How to Have One Class Read Another Classes Variable Java
We discussed the concept of classes and objects in the previous chapter. Before learning to create classes and objects using Java, let'due south see one more example.
We know that a student has a proper noun and a roll number. So, we can say that Educatee is a class, and name and curl number are the attributes of this class. Furthermore, we know that in that location can be many students and each student will have a proper noun and a roll number. Thus, all students are the objects of the Student class.
And then, you must have understood the basic concept of classes and objects. Now, permit'southward see how to create a grade and its objects.
Creating Classes and Objects in Java
A form is created using the class keyword.
class Student { String name ; int roll_no ; }
In the above declaration, Educatee is the name of a course. In other words, Student is a class.
In the trunk of the class, the attributes name and roll_no are divers.
So, we can say that each student has a proper name and a roll number.
Attributes are variables defined inside a class.
At present let's create an object of this class.
// Student class class Student { String name ; int roll_no ; } // Test (Chief) class class Test { public static void primary ( Cord [] args ) { // creating object st of class Student Student st = new Student (); } }
In the above case, Test is the main class containing the main method.
The course which has the main method within it is known every bit the primary class. Till now we were writing all our code in the main grade.
We created the Student class having two attributes - name and roll_no.
Student st = new Student() → We created an object st of the Student class. Nosotros tin can also say that st is a Student.
The terms object and instance are used interchangeably.
Any number of objects of a class can be created.
// Student grade course Student { String name ; int roll_no ; } // Test (Main) class grade Test { public static void chief ( String [] args ) { // creating objects st1 and st2 of form Student Student st1 = new Pupil (); Educatee st2 = new Student (); } }
Hither, nosotros created ii objects of the Pupil form - st1 and st2. Thus, st1 and st2 are two Students having some name and whorl number.
You lot now know how to create a class with attributes and its objects. Now let's movement forward and assign values to attributes.
Java Attributes
Nosotros know that attributes are variables defined in a grade. They specify the features (properties) of the class. For example, name and roll_no are the attributes/features of the Student form.
// Student class grade Student { String name = "John" ; } // Test (Main) class form Exam { public static void chief ( Cord [] args ) { // creating object st of class Student Student st = new Student (); Organization . out . println ( st . name ); } }
In this example, a class named Student is defined. It has an attribute name having the value "John".
Student st = new Student() → An object st of the Student class is created.
System.out.println(st.name) → The object st accesses the attribute name of the class through st.name and its value is printed.
Note that an object of a course tin can access the attributes of that class using the dot '.' operator. In the higher up case, the object st accesses the attribute name of the Student class through st.proper noun.
In the above example, nosotros can say that st is a Pupil whose name is "John".
Let's see another way to assign value to attributes.
// Student grade class Pupil { String name ; } // Test (Main) class course Test { public static void main ( String [] args ) { // creating object st of grade Student Student st = new Student (); // assigning value to aspect name st . name = "John" ; Organization . out . println ( st . name ); } }
Hither, the statement st.name = "John" assigned the value of the aspect name for the object st to "John".
Look at another instance.
// Pupil class class Educatee { Cord name = "John" ; } // Exam (Primary) grade class Test { public static void master ( Cord [] args ) { // creating objects st1 and st2 of form Student Student st1 = new Student (); Student st2 = new Student (); System . out . println ( "Student 1 Name: " + st1 . name ); Organization . out . println ( "Student 2 Name: " + st2 . name ); } }
2 objects st1 and st2 of the Student grade are created. These objects accessed the value of the attribute name through st1.name and st2.name respectively.
Thus, st1 and st2 are two Students having the same proper name i.e. "John".
Now in the above example, we can ascertain different names for both the objects as shown below.
// Student class class Educatee { String name ; } // Test (Main) class class Test { public static void main ( Cord [] args ) { // creating objects of grade Educatee Student st1 = new Student (); Student st2 = new Educatee (); // assigning values to attribute name for objects st1 . name = "John" ; st2 . proper noun = "Julie" ; Organisation . out . println ( "Pupil 1 Name: " + st1 . name ); Organization . out . println ( "Student ii Proper noun: " + st2 . name ); } }
The statement st1.proper noun = "John" assigned the value of the aspect name for the object st1 as "John" and the statement st2.name = "John" assigned the value of the attribute name for the object st2 as "Julie".
Thus, st1 is a Student having the name "John" and st2 is also a Student having the proper noun "Julie".
If you lot have understood till here, the rest of the topic will be an piece of cake walk for you. Let's move further.
Coffee Methods in Class
In the to a higher place examples, we divers variables (known equally attributes) in a class. Similarly, we tin can also define methods in a form. The variables and methods defined in a grade are chosen members of the course.
Let's define a method in our Student class.
