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Tutorial on Java classes and objects

 Java Classes and Objects:

Classes and objects both are treated in similar way in all programming languages. Java is not exception.

Understanding classes and objects with real world example

Consider a physical class room with lot of real world objects. What we Observe in class room..?

• Chairs 
• Table
•  Students
•  Teacher
•  Black board
• Markers and Erasers etc

So, a programmer needs to instruct the computer, how to create a class room object and it's subsequent objects. like, chair, table, teacher and student etc.

If programmer wants to create or develop a School application, he would need to create:

• class room object
• Office room object,
• Student object,
• Stationary object
• Play room object etc

Let us now create School application with above mentioned objects and classes using java programming language.

To create Objects, first of all, we have to create respective classes in java

public class School_Application{

     public Student students;

     public Teacher teachers;

     public ClassRoom classRoom;

      public School_Application(Student students, Teacher teachers, ClassRoom classRoom){

          this.students = students; 

          this.teachers = teachers; 

          this.classRoom = classRoom;

   }

}

Subsequently, programmer needs to create, Student, Teacher and ClassRoom classes and respective objects in main() method.

Objects in java are created using new operator. Example:

School_Application app = new School_Application(student, teachers, classRoom);

Happy coding! :)

Tutorial on coding and best practices

 Write code or write code in right way...

Writing a coding solution to a problem can be done in many ways. If the solution can be obtained with less number of code lines, then it is easy to understand and maintain.

But what if code is large with having millions of lines. Moreover maintenance and readability becomes difficult as the code base grows... Right? 

In this blog I want to share my views and experiences on writing code efficiently and effectively with less maintenance efforts.

Aspects to consider while coding:

• Naming convention
• Cyclomatic Complexity
• SOLID principles
• DRY Principle
• Managing dead code
• Comments

Naming conventions:

Every programming language mandates few norms in order to name variables, classes and functions etc. For ex: Java uses class names first letter to be a upper case and rest to be lower case letters. 

But if we dig little bit more, naming conventions are not limited to only upper case or lower case letters and using alphanumeric letters etc.

Naming a variable must be enough concise as naming your first child's name...

Few bad names for variables and constants:

          int final static TWO = 2;  [wrong]

          int final static MAX_LENGHT = 2; [Correct] 

  

Cyclomatic Complexity:

Cyclomatic complexity is a measure or metric to estimate complexity level of a snippet or block of code. 

Cyclomatic complexity and best practices:

• Never put more than three conditional checks (&& operator and || operator etc.) in one if condition
• Never exceed more than thirty lines in one function or method.
• Never exceed more than 1000 lines in one class or one file.

SOLID principles:

Usually developing a large and scalable application, requires better design. Adhering to SOLID principles yields better scalable and easily maintainable applications.

There are five SOLID principles. They are:

1. Single Responsibility Principle
2. Open/Closed Principle
3. Liskov's Substitution Principle
4. Interface Segregation Principle
5. Dependency Inversion Principle

DRY Principle:  

DRY stands for Do Not Repeat Yourself. Sometimes, There could be a need to use same or similar set of lines of code to be used multiple times in the same application. For ex. String utilities, DB connections or updating records in database etc.

Writing frequently used code snippets multiple times is not a good practice. Instead move this code to a function or method and use this method in places where ever required.

Simple use cases are:

• DB Connections
• File IO Operations
• String Utilities etc. 

Managing Dead Code:

Sometimes there are some lines or instructions in code base, which are completely not executed in all cases at all. This is considered as dead code. 

Dead code reduces the quality with respect to the readability.  

Comments:

Comments increases the readability of the program for programmers. Comments are written at class level, method level and variable level [Constants, instance variables and class variables] differently. 

The reason for this difference is, the pattern of comments is identified by API document generator tools and generates API documents.

Conclusion:

What all I tried to share through this post is, writing code for a specific problem can be done in different ways. But making sure that the program is more readable, maintenance free and easily scalable is also very important. 

The above few solutions or hacks will definitely help in developing better applications. 

Hope you enjoyed reading the post. Comments are welcomed. Thank you!! 

Happy coding!..

Tutorial and CHEAT SHEET for java List interface, LinkedList and ArrayList

Consider Below points before choosing List concrete implementations for your use case: 

• ArrayList is a linear data structure and  re-sizable/dynamic array

 

• Initial size can be given. and if not given the default size is 10

 

• It is always good practice to give initial size and advised not to give very large number as size

 

• Good programmers prefer ArrayList to LinkedList if retrieval operations is done more frequently than insertion and deletions of elements.

 

• Better choose LinkedList if insertion and deletion operations are done frequently compared to retrieval operations.

