Java array objects example

How to Create Array of Objects in Java

Java Array Of Objects, as defined by its name, stores an array of objects. Unlike a traditional array that store values like string, integer, Boolean, etc an array of objects stores OBJECTS. The array elements store the location of the reference variables of the object.

Class obj[]= new Class[array_length]

How to Create Array of Objects in Java?

Step 1) Open your code editor.
Copy the following code into an editor.

class ObjectArray < public static void main(String args[])< Account obj[] = new Account[2] ; //obj[0] = new Account(); //obj[1] = new Account(); obj[0].setData(1,2); obj[1].setData(3,4); System.out.println("For Array Element 0"); obj[0].showData(); System.out.println("For Array Element 1"); obj[1].showData(); >> class Account < int a; int b; public void setData(int c,int d)< a=c; b=d; >public void showData()< System.out.println("Value of a ="+a); System.out.println("Value of b /java-variables.html" data-lasso-id="471893">variables as shown below.
 
 
 
Step 5) Uncomment Line. 
Uncomment Line # 4 & 5. This step creates objects and assigns them to the reference variable array as shown below. Your code must run now.
 
 
 
Step 5) Uncomment Line. 
Uncomment Line # 4 & 5. This step creates objects and assigns them to the reference variable array as shown below. Your code must run now.
 
 
 
Output: 
 
For Array Element 0 Value of a =1 Value of b =2 For Array Element 1 Value of a =3 Value of b =4
 
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Array of Objects in Java With Example
 
In Java, like primitive values, we can also create an array of objects. Here we will discuss an array of objects in java with the simple example program.
 
There are three ways to create an array of objects in Java,
1) The Array of objects created with values.
2) The Array of objects created without explicit values or with default values.
3) Anonymous array
 
Array of objects creation in Java with explicit values
 
In this array object creation, the array contains 3 continuous memory locations with some starting base address, and that address is stored in “ex” referenced variable.
 
 
Differences in creating array objects with primitive types and referenced types
 
If we create an array object with a primitive type, then all its memory locations are of primitive type variables, so values are stored directly in those locations. If we create an array object with a referenced type, all its memory locations are of referenced type variables, so object reference is stored in those locations, not the values. Also see- One-dimensional array in Java 
 
In this array declaration in Java has a limitation i.e. we can use <> only with a variable declaration like Example[] arr = ; We can’t use this syntax as a variable assignment, It means array variable declared in one place and performing assignment in another place with <> .
 
Example[] arr; // array variable declared 
arr = ; // error
 
Example program-1:- Student Array Object Creation with Values
 
Program Description:- Develop a program to create an array object to store Student class objects with student_idNumber and student_name properties.
 
class Student < int idNum; String name; >class College< public static void main(String[] args) < // declaring, creating and // initializating array objects Student[] st = ; //displaying initialized values System.out.println("Initialized values of array:"); for(int i=0; i < st.length; i++)< System.out.print(st[i]+"\t"); >//initializing Student objects st[0].idNum = 9876; st[0].name = "Rocco"; st[1].idNum = 9865; st[1].name = "Jerry"; //displaying Student objects values after initialization System.out.println("\n\nStudent details:"); for (int i=0; i < st.length; i++) < System.out.println("Student" + (i+1) +" idNumber: "+st[i].idNum); System.out.println("Student"+(i+1)+" name: "+st[i].name); >> >
 
Student details:
Student1 idNumber: 9876
Student1 name: Rocco
Student2 idNumber: 9865
Student2 name: Jerry
 
 
In this array program in Java, First, we create a Student class with properties student id number and student name. In the College class, we used the student class properties, and In the college class, one Student array objects with 2 locations of Student type with the default value null. Later Student objects are initialized and finally displayed. To access idNumber of student1 we use st[0].idNum
 
If we want to create 5 locations of Student type but only initialized 2 of them at the time of declaration then we can create it as,
 
Objects of Array creation in Java without explicit values or with default values
 
class Example < int x = 10; int y = 20; >Example[] ex = new Example[4];
 
 
From the above statement array object is created with four Example type variables. All locations are initialized with default value null because the array object is created with referenced datatype Example.
 