// Student course course Student { String name = "John" ; void clarification () { System . out . println ( "This is a student" ); } } // Test (Master) form class Test { public static void main ( String [] args ) { // creating object st of class Educatee Educatee st = new Student (); // accessing variable proper name of class Organization . out . println ( st . proper name ); // accessing method description of class st . description (); } }
In this example, the Student grade has a variable (attribute) proper noun and a method description() divers in information technology. This variable and method are the members of the class.
Nosotros created an object st of this form. Thus, the object accessed the class aspect by writing st.proper noun and the class method by writing st.description().
At present let'southward look at another example.
class Student { String proper name ; void setName ( String northward ) { name = n ; } String getName () { return name ; } } class Test { public static void main ( String [] args ) { Student st = new Student (); st . setName ( "John" ); String st_name = st . getName (); System . out . println ( "Student Name: " + st_name ); } }
We created the Pupil class having an attribute name and methods getName() and setName().
Student st = new Student() → An object st of the course is created.
st.setName("John") → The object st calls the method setName() past passing "John" to it. Inside the setName() method, name = n assigns "John" to the variable name.
Cord st_name = st.getName() → The object st calls the method getName(). Inside the getName() method, render proper name returns the value of the variable proper name i.east. "John". Thus, the value of the variable st_name becomes "John".
By now, you know how to create classes and its objects. You also know that variables and methods tin can be defined in a class and can exist accessed by the objects of that class. Pretty uncomplicated uptil here, right?
At present allow's movement onto the side by side concept.
Java Constructor
A constructor is a special form method which gets called automatically whenever an object of the class is created. Its proper name is the same as the class name and it has no render type.
Constructor is a special type of method which is used to initialize an object. Information technology is invoked at the fourth dimension of object creation.
Since the constructor is called at the time of object creation, the role of the code that yous want to execute at the time of object creation should be written in information technology.
An example will help understand the use of a constructor.
class Pupil { String name ; // constructor Pupil () { name = "Unknown" ; } void setName ( String n ) { name = n ; } Cord getName () { return proper name ; } } form Test { public static void chief ( String [] args ) { // creating object st of Student grade Pupil st = new Student (); // press value of name of st System . out . println ( "Student Name: " + st . getName ()); // changing value of name of st st . setName ( "John" ); // press value of name of st Organization . out . println ( "Pupil Name: " + st . getName ()); } }
In this instance, the Educatee class is defined with an attribute proper noun, a constructor Student() and two methods getName() (returns the value of proper noun) and setName() (assigns a value to proper noun).
When we created the object st of the form Student, the constructor Student() automatically got chosen. Within the constructor, the value of the attribute name is initialized to "Unknown". Therefore, on printing the value of st.getName() after that, "Unknown" got printed.
Next, st.setName("John") chosen the setName() method to set the value of name to "John". Thus, on printing the value of st.getName() after that, "John" got printed.
Therefore, in the higher up example, nosotros are assigning the name of the objects as "Unknown" by default using the constructor.
Let'southward encounter another example.
form Student { String name ; int historic period ; // constructor Pupil () { name = "Unknown" ; age = 0 ; } void setDetails ( String n , int a ) { name = north ; historic period = a ; } void printDetails () { System . out . println ( "My name is " + name + " and my age is " + age ); } } grade Examination { public static void main ( String [] args ) { Student st1 = new Student (); Educatee st2 = new Student (); st1 . setDetails ( "John" , 25 ); st2 . setDetails ( "Julie" , 20 ); st1 . printDetails (); st2 . printDetails (); } }
Yous must have understood this program. When the objects st1 and st2 were created, the constructor Student() got called and assigned name every bit "Unknown" and historic period equally 0 for both the objects. Later on that, both the objects called the setDetails() method to gear up the corresponding values of name and historic period and the printDetails() method to impress their values.
Nosotros can also make a constructor with nothing in its body.
Student(){ };
If we don't explicitly create whatsoever constructor in a class, and so the compiler automatically creates a constructor with no parameter and empty torso.
Java Constructor Having Parameters
We can besides define constructors having parameters.
class Student { String name ; int historic period ; // constructor Educatee ( String north , int a ) { proper noun = n ; age = a ; } void printDetails () { Arrangement . out . println ( "My name is " + name + " and my age is " + age ); } } class Test { public static void master ( String [] args ) { Student st1 = new Student ( "John" , 25 ); Pupil st2 = new Educatee ( "Julie" , xx ); st1 . printDetails (); st2 . printDetails (); } }
Here, the constructor Educatee() has two parameters, the commencement parameter of type String and the second parameter of type int. Therefore, when creating an object of the Student class, nosotros have to laissez passer two arguments.
Expect at the following argument.
