Encapsulation

Encapsulation in Java or object oriented programming language is a concept which enforce protecting variables, functions from outside of class, in order to better manage that piece of code and having least impact or no impact on other parts of program duec to change in protected code. 

Encapsulation in Java is visible at different places and Java language itself provide many construct to encapsulate members.

 You can completely encapsulate a member be it a variable or method in Java by using private keyword and you can even achieve a lesser degree of encapsulation in Java by using other access modifier like protected or public. true value of encapsulation is realized in an environment which is prone to change a lot and we know that in software requirements changes every day at that time if you have your code well encapsulated you can better manage risk with change in requirement. Along with abstaction in java and polymorphism in Java, Encapsulation is a must know concept.

In this java tutorial  we will see How to use encapsulation in Java, advantage and disadvantage of Encapsulation and various design patterns and real life problems which makes use of Encapsulation object oriented concept.

What is Encapsulation in Java

Encapsulation is nothing but protecting anything which is prone to change. rational behind encapsulation is that if any functionality which is well encapsulated in code i.e maintained in just one place and not scattered around code is easy to change. this can be better explained with a simple example of encapsulation in Java.

we all know that constructor is used to create object in Java and constructor can accept argument. Suppose we have a class Loan has a constructor and than in various classes you have created instance of loan by using this constructor. now requirements change and you need to include age of borrower as well while taking loan. Since this code is not well encapsulated i.e. not confined in one place you need to change everywhere you are calling this constructor i.e. for one change you need to modify several file instead of just one file which is more error prone and tedious, though it can be done with refactoring feature of advanced IDE wouldn't it be better if you only need to make change at one place ? Yes that is possible if we encapsulate Loan creation logic in one method say createLoan() and client code call this method and this method internally crate Loan object. in this case you only need to modify this method instead of all client code.

Example of Encapsulation in Java

class Loan{
    private int duration;  //private variables examples of encapsulation
    private String loan;
    private String borrower;
    private String salary;
   

    //public constructor can break encapsulation instead use factory method
    private Loan(int duration, String loan, String borrower, String salary){
        this.duration = duration;
        this.loan = loan;
        this.borrower = borrower;
        this.salary = salary;
    }

   

    //no argument consustructor omitted here

  
   // create loan can encapsulate loan creation logic

    public Loan createLoan(String loanType){

     //processing based on loan type and than returning loan object

      return loan;

    }

   
}

In this same example of Encapsulation in Java you see all member variables are made private so they are well encapsulated you can only change or access this variable directly inside this class. if you want to allow outside world to access these variables is better creating a getter and setter e.g. getLoan() that allows you to do any kind of validation, security check before return loan so it gives you complete control of whatever you want to do and single channel of access for client which is controlled and managed.

Advantage of Encapsulation in Java and OOPS

Here are few advantages of using Encapsulation while writing code in Java or any Object oriented programming language:

1. Encapsulated Code is more flexible and easy to change with new requirements.

2. Encapsulation in Java makes unit testing easy.

3. Encapsulation in Java allows you to control who can access what.

4. Encapsulation also helps to write immutable class in Java which are a good choice in multi-threading

environment.

5. Encapsulation reduce coupling of modules and increase cohesion inside a module because all piece of one thing

are encapsulated in one place.

6. Encapsulation allows you to change one part of code without affecting other part of code.

What should you encapsulate in code

Anythign which can be change and more likely to change in near future is candidate of Encapsulation. This also helps to write more specific and cohesive code. Example of this is object creation code, code which can be improved in future like sorting and searching logic.

Design Pattern based on Encapsulation in Java

Many design pattern in Java uses encapsulation concept, one of them is Factory pattern which is used to create objects. Factory pattern is better choice than new operator for creating object of those classes whose creation logic can vary and also for creating different implementation of same interface. BorderFactory class of JDK is a good example of encapsulation in Java which creates different types of Border and encapsulate creation logic of Border. Singleton pattern in Java also encpasulate how you create instance by providing getInstance() method. since object

is created inside one class and not from any other place in code you can easily change how you create object without

affect other part of code.

Important points aboue encapsulation in Java.

1. "Whatever changes encapsulate it" is a famous design princple.

2. Encapsulation helps in loose coupling and high cohesion of code.

3. Encpasulation in Java is achieved using access modifier private, protected and public.

4. Factory pattern , Singleton pattern in Java makes good use of Encapsulation.

Access Specifiers

One of the techniques in object-oriented programming is encapsulation. It concerns the hiding of data in a class and making this class available only through methods. In this way the chance of making accidental mistakes in changing values is minimized. Java allows you to control access to classes, methods, and fields via so-called access specifiers.