 

• Lists are not sorted. if required use Collections.sort() to sort

 

• Collections.sort() uses optimized merge sort [tim sort] on ArrayList

 

• However legacy merge sort also can be used using a switch. But it is going to be deprecated  in near future 

• The major difference between ArrayList and LinkedList is RandomAccess behavior 

• ArrayLists uses Random access indexing where as LinkedLists uses

          Sequential access indexing
 

• LinkedList carries extra overhead in the form of reference to the next and previous elements other than element's data itself

• LinkedList uses Doubly LinkedList data structure internally

• Deque is the java DoubleLinkedList data structure implementation

• Before choosing List implementation ask yourself below questions 

                    a. Are you aware of initial size? 
                    b. Which operations are used more frequently? [insertion, deletion,
                       retrieval, search, sort are few among others]

• Search Algorithm: 

                  a. Collections class uses binary search if List is ArrayList
                  b. Collections class uses linear search if List is of type LinkedList

• ArrayList and LinkedList both uses fast fail iterations.

• If another thread tries to modify list while iterating, java runtime throws ConcurrentModificationException. This can be considered as fast fail behavior  

Tutorial on Atomicity with Java Programming Language

 Atomicity with Java

What is Atomicity

Atomicity, in computer science, is considered to be a property [ALL-OR-NOTHING], that the state of a resource in an operation is not controlled by a different flow of execution at the same time [Even in time slicing operations]

We know, Java is not pure object oriented programming language, due to primitive data types' design behavior.

Most of the Java experts, while designing and developing applications thrives hard, to create, most secured applications considering "Atomicity in multi threaded application".

Does Java language helps in developing Secured application..?

The answer is no, Java programming language does not give secured applications at all, especially in multi threaded environment.

As we know, Java supports 2 types of variables/data types. 

1. Primitive data types
2. User defined data types

Primitive data types:      

Java supports 8 different primitive data types. They are: 

1.  boolean    1 byte
2.  short        2 bytes
3.  int            4 bytes
4.  char         2 bytes
5.  float         4 bytes
6.  long         8 bytes
7.  double     8 bytes
8.  byte         1 byte

User Defined data types:

In Java, user defined data types are created using "new" operator. For example, to create Employee object, the following instruction is coded

Employee employee = new Employee();

Now, the employee in the above statement, is a reference to an Employee class and called as user defined data type.

Now, coming to the point [Why Java is not secured..?], the primitive data types, long and double are not atomic variables. That means they are not executed in a single operation. 

long variable's size is 64 bits. And it is divided into 32 bits + 32 bits while copying from one memory location to another memory location. and all user defined data types are based on primitive data types.

And these two primitive variables [long and double] breaks the atomicity law. 

And developers can not bypass long and double variables in their program to create application. It is almost impossible to create the application without these variables.

Especially, in multi threaded based application, these non atomic variables may give unpredictable results. 

Tutorial on file IO operations with java programming language

File Management in OS

File management includes files and folders creation, copy and paste operations, reading the file and writing into files and deleting the files and folders 

And also reading meta data about files and folders. It may include file free space, occupied space and read/write permissions etc.

These above operations are considered as file management and managed by respective operating system.

GUI that represents Windows File Manager

File Management with Java 

Though Java is platform independent, programming language depends on native file IO resources of operating system.

This is possible with the Java API support. These API are categorized into 2 types.

1. Readers and Writers: Readers and writers does the IO operations, character by character
2. InputStream and OutputStream: Where as InputStream and OutputStream does IO operations byte by byte

Below are simple Java programs that demonstrates different File IO operations.

Program for creating directory

package com.allabtjava.fileio;

import java.io.BufferedReader;

import java.io.BufferedWriter;

import java.io.File;

import java.io.FileReader;

import java.io.FileWriter;

import java.io.IOException;

import java.nio.file.Path;

public class FileManager {

public FileManager() {

}

public void makeDirectory(String path, String dirName) {

Path dirPath = Path.of(path, dirName);

File dir = new File(dirPath.toString());

boolean isCreated = dir.mkdir();

if (isCreated)

System.out.println("A new directory with the name " + dir.getPath() + " created!");

}

        public static void main(String args[]) throws IOException {

FileManager manager = new FileManager();

manager.makeDirectory("e:\\", "students");

}

}

Program for creating file:

package com.allabtjava.fileio;

import java.io.BufferedReader;

import java.io.BufferedWriter;

import java.io.File;

import java.io.FileReader;

import java.io.FileWriter;

import java.io.IOException;

import java.nio.file.Path;

public class FileManager {

public FileManager() {

}

public void createFile(String dirPath, String dirName, String fileName) throws IOException {

Path filePath = Path.of(dirPath, dirName, fileName);

File file = new File(filePath.toString());

boolean isFileCreated = file.createNewFile();

if (isFileCreated)

System.out.println("A new file with the name " + file.getPath() + " created!");

}

        public static void main(String args[]) throws IOException {

FileManager manager = new FileManager();

manager.createFile("e:\\", "students", "student_1");

