Friday, October 19, 2012

Difference between private, protected, public and package modifier or keyword in Java

private vs public vs protected vs package in Java
Java has four access modifier namely private, protected and public. package level access is default access level provided by Java  if no access modifier is specified. These access modifier is used to restrict accessibility of a class, method or variable on which it applies. We will start from private access modifier which is most restrictive access modifier and then go towards public which is least restrictive access modifier, along the way we will see some best practices while using access modifier in Java and some examples of using private and protected keywords.

private keyword in Java
private vs public vs package vs protected access in Java
private keyword or modifier in java can be applied to member field, method or nested class in Java. you can not use private modifier on top level class. private variables, methods and class are only accessible on the class on which they are declared. private is highest form of Encapsulation Java API provides and should be used as much as possible. Its best coding practice in Java to declare variable private by default. private method can only be called from the class where it has declared. As per Rules of method overriding in Java, private method can not be overridden as well. private keyword can also be applied to constructor and if you make constructor private you prevent it from being sub-classed. popular example of making constructor private  is Singleton class in Java which provides getInstance() method to get object instead of creating new object using constructor in Java. here are some differences between private and protected, public and package level access

package or default access level in Java
there is no access modifier called package instead package is a keyword which is used to declare package in Java, package is a directory on which a class in Java belongs. Package or default access level is second highest restrictive access modifier after private and any variable, method or class declared as package-private is only accessible on the package it belongs. good thing about default modifier is that, top level class can also be package-private if there is no class level access modifier.

protected keyword in Java
Difference between private and protected keyword is that protected method, variable or nested class not only accessible inside class, inside package but also outside of package on subclass. if you declare a variable protected means any one can use it if they extend your class. top level class can not be make protected as well.

public keyword in Java
public is the least restrictive access modifier in Java programming language and its bad practice to declare field, method or class by default public because once you make it public its very difficult to make any change on internal structure of class as it affect all clients using it. Making class or instance variable public also violated principle of Encapsulation which is not good at all and affects maintenance badly. instead of making variable public you should make it private and provided public getter and setter. public modifier can also be applied to top level class. In Java name of file must be same with public class declared in the file.

That's all difference between private, protected, package and public access modifier. As you have seen difference between private and public lies on how accessible a particular field, method or class would have. public means you can access it anywhere while private means you can only access it inside its own class.

Just to note all private, protected or public modifier are not applicable to local variables in Java. local variable can only be final in java.

Other Java fundamentals tutorials from Javarevisited

3 comments :

howtodoinjava.com said...

"Just to note all private, protected or public modifier are not applicable to local variables in Java."

Doesn't "local variable" term indicate that subject is local in scope. Access modifiers are only which can be shared. Right??

Javin @ JDBC Best practices said...

Indeed, but given its obvious that local variable are only accessible on block on which it declared, many programmer get confused if you ask can you use public, private or protected keyword on that.

Vikas Shukla said...

I am Confused very much with this question. In Some Sites it's mentioned Java Modifiers as Final, static, transient etc where as for access specifiers it mentioned as Public protected ,private ,default and vice versa ( java modifiers as public protected etc and java access specifiers as static, final etc..like it's mentioned in this site... Kindly elaborate which one is true..?

Post a Comment