Note that In this array object creation statement, Example class objects are not created rather only Example class type referenced variables are created to store Example class objects further.
 
Here Example class object is not created means that Example class non-static variables are not created. In the above syntax of array creation only we can specify size. It creates only variables, it will not allows us to store values or objects in the array creation syntax itself. If we want to store values or objects, we must perform the initialization operation in the next line separately as shown below:-
 
Example[] ex = new Example[4]; ex[0] = new Example(); ex[1] = new Example(); ex[2] = new Example(); ex[3] = new Example();
 
Q) How many String objects are created from the below statements?
 
Ans:- Zero. There is no String object created. It creates a String array object with 10 variables of string type with the default value “null”.
 
Example program-2 Student Array Object creation without explicit values or with the default value
 
class Student < int idNum; String name; >class Collage < static void displayDefault(Student[] s)< for(int i=0; i < s.length; i++)< System.out.print(s[i]+"\t"); >> static void displayStudent(Student[] s) < for (int i=0; i < s.length; i++) < System.out.println("Student" + (i+1) + " idNumber: "+s[i].idNum); System.out.println("Student"+(i+1)+ " name: "+s[i].name); >> public static void main(String[] args) < //Declaring, and creating array objects Student[] st = new Student[2]; //displaying default values of array System.out.println("Default values of array:"); displayDefault(st); //initializing Student array locations st[0] = new Student(); st[1] = new Student(); //displaying initialized values of array System.out.println("\n\nInitialized values of array:"); displayDefault(st); //displaying default values of Student objects System.out.println("\n\nDefault values of Student objects:"); displayStudent(st); //initializing Student objects st[0].idNum = 9876; st[0].name = "Rocco"; st[1].idNum = 9865; st[1].name = "Jerry"; //displaying Student objects values after initialization System.out.println("\nStudent details:"); displayStudent(st); >>
 
Default values of array:
null null
 
Default values of Student objects:
Student1 idNumber: 0
Student1 name: null
Student2 idNumber: 0
Student2 name: null
 
Student details:
Student1 idNumber: 9876
Student1 name: Rocco
Student2 idNumber: 9865
Student2 name: Jerry
 
 
In this array program in java, the Student array object with 2 locations of Student type with the default value null. Later they are initialized using new Student(); So, the Student object is also initialized with their default values. Finally, Student objects are updated and displayed.
 
Other Points on Array of objects
 
Source data type and destination data type must be compatible, else it leads to a compile-time error: incompatible types.
 
class A<> class B extends A <> class C extends B <> class D extends C <>
 
There is a class A, B is a subclass of A, C is a subclass of B, and D is sub class of C.
 
Q) If we declare an array object of a class is its class bytecodes are loaded into JVM?
 
Ans:- No, if we create an Example class array object, Example class bytecode are not loaded into JVM. If we are creating and adding an “Example object”, the Example class is loaded into JVM.
 
The below program does not load the Example class,
 
Below program loads Example class,
 
Program:- Find the output of the below program?
 
class Example < int a = 9; int b = 18; void m1()< a = 10; b = 20; >> class Test < static void m1(Example[] e)< e[1].m1(); >> class Array< public static void main(String[] args) < Example[] ex = ; Test.m1(ex); for(int i=0; i < ex.length; i++)< System.out.print(ex[i].a+"\t"); System.out.print(ex[i].b+"\n"); >> >
 
View Answer Answer:- Output:-
 
Program3:- Array of Employee Objects in Java
 
Program description:- Develop a program to create array objects to store employee objects with properties employee_number and name. You must initialize the employee object at the time of object creation itself and also should display all values of the employee object in a single column.
 
