Objects and Classes in Java

 

Objects in Java

Let us now look deep into what are objects. If we consider the real-world, we can find many objects around us, cars, dogs, humans, etc. All these objects have a state and a behavior. If we consider a dog, then its state is - name, breed, color, and the behavior is - barking, wagging the tail, running. If you compare the software object with a real-world object, they have very similar characteristics. Software objects also have a state and a behavior. A software object's state is stored in fields and behavior is shown via methods. So in software development, methods operate on the internal state of an object and the object-to-object communication is done via methods. 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:

• State: represents the data (value) of an object.
• Behavior: represents the behavior (functionality) of an object such as deposit, withdraw, etc.
• Identity: An object identity is typically implemented via a unique ID. The value of the ID is not visible to the external user. However, it is used internally by the JVM to identify each object uniquely.

Classes in Java

A class is a blueprint from which individual objects are created. 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:

• Fields
• Methods
• Constructors
• Blocks
• Nested class and interface

3 Ways to initialize object

There are 3 ways to initialize object in Java.

1. By reference variable
2. By method
3. By constructor

Initializing an object

The new operator instantiates a class by allocating memory for a new object and returning a reference to that memory. The new operator also invokes the class constructor.

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.

//Java Program to demonstrate having the main method in
//another class
//Creating Student class.
class Student{
int id;
String name;
}
//Creating another class TestStudent1 which contains the main method
class TestStudent1{
public static void main(String args[]){
Student s1=new Student();
System.out.println(s1.id);
System.out.println(s1.name);
}
}

Output:

 

 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.

Creating multiple objects by one type only (A good practice)

• In real-time, we need different objects of a class in different methods. Creating a number of references for storing them is not a good practice and therefore we declare a static reference variable and use it whenever required. In this case, wastage of memory is less. The objects that are not referenced anymore will be destroyed by Garbage Collector of java. Example:

          
Test test = new Test();
test = new Test();

• In inheritance system, we use parent class reference variable to store a sub-class object. In this case, we can switch into different subclass objects using same referenced variable. Example:

class Animal {} 
class Dog extends Animal {} 
class Cat extends Animal {} 
public class Test { 
// using Dog object Animal obj = new Dog(); 
// using Cat object obj = new Cat(); 
}

 

Anonymous objects

Anonymous objects are the objects that are instantiated but are not stored in a reference variable.

• They are used for immediate method calling.
• They will be destroyed after method calling.
• They are widely used in different libraries. For example, in AWT libraries, they are used to perform some action on capturing an event(eg a key press).
• In the example below, when a key is button(referred by the btn) is pressed, we are simply creating anonymous object of EventHandler class for just calling handle method.

btn.setOnAction(new EventHandler()
{
    public void handle(ActionEvent event)
    {
        System.out.println("Hello World!");
    }
});