Java offers four access specifiers, listed below in decreasing accessibility:

• public
• protected
• default (no specifier)
• private

We look at these access specifiers in more detail.

public

public classes, methods, and fields can be accessed from everywhere. The only constraint is that a file with Java source code can only contain one public class whose name must also match with the filename. If it exists, this public class represents the application or the applet, in which case the public keyword is necessary to enable your Web browser or appletviewer to show the applet. You use public classes, methods, or fields only if you explicitly want to offer access to these entities and if this access cannot do any harm. An example of a square determined by the position of its upper-left corner and its size:

public class Square {  // public class

  public x, y, size;   // public instance variables

}

protected

protected methods and fields can only be accessed within the same class to which the methods and fields belong, within its subclasses, and within classes of the same package, but not from anywhere else. You use theprotected access level when it is appropriate for a class's subclasses to have access to the method or field, but not for unrelated classes.

default (no specifier)

If you do not set access to specific level, then such a class, m

ethod, or field will be accessible from inside the same package to which the class, method, or field belongs, but not from outside this package. This access-level is convenient if you are creating packages. For example, ageometry package that contains Square and Tiling classes, may be easier and cleaner to implement if the coordinates of the upper-left corner of a Square are directly available to the Tiling class but not outside thegeometry package.

private

private methods and fields can only be accessed within the same class to which the methods and fields belong. private methods and fields are not visible within subclasses and are not inherited by subclasses. So, theprivate access specifier is opposite to the public access specifier. It is mostly used for encapsulation: data are hidden within the class and accessor methods are provided. An example, in which the position of the upper-left corner of a square can be set or obtained by accessor methods, but individual coordinates are not accessible to the user.

public class Square {   // public class

  private double x, y   // private (encapsulated) instance variables

  public setCorner(int x, int y) {  // setting values of private fields

    this.x = x;

    this.y = y;

  }

  public getCorner() {  // setting values of private fields

    return Point(x, y);

  }

}

Summary of Access Specifiers

The following table summarizes the access level permitted by each specifier.

Situation	public	protected	default	private
Accessible to class  from same package?	yes	yes	yes	no
Accessible to class  from different package?	yes	no, unless it is a subclass	no	no

Note the difference between the default access which is in fact more restricted than the protected access. Without access specifier (the default choice), methods and variables are accessible only within the class that defines them and within classes that are part of the same package. They are not visible to subclasses unless these are in the same package. protected methods and variables are visible to subclasses regardless of which package they are in.

Packages

Packages are used in Java in-order to prevent naming conflicts, to control access, to make searching/locating and usage of classes, interfaces, enumerations and annotations easier etc.

A Package can be defined as a grouping of related types(classes, interfaces, enumerations and annotations ) providing access protection and name space management.

Some of the existing packages in Java are::

·         java.lang - bundles the fundamental classes

·         java.io - classes for input , output functions are bundled in this package

Programmers can define their own packages to bundle group of classes/interfaces etc. It is a good practice to group related classes implemented by you so that a programmers can easily determine that the classes, interfaces, enumerations, annotations are related.

Since the package creates a new namespace there won't be any name conflicts with names in other packages. Using packages, it is easier to provide access control and it is also easier to locate the related classed.

Creating a package:

When creating a package, you should choose a name for the package and put a package statement with that name at the top of every source file that contains the classes, interfaces, enumerations, and annotation types that you want to include in the package.

The package statement should be the first line in the source file. There can be only one package statement in each source file, and it applies to all types in the file.

If a package statement is not used then the class, interfaces, enumerations, and annotation types will be put into an unnamed package.

Example:

Let us look at an example that creates a package called animals. It is common practice to use lowercased names of packages to avoid any conflicts with the names of classes, interfaces.

Put an interface in the package animals:

/* File name : Animal.java */

package animals;

interface Animal {

   public void eat();

   public void travel();

}

Now put an implementation in the same package animals:

package animals;

/* File name : MammalInt.java */

public class MammalInt implements Animal{

   public void eat(){

      System.out.println("Mammal eats");

   }

   public void travel(){

      System.out.println("Mammal travels");

   }

   public int noOfLegs(){

      return 0;

   }

   public static void main(String args[]){

      MammalInt m = new MammalInt();

      m.eat();

      m.travel();

   }

}

Now you compile these two files and put them in a sub-directory called animals and try to run as follows:

$ mkdir animals

$ cp Animal.class  MammalInt.class animals

$ java animals/MammalInt

Mammal eats

Mammal travels

The import Keyword:

If a class wants to use another class in the same package, the package name does not need to be used. Classes in the same package find each other without any special syntax.

