What is a variable associated with a class or with an instance of a class?

In class-based, object-oriented programming, a class variable is a variable defined in a class of which a single copy exists, regardless of how many instances of the class exist.[1][2][3][5]

A class variable is not an instance variable. It is a special type of class attribute (or class property, field, or data member). The same dichotomy between instance and class members applies to methods ("member functions") as well; a class may have both instance methods and class methods.

Static member variables and static member functions[edit]

In some languages, class variables and class methods are either statically resolved, not via dynamic dispatch, or their memory statically allocated at compile time (once for the entire class, as static variables), not dynamically allocated at run time (at every instantiation of an object). In other cases, however, either or both of these are dynamic. For example, if classes can be dynamically defined (at run time), class variables of these classes are allocated dynamically when the class is defined, and in some languages class methods are also dispatched dynamically.

Thus in some languages, static member variable or static member function are used synonymously with or in place of "class variable" or "class function", but these are not synonymous across languages. These terms are commonly used in Java, C# [5] , and C++, where class variables and class methods are declared with the static keyword, and referred to as static member variables or static member functions.

Example[edit]

struct Request {

    static int count;
    int number;

    Requestobject() {
        number = count; // modifies the instance variable "this->number"
        ++count; // modifies the class variable "Request::count"
    }

};

int Request::count = 0;

In this C++ example, the class variable Request::count is incremented on each call to the constructor, so that Request::count always holds the number of Requests that have been constructed, and each new Request object is given a number in sequential order. Since count is a class variable, there is only one object Request::count; in contrast, each Request object contains its own distinct number field.

In class-based, object-oriented programming, an instance variable is a variable defined in a class (i.e. a member variable), for which each instantiated object of the class has a separate copy, or instance. An instance variable has similarities with a class variable,[1] but is non-static. An instance variable is a variable which is declared in a class but outside of constructors, methods, or blocks. Instance variables are created when an object is instantiated, and are accessible to all the constructors, methods, or blocks in the class. Access modifiers can be given to the instance variable.

An instance variable is not a class variable although there are similarities. It is a type of class attribute (or class property, field, or data member). The same dichotomy between instance and class members applies to methods ("member functions") as well; a class may have both instance methods and class methods.

Each instance variable lives in memory for the lifetime of the object it is owned by.[2]

Variables are properties an object knows about itself. All instances of an object have their own copies of instance variables, even if the value is the same from one object to another. One object instance can change values of its instance variables without affecting all other instances. Instance variables can be used by all methods of a class unless the method is declared as static.[3]

Example[edit]

struct Request {

    static int count1; // variable name is not important
    int number;

    Request() {
        number = count1; // modifies the instance variable "this->number"
        ++count1; // modifies the class variable "Request::count1"
    }

};

int Request::count1 = 0;

In this C++ example, the instance variable Request::number is a copy of the class variable Request::count1 where each instance constructed is assigned a sequential value of count1 before it is incremented. Since number is an instance variable, each Request object contains its own distinct value; in contrast, there is only one object Request::count1 available to all instances with the same value.

Instance variables are specific to a particular instance of a class. For example, each time you create a new class object, it will have its copy of the instance variables.

Instance variables are the variables that are declared inside the class but outside any method.

There are a few features of instance variables that you should know about. We’ll go over those in this post. We’ll also take a closer look at the instance variables and why you might want to use them. We’ll also show you how to declare and initialise them and discuss some of the pros and cons of using them. And finally, we’ll compare instance and class variables and discuss when you might want to use each.

Instance Variables in Java:

You’ve probably heard the terms instance variable and class variable before, but what do they mean? Before diving deep into instance variables, let us see what the variables are.

There are three main variables in Java: 

1 Local variable

2 Instance variables

3 Class/Static variables.

In this blog, let us focus on the Instance variable and instance methods in java.

Instance variables are specific to a certain instance of a class. That might sound confusing, but it’s pretty simple.

Class variables are usually shared by all instances of the class in java. In other words, they belong to the class, not to any particular class instance.

Instance variables are declared inside the body of the class.

Syntax:

  ;
   = ;
public class Employee {
public String Name; // Name is an instance variable with public access modifier
private int salary ; // salary is an instance variable with private access modifier.
public static String company; // Company is not an instance variable as it is Static, and the value it holds is class specific but not instance.
}

Instance Initializer Block in Java:

An instance variable is a variable that is specific to a certain object. It is declared within the curly braces of the class but outside of any method. The value of an instance variable can be changed by any method in the class, but it is not accessible from outside the class.

Instance variables are usually initialised when the object is created. This is done with an instance initializer block, a special block of code run when an object is created.

Instance variables can have initializers, a block of code executed when an instance of the class is created. Class variables cannot have initializers.

As you can see, instance variables have a lot of features that class variables don’t have. You’ll most likely want to use instance variables instead of class variables.

Syntax

public class TempClass{
 // Initializer block 1
{
//statements
}
 // Initializer block 2
{
//statements
}
}

Features of Java Instance Initializer:

• Multiple initializers can be defined in a single class
• The initializers execute in the specified order in the class from top to bottom
• Constructors statements are executed after the instance initializers have been executed

Advantages and Disadvantages of Instance Variables in Java:

Instance variables are variables that are specific to a certain instance of a class. This means that each object in Java has its copy of the instance variables associated with it.