Pupil st1 = new Student("John", 25);
This statement creates an object st1 and calls the Student() constructor past assigning the value "John" to n and 25 to a. In the constructor, n ("John") is assigned to proper noun and a (25) is assigned to age. The same is done for the object st2 likewise.
Using public and private Modifiers
We will discuss modifiers in the chapter Access Modifiers. Let's just learn most a few things which are necessary for now. Members (variables and methods) of a class are usually alleged as private or public.
Declaring a course variable/method as private means that the variable/method can be accessed only inside the class. Accessing a private variable/method outside the class will issue in an error. Whereas, declaring a grade variable/method every bit public means that the variable/method can be accessed exterior the class in which it is defined.
Therefore, we can't admission a individual variable/method through the grade object outside the class.
Usually, class variables (attributes) are declared private and class methods are alleged public. (We will acquire about the reason for doing so in the Encapsulation chapter)
Allow's make all class variables as individual and class methods equally public in the final example.
class Student { private String name ; private int age ; // constructor public Student ( String n , int a ) { name = n ; age = a ; } public void printDetails () { System . out . println ( "My name is " + proper name + " and my age is " + age ); } } class Test { public static void principal ( String [] args ) { Pupil st1 = new Student ( "John" , 25 ); st1 . printDetails (); } }
In this example, the object st1 was able to access the method printDetails() considering this method is alleged as public.
Now allow's try to access a private attribute outside the class in the above example.
course Student { private String name ; private int age ; // constructor public Student ( String n , int a ) { name = north ; age = a ; } public void printDetails () { Organisation . out . println ( "My proper noun is " + name + " and my age is " + age ); } } grade Test { public static void chief ( String [] args ) { Student st1 = new Student ( "John" , 25 ); Organisation . out . println ( st1 . proper name ); } }
We got an error. This is because the object st1 tried to access the individual attribute name outside the Educatee class.
From now on, let's make a habit of declaring class attributes as private and class methods every bit public in normal weather. In the rest of the examples, we will follow this design.
Let's see another example.
grade Rectangle { private int length ; private int breadth ; public Rectangle ( int l , int b ) { length = 50 ; breadth = b ; } public int getArea () { return length * breadth ; } } form Test { public static void main ( Cord [] args ) { Rectangle rect = new Rectangle ( 2 , four ); System . out . println ( rect . getArea ()); } }
This plan has a class named Rectangle which has length and breadth equally attributes and a method getArea() that returns the surface area of the rectangle. Try to empathise the rest of the code yourself.
Coffee static
Declaring a variable/method divers in a course as static ways that the variable/method can be accessed without making an object of the class in which information technology is defined. In other words, static is used and then that we can access any variable or method in a class without making an object of that grade.
Let'due south understand with an instance.
class Rectangle { public static void printArea ( int l , int b ) { System . out . println ( l * b ); } } course Exam { public static void main ( String [] args ) { Rectangle . printArea ( 2 , iv ); } }
Hither, the method printArea() is declared every bit static. As a outcome, we were able to access it by directly using the class name Rectangle, without creating an object of the course.
Now permit'due south get to the last topic of this chapter.
Returning and passing object in a method
Yes, we tin return or laissez passer object(s) to a method.
Let's see how.
form Business relationship { public int balance ; public Account () { balance = 0 ; } public static Account getAcc ( Business relationship a , Account b ) { Business relationship air conditioning = new Account (); ac . residuum = a . balance + b . rest ; return ac ; } } class Examination { public static void principal ( String [] args ) { Account a1 = new Business relationship (); a1 . rest = 50 ; Business relationship a2 = new Business relationship (); a2 . balance = sixty ; Account a3 = Account . getAcc ( a1 , a2 ); Organisation . out . println ( a3 . balance ); } }
In this example, the getAcc() method is taking two Business relationship objects as parameters and returning an Business relationship object. Note that the attribute balance is defined as public considering nosotros want the objects to straight admission this aspect in this example.
Account a3 = Account.getAcc(a1, a2);
In the above statement, the getAcc() method is taking the objects a1 and a2 and is returning a new object of the Account course. The returned object is assigned to a3.
public static Account getAcc ( Account a , Account b ) { Account ac = new Account (); ac . balance = a . residue + b . rest ; render ac ; }
Inside the getAcc method, a new object ac is created. And so the sum of balances of the two parameter objects is assigned to the residue of ac, and finally ac is returned.
Then this completes the topic - classes and objects. If you have understood this chapter, then you take surpassed many beginners who find information technology difficult.
This might be a heavy chapter, then go through it thoroughly and ask doubts in the discussion forum. Besides practice questions on classes and objects from the practice section earlier moving onto the adjacent affiliate.
If You Are Working On Something That You Really Intendance About, You lot Don't Have To Be Pushed. The Vision Pulls Y'all.
- Steve Jobs
Source: https://www.codesdope.com/course/java-classes-and-objects/
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