}

}

Listing all files available in given folder

package com.allabtjava.fileio;

import java.io.BufferedReader;

import java.io.BufferedWriter;

import java.io.File;

import java.io.FileReader;

import java.io.FileWriter;

import java.io.IOException;

import java.nio.file.Path;

public class FileManager {

public FileManager() {

}

     public void listFiles(String dirPath, String dirName) {
Path dir = Path.of(dirPath, dirName);
File file = new File(dir.toString());
String[] listOfAllFiles = file.list();
System.out.println("List of all Files in the directory/folder - " + dirName + ": ");
for (String fileName : listOfAllFiles) {
System.out.println(fileName);
}
}

        public static void main(String args[]) throws IOException {

FileManager manager = new FileManager();

manager.listFiles("e:\\", "pinterest");

}

}

Delete a directory

package com.allabtjava.fileio;

import java.io.BufferedReader;

import java.io.BufferedWriter;

import java.io.File;

import java.io.FileReader;

import java.io.FileWriter;

import java.io.IOException;

import java.nio.file.Path;

public class FileManager {

public FileManager() {

}

     public void deleteDirectory(String dirPath, String dirName) {

Path dir = Path.of(dirPath, dirName);

File dirToBeDeleted = new File(dir.toString());

if (dirToBeDeleted.exists()) {

boolean isDeleted = dirToBeDeleted.delete();

if(isDeleted)

System.out.println("File deleted!");

else

System.out.println("Could not delete file. File is being used by other program or application.");

}

}

        public static void main(String args[]) throws IOException {

FileManager manager = new FileManager();

manager.deleteDirectory("e:\\", "pinterest");

}

}

Read a file content

package com.allabtjava.fileio;

import java.io.BufferedReader;

import java.io.BufferedWriter;

import java.io.File;

import java.io.FileReader;

import java.io.FileWriter;

import java.io.IOException;

import java.nio.file.Path;

public class FileManager {

public FileManager() {

}

     public void readFile(String dirPath, String dirName, String fileName) throws IOException {

Path filePath = Path.of(dirPath, dirName, fileName);

File file = new File(filePath.toString());

FileReader reader = new FileReader(file.getAbsolutePath());

BufferedReader bufferedRead = new BufferedReader(reader);

String line = null;

while((line = bufferedRead.readLine()) != null) {

System.out.println(line);

}

bufferedRead.close();

}

        public static void main(String args[]) throws IOException {

FileManager manager = new FileManager();

manager.readFile("e:\\", "pinterest", "demo.txt");

}

}

Write content into file

package com.allabtjava.fileio;

import java.io.BufferedReader;

import java.io.BufferedWriter;

import java.io.File;

import java.io.FileReader;

import java.io.FileWriter;

import java.io.IOException;

import java.nio.file.Path;

public class FileManager {

public FileManager() {

}

     public void writeToFile(String dirPath, String dirName, String fileName) throws IOException {

Path filePath = Path.of(dirPath, dirName, fileName);

File file = new File(filePath.toString());

FileWriter writer = new FileWriter(file);

BufferedWriter bufferedWriter = new BufferedWriter(writer);

bufferedWriter.write("allabtjava.com is a web site, has information about java technologies \n");

bufferedWriter.write("This article explains about, files and IO management with java programming language.");

bufferedWriter.close();

}

        public static void main(String args[]) throws IOException {

FileManager manager = new FileManager();

manager.writeToFile("e:\\", "pinterest", "demo.txt");

}

}

Tutorial on Java NullPointerException

NullPointerException causes and reasons

Since Java is object oriented programming language, every thing is considered to be an object. and there is always a scope of an object to be null or "No value". 

Since Java supports primitive data types, Java is not considered to be pure Java object oriented programming language. 

Few Notable points about NullPointerException

1. NullPointerExceptions is unchecked exception
2. Please read post Exception handling and java keywords before reading this article
3. Usually if program tries to fetch the value from reference and finds nothing as value, then program throws NullPointerException
4. 'Zero' is not null
5. And empty String is not considered to be null
6. It is not the subclass of RuntimeException class

Demo code that throws NullPointerException 

package com.allabtjava.blog;

public class DemoNullPointerException {

class BeanDemo {

String name;

Integer ID;

public String getName() {

return name;

}

public void setName(String name) {

this.name = name;

}

public Integer getID() {

return ID;

}

public void setID(Integer iD) {

ID = iD;

}

}

public DemoNullPointerException() {

}

public void createNullPointerException() {

BeanDemo bean = new BeanDemo();

bean.setName("All about Java!");

System.out.println(bean.getName());

if(bean.getID() == null) {

throw new NullPointerException();

}else {

System.out.println(bean.getID());

}

}

public static void main(String args[]) {

new DemoNullPointerException().createNullPointerException();

}

}

Output

All about Java!

Exception in thread "main" java.lang.NullPointerException

at com.allabtjava.blog.DemoNullPointerException.createNullPointerException(DemoNullPointerException.java:36)

at com.allabtjava.blog.DemoNullPointerException.main(DemoNullPointerException.java:44)