Example:

Here a class named Boss is added to the payroll package that already contains Employee. The Boss can then refer to the Employee class without using the payroll prefix, as demonstrated by the following Boss class.

package payroll;

public class Boss

{

   public void payEmployee(Employee e)

   {

      e.mailCheck();

   }

}

What happens if Boss is not in the payroll package? The Boss class must then use one of the following techniques for referring to a class in a different package.

·         The fully qualified name of the class can be used. For example:

payroll.Employee

·         The package can be imported using the import keyword and the wild card (*). For example:

import payroll.*;

·         The class itself can be imported using the import keyword. For example:

import payroll.Employee;

Note: A class file can contain any number of import statements. The import statements must appear after the package statement and before the class declaration.

The Directory Structure of Packages:

Two major results occur when a class is placed in a package:

·         The name of the package becomes a part of the name of the class, as we just discussed in the previous section.

·         The name of the package must match the directory structure where the corresponding bytecode resides.

Here is simple way of managing your files in java:

Put the source code for a class, interface, enumeration, or annotation type in a text file whose name is the simple name of the type and whose extension is .java. For example:

// File Name :  Car.java

package vehicle;

public class Car {

   // Class implementation.  

}

Now put the source file in a directory whose name reflects the name of the package to which the class belongs:

....\vehicle\Car.java

Now the qualified class name and pathname would be as below:

·         Class name -> vehicle.Car

·         Path name -> vehicle\Car.java (in windows)

In general a company uses its reversed Internet domain name for its package names. Example: A company's Internet domain name is apple.com, then all its package names would start with com.apple. Each component of the package name corresponds to a subdirectory.

Example: The company had a com.apple.computers package that contained a Dell.java source file, it would be contained in a series of subdirectories like this:

....\com\apple\computers\Dell.java

At the time of compilation, the compiler creates a different output file for each class, interface and enumeration defined in it. The base name of the output file is the name of the type, and its extension is.class

For example:

// File Name: Dell.java

package com.apple.computers;

public class Dell{

     

}

class Ups{

     

}

Now compile this file as follows using -d option:

$javac -d . Dell.java

This would put compiled files as follows:

.\com\apple\computers\Dell.class

.\com\apple\computers\Ups.class

You can import all the classes or interfaces defined in \com\apple\computers\ as follows:

import com.apple.computers.*;

Like the .java source files, the compiled .class files should be in a series of directories that reflect the package name. However, the path to the .class files does not have to be the same as the path to the .java source files. You can arrange your source and class directories separately, as:

<path-one>\sources\com\apple\computers\Dell.java

<path-two>\classes\com\apple\computers\Dell.class

By doing this, it is possible to give the classes directory to other programmers without revealing your sources. You also need to manage source and class files in this manner so that the compiler and the Java Virtual Machine (JVM) can find all the types your program uses.

The full path to the classes directory, <path-two>\classes, is called the class path, and is set with the CLASSPATH system variable. Both the compiler and the JVM construct the path to your .class files by adding the package name to the class path.

Say <path-two>\classes is the class path, and the package name is com.apple.computers, then the compiler and JVM will look for .class files in <path-two>\classes\com\apple\comptuers.

A class path may include several paths. Multiple paths should be separated by a semicolon (Windows) or colon (Unix). By default, the compiler and the JVM search the current directory and the JAR file containing the Java platform classes so that these directories are automatically in the class path.

Set CLASSPATH System Variable:

To display the current CLASSPATH variable, use the following commands in Windows and Unix (Bourne shell):

·         In Windows -> C:\> set CLASSPATH

·         In Unix -> % echo $CLASSPATH

To delete the current contents of the CLASSPATH variable, use :

·         In Windows -> C:\> set CLASSPATH=

·         In Unix -> % unset CLASSPATH; export CLASSPATH

To set the CLASSPATH variable:

·         In Windows -> set CLASSPATH=C:\users\jack\java\classes

·         In Unix -> % CLASSPATH=/home/jack/java/classes; export CLASSPATH