The main difference between instance variable and local variable is that instance variable is a variable that is declared in a class but outside a method, while a local variable is a variable declared within a method or a constructor.
Object-Oriented Programming (OOP) is a major programming paradigm used in software development. It allows the programmer to model real-world scenarios using objects. Therefore, it is easier to build enterprise level applications using OOP. Thus, programming languages such as Java supports OOP paradigm. Moreover, a variable is a memory location to store data in these programs. There are various types of variable, and two of them are instance variable and local variable.
Key Areas Covered
1. What is Instance Variable
– Definition, Functionality
2. What is Local Variable
– Definition, Functionality
3. Difference Between Instance Variable and Local Variable
– Comparison of Key Differences
Key Terms
Class, Constructor, Object, Method, Instance Variable, Local Variable
What is Instance Variable
An instance variable is a variable declared in a class, but outside a method. These variables represent the object state throughout the class. Any object of that class has its own copy of that instance variable. Therefore, you cannot find a modification in one object’s instance variable in the instance variable of another object. These variables are visible to all constructors and methods of the class.
Moreover, it is possible to use access modifiers with an instance variable. For example, private instance variables are only accessible within the same class, whereas public instance variables are accessible by any class. Also, the new keyword helps to create objects, and the memory allocates for the instance variables. In other words, memory is allocated in a heap for objects, including its instance variables. Besides, the destruction of the object leads to the destruction of the instance variable.
Furthermore, instance variables can have default values. The default value for an object is null. The default value of a Boolean is false whereas for an integer it is 0. The programmer can assign values to the instance variables during declaration or within a constructor.
An example program with instance variables is as follows.
Figure 1: Java program with instance variables
Here, in the Student class, id and name are local variables. They are private variables, so they are accessible within the class. The constructor provides values to the instance variables. The display method has two statements to print the id and name. In the main method, there is an object of type Student. Then the programmer can call the display method using that object. Finally, the console displays the id and name. Student constructor and methods display and main can access id and name as they are local variables.
What is Local Variable
A local variable is a variable that is declared inside a method or a constructor. Local variables are created when entering the method or a constructor. Similarly, exiting the method or a constructor destroys these variables. Therefore, local variables are only visible within the declared method or the constructor.
Moreover, it is not possible to use access modifiers for local variables. Also, those variables do not have default values. Therefore, after declaring the local variables, the programmer has to assign values to them before using them in the program.
An example program with local variables is as follows.
Figure 2: Java program with a local variable
Here, the method calculateValue is inside the Test class. It has a local variable: value. And, it’s initial value is 0. Then, that value increases by 10. Also, in the main method, there is an object of type Test. The next step is calling the calculateValue method using this object. Finally, the console displays the new value.
Difference Between Instance Variable and Local Variable
Definition
An instance variable is a variable that is bound to the object itself while the local variable is a variable that is typically used in a method or a constructor. Hence, this is the main difference between instance variable and local variable.
Access Modifiers
Moreover, it is possible to use access modifiers for instance variables, whereas it is not possible to use access modifiers for the local variables.
Default variables
Also, while instance variables can have default values, local variables do not have default values.
Creation
Furthermore, instance variables are created when creating an object, whereas local variables are created when entering the method or a constructor. Thus, this is another difference between instance variable and local variable.
Destruction
Importantly, the destruction of the object leads to the destruction of the instance variable while exiting the method or a constructor leads to the destruction of local variables. Hence, this is also an important difference between instance variable and local variable.
Conclusion
In brief, programming languages such as Java support instance and local variables. An instance variable is a variable that is declared in a class but outside a method while the local variable is a variable declared within a method or a constructor. Thus, this is the main difference between instance variable and local variable.
References:
1.“Variable in Java | Core Java Tutorial.” Studytonight, Available here.
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