Just like with primitive data types, arrays, in Java, can be created with objects. Infact, objects are reference types. In the following program, Student objects arrays are created in different styles and their values are printed.
Example on Objects Array
public class Student
{
int marks;
public static void main(String args[])
{
Student std1[] = new Student[3]; // creating array of Student reference variables
// std1[0].marks = 40; // raises NullPointerException
std1[0] = new Student(); // converting each student reference varaibles into Student object
std1[1] = new Student();
std1[2] = new Student();
std1[0].marks = 40; // assigning values
std1[1].marks = 50;
std1[2].marks = 60;
System.out.println(std1[0].marks); // prints 40
System.out.println(std1[1].marks); // prints 50
System.out.println(std1[2].marks); // prints 60
// this can be done like this also in a loop
Student std2[] = new Student[3]; // creating array of Student reference variables
int x = 70;
for(int i = 0; i < std2.length; i++, x+=10) // converting reference variables into objects in a loop
{
std2[i] = new Student();
std2[i].marks = x; // assign some marks in loop itself
}
for(int i = 0; i < std2.length; i++) // print the value in a loop
{
System.out.println(std2[i].marks); // prints 70, 80, 90
}
// this can be done like this also
Student std3 = new Student();
std3.marks = 45;
Student std4 = new Student();
std4.marks = 55;
Student std5 = new Student();
std5.marks = 65;
Student std6[] = {std3, std4, std5};
for(int i = 0; i < std6.length; i++) // print the value in a loop
{
System.out.println(std6[i].marks); // prints 45, 55, 65
}
// this can be done like this also; storing anonymous objects in the array
Student std7[] = {new Student(), new Student(), new Student()};
std7[0].marks = 66;
std7[1].marks = 76;
std7[2].marks = 86;
for(int i = 0; i < std7.length; i++) // print the value in a loop
{
System.out.println(std7[i].marks); // prints 66, 76, 86
}
}
}
Student std1[] = new Student[3]; // creating array of Student reference variables
// std1[0].marks = 40; // raises NullPointerException
In the above statement, many think that std1[] is an array with 3 Student objects. No, not at all. std1[] is an array with 3 Student reference variables. That is, you have created an array of 3 reference variables. std1[0].marks = 40; gives NullPointerException as std1[0] is a reference variable and not object. std1[0] should be converted into an object and then used. It is done in the next statement.
std1[0] = new Student(); // converting each student reference
In the above statement, std1[0] is converted into object just like any other by calling Student constructor with new keyword. Like this, each array element should be converted into Student object.
Student std2[] = new Student[3];
Just for a change, again std2[] array of 3 reference variables is created. Each reference variable is converted into object in a for loop and in the same for loop values are also assigned.
Student std6[] = {std3, std4, std5};
In the above statement, std6[] array contains real objects.
Student std7[] = {new Student(), new Student(), new Student()};
std7[0].marks = 66;
std7[1].marks = 76;
std7[2].marks = 86;
Now std7[] is a Student array with 3 anonymous objects Student. std7[0] contains first Student anonymous objects. Marks are assigned.
Shall we go a little deeper into Objects Array. Observe the following code. Before going into the notes, think yourself how the program works.
class Student
{
int marks = 50;
}
class Employee
{
double salary = 5555.55;
}
class Officer
{
String rank = "Gazetted Officer";
}
public class ArrayOfClasses
{
public static void main(String args[])
{
Object obj1[] = {new Student(), new Employee(), new Officer()};
Student std1 = (Student) obj1[0];
System.out.println(std1.marks); // prints 50
Employee emp1 = (Employee) obj1[1];
System.out.println(emp1.salary); // prints 5555.55
Officer off1 = (Officer) obj1[2];
System.out.println(off1.rank); // prints Gazetted Officer
}
}
Object obj1[] = {new Student(), new Employee(), new Officer()};
We know an array stores similar types. But the above array obj1 stores dissimilar objects of Student, Employee and Officer. How it is possible?
Object is the root class of all classes of Java. The rule says, a subclass object can be assigned to a super class object implicitly. By this rule, Student object, Employee object and Officer object are implicitly converted to Object class.
Student std1 = (Student) obj1[0];
System.out.println(std1.marks);
obj1[0], the first element of Object array obj1 represents internally a Student object. To get back, the Student object from the Object, explicit cast it to Student. That is what is done in the above statement. As proof of conversion, we calling Student class marks with Student object std1. Similarly with Employee and Officer classes.