparable interface to compare objects based on age. The compareTo method effectively organizes the Person objects by utilizing Collections.sort(people).
Differences Between the Comparator and Comparable Interface in Java
Below are the distinctions between Comparator and Comparable in Java:
Feature
Comparator
Comparable
Purpose
Used for custom arrangement of objects.
Specifies the inherent order of objects.
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Position of Sorting Logic
Outside the class, in a distinct class or method.
Specified within the class itself.
Method Employed
compare(T o1, T o2)
compareTo(T o)
Versatility
Highly versatile. Can create multiple comparators.
Restricted versatility. Only a single natural order is permissible.
Application
Can arrange by various attributes using different comparators.
Arrangements are based on one attribute (natural ordering).
Altering the Class
Does not necessitate modification of the custom class.
Requires alterations to the class to implement Comparable.
Multiple Sorting Parameters
Can specify numerous sorting parameters with distinct comparators.
Can only specify one natural sorting order.
Sorting Illustration
Collections.sort(list, new Comparator<>() {…})
Collections.sort(list) or list.sort() (if compareTo is implemented).
Frequent Usage
Sorting by various properties, sorting in reverse order, and custom sorting mechanisms.
Sorting by a default attribute (e.g., sorting by age or name).
Default Arrangement
No natural order is established. You must define a comparison explicitly.
Provides the natural ordering of objects within the class.
Conclusion
In Java, arranging an ArrayList of custom entities can be accomplished by employing both the Comparator and Comparable interfaces. The Comparator facilitates custom sorting defined externally to the object class, offering flexibility for various sorting criteria. Conversely, the Comparable interface establishes the natural sorting order within the object class itself. Both approaches assist in efficiently organizing and sorting data in Java.
To delve deeper into Java, you may refer to our Java Course.
Sort ArrayList of Custom Objects by Attribute – FAQs
Q1. How to arrange the list of custom items in Java?
You can also arrange a list of custom objects utilizing the Collections.sort() method.
Q2. How to arrange an ArrayList based on a parameter?
An ArrayList can be sorted by employing the sort() method of the Collections Class in Java. This sort() method takes the collection to be sorted as the argument and returns a Collection sorted in ascending order.
Q3. Which interface should be implemented for sorting?
The Comparable interface can be utilized to offer a singular method of sorting, while the Comparator interface is used to provide various sorting methods.
Q4. Can an ArrayList be a parameter?
Yes, you can pass an ArrayList of custom objects as an argument in Java.
Q5. What is the quickest way to sort a list?
QuickSort is typically regarded as the fastest sorting algorithm. Its performance is usually expressed in O(N × log N).
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