Table of Contents
Hello fellow developer! Do you work with multiple objects in your Java applications? Arrays are here to help.
An array of objects is your secret recipe for organizing many objects elegantly in code.
In this guide, I will uncover all the secrets behind arrays of objects so you can boost your Java application development skills.
Here‘s what you‘ll learn:
What Exactly is an Array of Objects?
Simply put, an array of objects holds references to multiple objects of a given class.
Let me break this down:
- Array – A data structure that stores elements of the same data type, like an object
- Object – A real-world entity with state (fields) and behavior (methods)
- Reference – Handle or address pointing to an object in memory
For example, a Student array called students will store handles pointing to many Student object instances.
We work with those objects through references as if working with the real objects directly!
This sets up the foundation before we dive deeper.
Why Use Arrays of Objects in Java?
I know what you are thinking – why bother with array of objects? Why not create objects normally?
Well, here are some great benefits arrays provide when working with objects:
1. Organized Group: Arrays allow storing a fixed group of similar objects together instead of separate references. This greatly reduces complexity in code with multiple objects.
2. Flexible Access: We can access any object randomly just by its index. This makes operations like search & sort easy.
3. Iteration: Objects have a defined order in array. We can now iterate the objects easily with either index or enhanced for loop.
4. Dynamic Code: Behavior code written for an object automatically works for all objects stored in array!
Clearly, arrays make managing a group of objects convenient. Developing apps that handle many objects becomes super smooth.
You‘ll see some real-world examples ahead.
With basics cleared, let me walk you through the exciting stuff now!
Creating or Declaring an Array of Objects
Creating an array of objects follows the same syntax rules as primitive arrays:
ClassName[] arrayRefVar; //preferred way
//or
ClassName arrayRefVar[]; Here ClassName depicts the Class whose objects need to be stored and arrayRefVar will reference the array object itself.
For example, Student object array:
Student[] students; This only declares students as a Student array reference in code. No actual array is created yet.
Instantiating an Array of Objects
The array declaration needs to be instantiated with memory allocated for it to store objects:
students = new Student[5];This creates a Student array that can hold references to 5 such objects.
However, do note all array elements contain null initially and not actual objects. We will see how to initialize next.
I hope you know the crucial difference between array declaration and instantiation now!
Initializing an Array of Objects
We have the array ready to hold references to Student objects. But first we need to initialize elements with actual Student instances before using it.
There are a couple of ways to do this:
1. Using Constructors
We can explicitly call Student constructor and initialize each index one by one:
students[0] = new Student("John");
students[1] = new Student("Sarah"); This fully initializes two array elements, rest remain null.
2. Using Loops
Calling constructors manually is fine for smaller arrays. But for large arrays, using loops is easier:
for(int i=0; i<students.length; i++) {
students[i] = new Student(); //default constructor
} This builds Student instances inside the entire array in one go!
We can pass constructor parameters as well for custom objects.
This was easy! Now onto fun stuff.
Accessing Array Elements
With valid Student references inside, we can access any object quickly using index:
Student firstStudent = students[0];This returns the first Student reference from array into firstStudent variable.
And then we can operate easily on public methods:
firstStudent.getName(); //access state
firstStudent.attendLecture(); //access behaviorThis access by index approach helps pick random objects situated at different positions in arrays.
Iterating Array of Objects with for-each Loop
Since arrays provide ordered storage, we can loop through objects in them with:
1. Normal for loop
Using index manually:
for(int i=0; i< students.length; i++) {
Student s = students[i];
//use s
}2. Enhanced for-each
Syntax automatically handles index:
for(Student s : students) {
//use s
}This simplified loop avoids managing index manually making code more readable.
Inside loop, s represents the individual Student reference traversed in each turn.
And any operation on s, be it state access or behavior call is applicable to every stored Student. Very easy!
This itself opens arrays of objects to many use cases.
Finding Size of Array
It is essential to know the maximum capacity allocated to array. The length property helps find that out:
System.out.println(students.length); //prints 5The array size helps put boundary checks on loops operating on arrays.
Sorting an Array of Objects
Sorting objects or custom types requires us to specify an order. We define this sorting logic in compareTo method of Comparable.
Let‘s sort students by their names alphabetically:
class Student implements Comparable<Student> {
private String name;
//..other fields & methods
public int compareTo(Student other) {
return this.name.compareTo(other.name);
}
}The interface forces implementing the comparison function. And Arrays.sort() applies it:
Arrays.sort(students); //sort students array by nameThat‘s it! The array gets sorted based on our custom logic.
Check this example below with diagrams for better understanding.
I have also explained custom sorting using Comparator when modification of original class via Comparable is not possible.
With basics covered, let me reveal some real-world examples demonstrating arrays of objects.
Real-World Applications of Array of Objects
We know arrays group objects safely together for easier access. But where do they shine in real software applications?
As a developer, you must know common scenarios where array of objects becomes inevitable.
Here are two standard use cases:
1. Online Shopping Cart
In an ecommerce app, the shopping cart holds purchased Item objects before checkout.
As user browses & adds items randomly, underlying cart is essentially an array holding Item objects in no order.
class ShoppingCart {
private Item[] items;
void addItem(Item item) {
items[index] = item; //add
}
double calculateTotal() {
double total = 0;
for(Item it : items) {
total += it.getPrice();
}
return total;
}
}This keeps adding different Item instances randomly based on user actions. Finally to print bill, calculate total loops through array uniformly accessing each object.