class Employee < private int eNo; private String eName; // constructor public Employee(int eNo, String eName)< this.eNo = eNo; this.eName = eName; >// method to display employee details public void display() < System.out.println("Employee Number: "+eNo); System.out.println("Employee Name: "+eName); >> class Company < public static void main(String[] args) < // declaring, and creating array objects Employee[] emp = new Employee[5]; // displaying default values of array System.out.println("Default values of array:"); for (int i=0; i < emp.length; i++) < System.out.print(emp[i] + " "); >// initializing Employee array locations // and Employee objects emp[0] = new Employee(1025,"Emma"); emp[1] = new Employee(9866,"Olivia"); // displaying Employee objects values // after initialization System.out.println("\nEmployee details:"); for(int i=0; i < emp.length; i++) < Employee e = emp[i]; System.out.println("\nArray location: "+e); //checking array location is null or not if(e != null) e.display(); >> >
 
Default values of array:
null null null null null
 
Array location: [email protected]Employee Number: 1025
Employee Name: Emma
 
Array location: [email protected]Employee Number: 9866
Employee Name: Olivia
 
In the above program, for displaying Employee objects we can use the below code also but the below code has a performance issue.
 
// displaying Employee objects values after initialization // Performance issue System.out.println("\nEmployee details:"); for(int i=0; i < emp.length; i++) < System.out.println("\nArray location: "+emp[i]); //checking array location is null or not if(emp[i]!=null) emp[i].display(); >
 
The above code has a performance issue because we are accessing the same object locations multiple times. These locations are stored in the JVM heap area. Instead of using this code, We should declare a local object ‘e’ of Employee type which will store the value of emp [i] . Now inside for loop, each time we will use the local object instead of accessing the object directly.
 
Program4
 
Program description:- Develop a Java Program to create an array object for storing Employee objects given in the table. In the above table, after some time employees of department Java (both Core & Advance Java) and HTML changed their department to Python, except employee having the name “Amelia”. The employees who changed their department got an increment of 500 in their salary. Display the original and modified details of Employee objects.
 
class Employee < // variables private int no; private String name; private double sal; private String dept; //Constructor public Employee(int no, String name, double sal, String dept)< this.no = no; this.name = name; this.sal = sal; this.dept = dept; >//Setter and getter methods public String getName() < return name; >public double getSal() < return sal; >public void setSal(double sal) < this.sal = sal; >public String getDept() < return dept; >public void setDept(String dept) < this.dept = dept; >// method to display employee details public void display() < System.out.printf("Employee details: "); System.out.printf("%d, %s, %.2f, %s\n",no, name, sal, dept); >>
 
class Test < public static void main(String[] args) < Employee[] emp = new Employee[9]; emp[0] = new Employee(1001, "Olivia", 5000, "Core Java"); emp[1] = new Employee(1002, "Amelia", 5500, ".Net"); emp[2] = new Employee(1003, "Ella", 4500, "Oracle"); emp[3] = new Employee(1004, "Amelia", 8000, "Core Java"); emp[4] = new Employee(1005, "Grace", 5000, "HTML"); emp[5] = new Employee(1006, "Olivia", 6500, "C++"); emp[6] = new Employee(1007, "Isla", 6000, "Advance Java"); emp[7] = new Employee(1008, "Ada", 7500, "Oracle"); emp[8] = new Employee(1009, "Myla", 7000, "C++"); //displaying Original values System.out.println("Original Employee details: "); for(Employee e : emp)< e.display(); >//Modifying values for(Employee e: emp) < if((e.getDept().toUpperCase().contains("JAVA") || e.getDept().toUpperCase().contains("HTML")) && (!e.getName().equalsIgnoreCase("Amelia")))< e.setDept("Python"); e.setSal(e.getSal()+500); >> //displaying Modified values System.out.println("\nModified Employee details: "); for(Employee e : emp) < e.display(); >> >
 
Original Employee details:
Employee details: 1001, Olivia, 5000.00, Core Java
Employee details: 1002, Amelia, 5500.00, .Net
Employee details: 1003, Ella, 4500.00, Oracle
Employee details: 1004, Amelia, 8000.00, Core Java
Employee details: 1005, Grace, 5000.00, HTML
Employee details: 1006, Olivia, 6500.00, C++
Employee details: 1007, Isla, 6000.00, Advance Java
Employee details: 1008, Ada, 7500.00, Oracle
Employee details: 1009, Myla, 7000.00, C++
 
