Objects and Classes in Java
In this page, we will learn about Java objects and classes. In object-oriented programming technique, we design a program using objects and classes.
An object in Java is the physical as well as logical entity whereas a class in Java is a logical entity only.
What is an object in Java
An entity that has state and behavior is known as an object e.g. chair, bike, marker, pen, table, car etc. It can be physical or logical (tangible and intangible). The example of an intangible object is the banking system.
An object has three characteristics:
For Example, Pen is an object. Its name is Reynolds; color is white, known as its state. It is used to write, so writing is its behavior.
An object is an instance of a class. A class is a template or blueprint from which objects are created. So, an object is the instance(result) of a class.
What is a class in Java
A class is a group of objects which have common properties. It is a template or blueprint from which objects are created. It is a logical entity. It can't be physical.
A class in Java can contain:
Syntax to declare a class:
Instance variable in Java
A variable which is created inside the class but outside the method is known as an instance variable. Instance variable doesn't get memory at compile time. It gets memory at runtime when an object or instance is created. That is why it is known as an instance variable.
Method in Java
In Java, a method is like a function which is used to expose the behavior of an object.
Advantage of Method
new keyword in Java
The new keyword is used to allocate memory at runtime. All objects get memory in Heap memory area.
Object and Class Example: main within the class
In this example, we have created a Student class which has two data members id and name. We are creating the object of the Student class by new keyword and printing the object's value.
Here, we are creating a main() method inside the class.
File: Student.javaTest it Now
Object and Class Example: main outside the class
In real time development, we create classes and use it from another class. It is a better approach than previous one. Let's see a simple example, where we are having main() method in another class.
We can have multiple classes in different java files or single java file. If you define multiple classes in a single java source file, it is a good idea to save the file name with the class name which has main() method.
File: TestStudent1.javaTest it Now
3 Ways to initialize object
There are 3 ways to initialize object in java.
1) Object and Class Example: Initialization through reference
Initializing an object means storing data into the object. Let's see a simple example where we are going to initialize the object through a reference variable.
File: TestStudent2.javaTest it Now
We can also create multiple objects and store information in it through reference variable.
File: TestStudent3.javaTest it Now
101 Sonoo 102 Amit
2) Object and Class Example: Initialization through method
In this example, we are creating the two objects of Student class and initializing the value to these objects by invoking the insertRecord method. Here, we are displaying the state (data) of the objects by invoking the displayInformation() method.
File: TestStudent4.javaTest it Now
111 Karan 222 Aryan
As you can see in the above figure, object gets the memory in heap memory area. The reference variable refers to the object allocated in the heap memory area. Here, s1 and s2 both are reference variables that refer to the objects allocated in memory.
3) Object and Class Example: Initialization through a constructor
We will learn about constructors in java later.
Object and Class Example: Employee
Let's see an example where we are maintaining records of employees.
File: TestEmployee.javaTest it Now
101 ajeet 45000.0 102 irfan 25000.0 103 nakul 55000.0
Object and Class Example: Rectangle
There is given another example that maintains the records of Rectangle class.
File: TestRectangle1.javaTest it Now
What are the different ways to create an object in Java?
There are many ways to create an object in java. They are:
We will learn these ways to create object later.
Anonymous simply means nameless. An object which has no reference is known as an anonymous object. It can be used at the time of object creation only.
If you have to use an object only once, an anonymous object is a good approach. For example:
Calling method through a reference:
Calling method through an anonymous object
Let's see the full example of an anonymous object in Java.
Factorial is 120
Creating multiple objects by one type only
We can create multiple objects by one type only as we do in case of primitives.
Initialization of primitive variables:
Initialization of refernce variables:
Let's see the example:Test it Now
Real World Example: Account
File: TestAccount.javaTest it Now
832345 Ankit 1000.0 Balance is: 1000.0 40000.0 deposited Balance is: 41000.0 15000.0 withdrawn Balance is: 26000.0