Why We Should override hashCode method when we override equals method

Hello Friends,

In one of my previous post, I wrote about Why Should we override equals method in Java.In this article, let us try to find out Why overriding hashcode method is important and must when we override equals method.


Top 25 Unix Commands With Examples

Hello Friends,

In this tutorial, we will learn top Unix commands which are handy in day to day work, especially from Java developer point of view.



1. How to create a new file in Unix

    touch <fileName>
 
    Example :
    touch abc.txt

2. How to see content of file in Unix(without able to edit)

    cat <FileName>

    Example :
    cat abc.txt

3. How to open file in editor mode

    vi <File Name>

    Example :
    vi abc.txt

4. How to edit File and add content in File in Unix

    a) Open file with following command
        vi abc.txt
    b)   Press I ,it will bring file in edit mode.
    c)   Type whatever you enter in file.
    d)   Type :wq! and enter.This will save content you entered in file.

5. How to find a File/Directory with name starting with particular character in Unix

   ls -ltr | grep <character>*

   Example : Find file/directory starting with name "ab"
   ls -lrt | grep ab*  

6. How to give/change permissions of File or Directory

   File permissions in numeric format and their meaning :
   0 – no permissions
   1 – execute only
   2 – write only
   3 – write and execute
   4 – read only
   5 – read and execute
   6 – read and write
   7 – read, write and execute

  By default, when we create a File in Unix ,it is created with permission 666(read/write).
  By default, when we create a Directory in Unix, it is created with permission  777(read/write/execute).
Along with numeric notation, Unix permissions can also be represented by following characters:

Reference       Class      Description

u                     user        the owner of the file
g                     group     users who are members of the file's group
o                     others     users who are not the owner of the file or members of the group
a                     all           all three of the above, is the same as ugo
r                     read         read a file or list a directory's contents
w                   write        write to a file or directory
x                    execute    execute a file

6.1. How to make File read only in Unix

    chmod  <permission> <fileName>
  
    Example :
    chmod 444 abc.txt

6.2. How to give File Read/Write/Execute Permissions in Unix

    chmod <Permission> <FileName>
 
     Example :
    chmod 777 abc.txt

7. How to find list of all the links in a directory in Unix

    ls -lrt | grep "^l"
    ^ - This signifies start of each line
    l  - Each symbolic link has "l'' in the beginning of line like below :
    lrwxrwxrwx  1 gg99a weblog 3 Dec  9 22:20 latest -> 1.3
   So this command prints all lines from result of ls -lrt,which starts with "l" and all symolic link's lines      start with "l", hence the result.

8. How to see content of current directory

   ls

   This will not give details about Files or directories.

9. How to see content of current directory with details such as
   - the exact size of the file
   - who owns the file
   - who has the right to look at it
   - when it was last modified.
 
   ls -lrt

10. How to see all files  including the ones whose filenames begin in a dot
  
      ls -a
 
     This however does not give details as mentioned in point 9.

11. How to see List of all background process
 
     Jobs

12. How to bring a background process to foreground
 
     Fg

13. How to kill a process

      kill <ProcessId>

14. How to see current date in Unix

      date

15. How to see History of commands executed
 
      history
 
      Using only "history" will give you all the commands executed in current session.
      If you want to see specific type of command, use grep along with history as below :
   
      For example ,if you have executed command to create directory multiple times, you can  execute       history command as below. It will list mkdir command all the times you have executed previously  in current session.

     history | grep  mk

16. How to clear the console in Unix
 
     clear

17. How to check ,how much space is left on the current drive in Unix

     df -h

18. How to see which process is taking how much time in Unix

    top
  
    Press shift + O to get all options by which you can sort.For example ,to sort by PID,press 'a' and          then  press any key.It will sort all the processes by PID.

19. How to copy File from one host to another in Unix
   
19.1 When you are sending files from current machine to remote machine
  
        scp <current machine path to file,which we want to send to remote machine> user@remote                  machine:/remote path
  
    Example : scp /var/log/was/abc.txt GA99@test.com:/var/log/remoteDirectory

19.2 When you want to retrieve file(s) from remote machine
  
        scp user@remote machine:/remote path to file  <current machine path where we want to                      download the file>
     
        Example :

        scp user@remote machine:/var/log/remoteDirectory/abc.txt  /var/log/was

20. How to find ,how many times a word is there in File

    grep -c  <word to find> <FileName>
    Example :
    grep -c Hi abc.txt
    Above command will give count of word Hi in File abc.txt.

21. How to execute previously executed command in Unix

      !<Previous Command>
     This is really handy when your command includes lots of parameters and options.You need not            type all those parameters and options again with this shorthand.
 
     Example : Say I executed following command:
 
     ls -lrt | grep abc

    Now I want to execute this command again, I just have to type following to execute above                  command again :

   !ls
22. How to find all the files with certain content in Unix

      find . –name <type of files to be searched> | xargs grep <content to be matched>
  
      Example :
    
      find . -name "*.txt" | xargs grep "Hi"

23. How to find file(s) which have been modified since last day, today or more than 1 day

  23.1 Find all files in current directory and sub directories which has been modified exactly 1 day                back.

         find . -mtime 1

23.2  Find all files in current directory and sub directories which has been modified more than 1 day before.
   
        find . -mtime +1

23.3) Find all files in current directory and sub directories which has been modified less than 1 day before.

       find . -mtime -1

24. How to create Soft link in Unix

     ln -s <Directory which soft link will point to> <Soft link name>
 
     Example :

     ln -s 1.0 latest
  
    So here you are pointing latest soft link to directory 1.0.

24. How to update Soft link in Unix
 
      ln -nsf <New directory> <Soft link>
 
      Example : Say you want to point latest now to new directory 1.1,then you will execute following                         command :

    ln -nsf 1.1 latest

25.How to remove Soft link in Unix

    rm <Soft link name>


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