Modified Employee details:
Employee details: 1001, Olivia, 5500.00, Python
Employee details: 1002, Amelia, 5500.00, .Net
Employee details: 1003, Ella, 4500.00, Oracle
Employee details: 1004, Amelia, 8000.00, Core Java
Employee details: 1005, Grace, 5500.00, Python
Employee details: 1006, Olivia, 6500.00, C++
Employee details: 1007, Isla, 6500.00, Python
Employee details: 1008, Ada, 7500.00, Oracle
Employee details: 1009, Myla, 7000.00, C++
 
Explanation
 
In this program, Every property of the Employee is declared with an access modifier “private”. Outside of the Employee class, these properties can’t be directly accessed. To initialize the Employee object we used the constructor. After that original Employee objects are displayed. Now, remaining problem is, that employees of department Java (both Core & Advance Java) and HTML changed their department to Python, Except employees having the name “Amelia”. The employees who changed their department got an increment of 500 in their salary.
 
We need to check every employee’s department and name. If the department of an employee is Java or HTML and his/her name is not “Amelia” then modify their department to python using the setter method setDept(-) , and also modify their salary using the setter method setSal(-) . To modify their salary using the setter method setSal() the existing salary should be incremented by 500. The getter method getSal() returns the existing or original salary, 500 added with it and passed to the setter method setSal() . Finally, the Employee department changed and their salary also incremented. At last Modified values are displayed using a for-each loop.
 
Array of objects declaration as final in Java
 
We can declare a class referenced variable as final but in this case also only the class reference variable will become final not its’s object variables.
 
class Test < int a; int b; >final Test ts = new Test();
 
Here, only class reference variable ts is final not the variables x and y. It is also possible to declare an object’s variable as final. For this, we must declare the variables as final in the class definition.
 
class Test < final int a; final int b; >final Test ts = new Test();
 
Now, all three variables are final. We can change the value of variables a and b after initialization. Similarly, we can’t assign the new class to the referenced variable ts.
 
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Array of Objects in Java
 
You might have already created array of integers, strings, floats, etc., which are primitive and mostly well known datatypes. In this tutorial, we will learn how to create an Array of user defined datatype or simply called, class objects.
 
You can create an array of objects just like how you create an array of integers. The syntax remains same.
 
className arrayName[] = new className[size];
 
 
className is the name of class, whose objects we store in the array.
 
arrayName is the name of the array using which we will access its elements.
 
new keyword that allocates memory based on the size of array.
 
 
Example – Java Array of User Defined Class Type
 
In this example, we will define a class named Car, and in our main method, we will create an array of objects of type Car.
 
Java Program
 
public class ArrayExample < public static void main(String[] args) < Car cars[] = new Car[4]; cars[0] = new Car("Toyota", 56600); cars[1] = new Car("Honda", 63500); cars[2] = new Car("Tata", 87400); cars[3] = new Car("Hyundai", 63000); for(Car car: cars) car.printDetails(); >> class Car < public String name; public int miles; public Car(String name, int miles) < this.name = name; this.miles = miles; >public void printDetails() < System.out.println(name+" - "+miles); >>
 
Toyota - 55000 Honda - 55000 Tata - 55000 Hyundai - 55000
 
Example – Java Array of Objects
 
In this example, we will define a class named Car, and in our main method, we will create an array of objects of type Car, by assigning a list of objects to the array variable.
 
Java Program
 
public class ArrayExample < public static void main(String[] args) < Car cars[] = < new Car("Toyota", 56600), new Car("Honda", 63500), new Car("Tata", 87400), new Car("Hyundai", 63000) >; for(Car car: cars) car.printDetails(); > > class Car < public String name; public int miles; public Car(String name, int miles) < this.name = name; this.miles = miles; >public void printDetails() < System.out.println(name+" - "+miles); >>
 
Toyota - 55000 Honda - 55000 Tata - 55000 Hyundai - 55000
 
Conclusion
 
In this Java Tutorial, we learned how to create a Java Array of Objects that are user defined data type.
 
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