Such scenarios with unknown number of objects arising dynamically are perfectly fit for arrays treatment!
2. Task Scheduling
In an OS or app server handling tons of user requests coming randomly. Underlying executors use thread pools to schedule all tasks in a queue.
This Job Queue is implemented internally as an array of Task objects or Runnables:
class TaskScheduler {
private Runnable[] taskQueue;
void submitTask(Runnable task) {
taskQueue[index] = task; //add randomly
}
void executeAllPending() {
for(Runnable task : taskQueue) {
//start thread on each task
}
}
}Again tasks get added dynamically as users fire them randomly. Finally server loops queue uniformly to finish all.
This demonstrates the true power of arrays holding objects!
I‘m sure this clears any doubt on where object arrays fit in building real apps.
Now that you have understood arrays of objects conceptually, let‘s solidify the knowledge with some examples of common operations.
Performing Operations on Array of Objects
Let‘s explore some practical scenarios of working with object arrays in Java:
Passing Object Array to Method
We can pass array to method similar to primitives:
class StudentsUtil {
void print(Student[] students) {
for(Student s : students) {
System.out.print(s);
}
}
}
class Main {
public static void main(String[] args) {
Student[] students = new Student[5];
StudentsUtil util = new StudentsUtil();
util.print(students); //pass array
}
}This allows reusing logic across methods by passing group of objects together!
Returning Array from Method
Similarly, methods can return object arrays back to callers:
class StudentsUtil {
Student[] getHSStudents() {
Student[] hsStudents = {..};
return hsStudents;
}
}
Student[] highSchoolers = utils.getHSStudents(); Our method prepared and returned a meaningful array for reuse at calling place!
Copying Arrays
We can create copies of arrays using:
1. array.clone()
Creates a shallow copy of object array:
Student[] clone = original.clone();2. Arrays.copyOf()
Static method also provides cloning functionality:
Student[] clone = Arrays.copyOf(original); But in both cases, only references or handles get copied to new array. Objects still point to same instances.
3. Manual copy
To create brand new instances:
for(int i=0; i<original.length; i++) {
clone[i] = new Student(original[i].getName());
} This initializes each object separately so we get fresh objects!
Hope these examples give you a broader vision of arrays usage in everyday coding. Let‘s tackle some common questions next.
Frequently Asked Questions on Array of Objects
We know enough theory and basics about arrays of objects. But as a developer you might still have some queries around its usage in Java.
Let me try answering few common ones:
1. Can objects stored in array be of different types?
No. Array is a homogeneous data structure allowing only one kind of objects based on declaration.
For example, a Employee array can store only various Employee objects.
Employee[] employees = new Employee[5] //right
Employee[] records = {Employee, Student, Intern} //wrongThis ensures type safety inside arrays.
2. How is array of objects different from arraylist?
Array has fixed size defined at creation while ArrayList handles dynamic object additions. But arrays allow random index based access crucial for systems like cache.
So selection depends on use case. Both organize objects together though!
3. How do I print array elements without looping?
Use Arrays.toString() method to print array objects directly:
System.out.println(Arrays.toString(employees))
// [Employee1, Manager2, Employee3]It internally loops and handles object state printing!
I hope these queries clarify arrays usage further. Feel free to reach out for any doubts!
With this, we come to an end of this guide explaining arrays for organizing objects. Let‘s quickly summarize the key takeways.
Key Takeaways of Arrays of Objects in Java
We managed to cover a lot of ground on array of objects. Here are the key highlights:
- Array holds objects reference rather than objects themselves
- Provides ordered and random access of stored objects
- Enhanced for loop simplifies array traversal and operations
- Classes need to implement Comparable to leverage automatic sorting
- Passing arrays to methods allows reusing logic on group of objects
- Real-world apps handle number of dynamic objects elegantly via arrays
I hope you enjoyed this guided tour of arrays for objects. Please try the concepts out through some hands-on programs.
Happy coding!
Example Program Demonstrating Array of Objects
Let me finally conclude by showing an array of objects program in action covering all major operations:
import java.util.Arrays;
class Employee {
private int id;
private String name;
Employee(int id, String name){
this.id = id;
this.name = name;
}
//Getters, toString
}
public class Company {
public static void main(String[] args) {
// Declare array
Employee[] employees;
// Instantiate
employees = new Employee[3];
// Initialize using constructor
employees[0] = new Employee(1, "Sam");
employees[1] = new Employee(2, "John");
employees[2] = new Employee(3, "Raj");
// Array Size
System.out.println(employees.length); // 3
// Sort array - customComparator
Arrays.sort(employees, (a, b) -> a.getName().compareTo(b.getName()));
// Access element
Employee secondEmp = employees[1];
// Print array
System.out.println(Arrays.toString(employees));
// Iterate array
for(Employee e : employees) {
System.out.println(e);
}
}
}Feel free to tweak programs based on this template mixing various operations!
Summary – The Power of Arrays for Objects
We have successfully explored arrays for storing objects references safely in Java. I hope you have realized how significantly it eases applications dealing with many real-world objects arising dynamically.
Be it an online shopping cart or executing task scheduler, object arrays turn random unorganized objects into an ordered uniform group. This allows easy sequential access and reuse of behavior code.
Key takeaway – prefer object arrays whenever your program needs to handle multiple such entities in future together. Arrays offer great flexibility to add varying number of objects dynamically.
That was easy right? Please apply these learnings on some mini projects. Happy object array programming!