Java Objects

An object in Java is a basic unit of Object-Oriented Programming and represents real-life entities. Objects are the instances of a class that are created to use the attributes and methods of a class. A typical Java program creates many objects, which as you know, interact by invoking methods. An object consists of :

State: It is represented by attributes of an object. It also reflects the properties of an object.
Behavior: It is represented by the methods of an object. It also reflects the response of an object with other objects.
Identity: It gives a unique name to an object and enables one object to interact with other objects.

Example of an object: dog

Java Objects

Objects correspond to things found in the real world. For example, a graphics program may have objects such as “circle”, “square”, and “menu”. An online shopping system might have objects such as “shopping cart”, “customer”, and “product”.

Note: When we create an object which is a non primitive data type, it’s always allocated on the heap memory.

Declaring Objects (Also called instantiating a class)

When an object of a class is created, the class is said to be instantiated. All the instances share the attributes and the behavior of the class. But the values of those attributes, i.e. the state are unique for each object. A single class may have any number of instances.

Example:

Java Object Declaration

As we declare variables like (type name;). This notifies the compiler that we will use the name to refer to data whose type is type. With a primitive variable, this declaration also reserves the proper amount of memory for the variable. So for reference variables , the type must be strictly a concrete class name. In general, we can’t create objects of an abstract class or an interface.

Dog tuffy;

If we declare a reference variable(tuffy) like this, its value will be undetermined(null) until an object is actually created and assigned to it. Simply declaring a reference variable does not create an object.

Initializing a Java 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.

Example:

Java

// Class Declaration
 
public class Dog {
    // Instance Variables
    String name;
    String breed;
    int age;
    String color;
 
    // Constructor Declaration of Class
    public Dog(String name, String breed, int age,
               String color)
    {
        this.name = name;
        this.breed = breed;
        this.age = age;
        this.color = color;
    }
 
    // method 1
    public String getName() { return name; }
 
    // method 2
    public String getBreed() { return breed; }
 
    // method 3
    public int getAge() { return age; }
 
    // method 4
    public String getColor() { return color; }
 
    @Override public String toString()
    {
        return ("Hi my name is " + this.getName()
                + ".\nMy breed,age and color are "
                + this.getBreed() + "," + this.getAge()
                + "," + this.getColor());
    }
 
    public static void main(String[] args)
    {
        Dog tuffy
            = new Dog("tuffy", "papillon", 5, "white");
        System.out.println(tuffy.toString());
    }
}

Output

Hi my name is tuffy.
My breed,age and color are papillon,5,white

Initialize by using method/function:

Java

public class GFG {
    // sw=software
    static String sw_name;
    static float sw_price;
 
    static void set(String n, float p)
    {
        sw_name = n;
        sw_price = p;
    }
 
    static void get()
    {
        System.out.println("Software name is: " + sw_name);
        System.out.println("Software price is: "
                           + sw_price);
    }
 
    public static void main(String args[])
    {
        GFG.set("Visual studio", 0.0f);
        GFG.get();
    }
}

Output

Software name is: Visual studio
Software price is: 0.0

This class contains a single constructor. We can recognize a constructor because its declaration uses the same name as the class and it has no return type. The Java compiler differentiates the constructors based on the number and the type of the arguments. The constructor in the Dog class takes four arguments. The following statement provides “tuffy”, “papillon”,5, and “white” as values for those arguments:

Dog tuffy = new Dog("tuffy","papillon",5, "white");

The result of executing this statement can be illustrated as :

Memory Allocation of Java Objects

Note: All classes have at least one constructor. If a class does not explicitly declare any, the Java compiler automatically provides a no-argument constructor, also called the default constructor. This default constructor calls the class parent’s no-argument constructor (as it contains only one statement i.e super();), or the Object class constructor if the class has no other parent (as the Object class is the parent of all classes either directly or indirectly).

Classes and Objects in Java

In Java, classes and objects are basic concepts of Object Oriented Programming (OOPs) that are used to represent real-world concepts and entities. The class represents a group of objects having similar properties and behavior. For example, the animal type Dog is a class while a particular dog named Tommy is an object of the Dog class.

In this article, we will discuss Java objects and classes and how to implement them in our program.

Java Objects

Declaring Objects (Also called instantiating a class)

Initializing a Java object

Java

Java

Classes and Objects in Java

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Java Objects

Declaring Objects (Also called instantiating a class)

Initializing a Java object

Java

Java

Classes and Objects in Java

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