There are a few advantages to using instance variables in Java. Firstly, they provide privacy and security for the data contained within them. Secondly, they make it easy to track state changes within an object, as each instance variable will keep track of its changes.

However, there are also a few disadvantages to using instance variables in Java. Firstly, they can be more difficult to debug than class variables, as each object has its own set of instance variables. Secondly, they can lead to memory leaks if not properly handled.

Default Values of Instance Variables in Java:

Instance variables in Java have several properties you should be aware of. Let’s take a look at each one.

The first property is that instance variables are by default public. This means that any class in your application can access them. You can, however, make an instance variable private, which would restrict access to it only to the class in which it is declared.

The second property is that instance variables are initialized to their default values when they are declared. The default value for an int variable is 0, for a boolean variable, it is false, and for a String variable, it is null.

You can override the default value of an instance variable by initializing it in the constructor of the class in which it is declared. You can also make an instance variable final, preventing it from being changed after it is initialized.

The default value for the respective datatypes are as follows:

Difference Between Member Variables and Local Variables in Java:

Instance variables are specific to a certain instance of a class. This means that each class object will have its copy of the instance variables.

All objects of a class share member variables. This means that all objects will have the same values for these variables.

Local variables are variables that are specific to a certain block of code. This means that each time the code block is executed, a new set of local variables will be created.

public class Foo
{
 private int f1; //This is a  Member variable
 public void Bar()
   {
       int b1; // This is a local variable
       //Bar() can see b1 and f1
    }
public void Baz()
    {
       //Baz() can only see f1
    }
}

It’s important to understand the difference between member and instance variables, as it can have a significant impact on how your code is written. For example, if you want to change the value of a member variable, you only need to do so in one place. In contrast, if you want to change the value of an instance variable, you need to do so in every object that uses that variable.

Declare an Instance Variable:

Instance variables are specific to each instance of a class. This means that each object in a Java program has its copy of the instance variables defined for that class.

To declare an instance variable, you use the keyword, access specifier –  “private,” “public,” etc., then the variable’s data type, followed by the variable’s name. Let us look at one example: 

import java.io.*;
public class Student {
 // the instance variable name is visible for any child class.
   public String name;
 // the grade variable is visible in Student class only.
   private double grade;
 // The value for the name variable is assigned in the constructor 
   public Student (String stdName) {
      name = stdName;
   }
 // The variable grade is assigned a value now 
   public void setgrade(double stdgrade) {
      grade = stdgrade;
   }
//The following method will print the details of the Student
   public void printstd() {
      System.out.println("name  : " + name );
      System.out.println("grade :" + grade);
   }
   public static void main(String args[]) {
      Student stdOne = new Student("Shravan");
      stdOne.setgrade(10);
      stdOne.printstd();
   }
}

O/p:

name  : Shravan

grade :10.0

Difference between instance and static variables:

Instance variables are variables that are specific to a particular object. They are created when an object is instantiated and destroyed when the object is garbage collected.

Static variables are the variables that are shared by all objects of a class. They are created when the class is loaded and destroyed when it is unloaded.

Instance Variable Hiding in Java

Instance variables are hidden by default. This means that they are not accessible from other classes. To make an instance variable accessible from other classes, you must mark it as public.

Difference Between Class Variables and Instance Variables in Java

Instance variables are variables that are specific to a particular instance of a class. This means that each object in Java has its own set of instance variables, and the values of these variables can vary from one object to the next.

All objects of a given class share class variables. This means that the values of these variables will be the same for every object of that class.

The main difference between class and instance variables is that any class object can access class variables. In contrast, instance variables can only be accessed by the associated object.

Instance variables can be initialized when you create the object, or you can set their values later using a setter method. Hence, in brief, Instance variables are specific to a certain object. That means that each object has its copy of the variable, and the variable’s value can change depending on its associated object. Contrast this with class variables, which are shared by all objects of a given class.

Features of an instance variable

Instance variables have the following features:

• They are declared within the class but outside any method.
• They are preceded by the access specifier like private, public, etc.
• The value of an instance variable can be changed by calling a setter method.
• The value of an instance variable can be accessed by calling a getter method.
• It is not necessary to initialize an instance variable. It will take the default values as shown above for the respective data type.

Differences between static and instance methods in java

Instance variables are variables associated with a specific instance of a class. Unlike static variables, instance variables are not shared by all instances of a class. Each instance has its copy of the variable and can modify its value independently of other instances.

Instance methods are specific to a particular instance of a class. They are invoked by using the instanceof keyword, as in obj.instanceMethod(). Unlike static methods, instance methods can access the instance variables of the object on which they are invoked.

When should one use instance methods in java?

You should use instance methods when you need to access the state or behaviour of a particular instance. Static methods in java are more appropriate when you need to operate on the class as a whole or when you don’t need to access any instance variables.

Conclusion

When you create a class, you create a template for objects. An instance variable is a class property that can be different for each object. You create an instance variable by declaring it in the class definition, outside of any method.

Instance variables are important because they allow each object to have its own copy of the data. This makes your program more flexible and efficient.

An instance variable is automatically associated with each object of its class.

Instance variables are declared with the keyword “private” by default. However, it is possible to make an instance variable public or protected.

The value of an instance variable can be changed only within the method in which it is declared.

When the class is first loaded, instance variables are automatically initialized with their default values.

What is a variable which is an instance of a class?

What is class variables and instance variables?

What type of variables are shared by all instances of a class?

Which variables are class variables?