Java 8 : Functions

Functional programming in Java, a useful feature was introduced with Java 8 (java.util.function package).

With that, we have the ability to define a function, pass functions to a method as arguments or use already defined methods as functions.

I'll use a simple example to explain a function.

Function  myFunction = n -> n * n

I have defined a function here, named as "myFunction". It takes an argument of type Integer and returns an Integer (the square of the number passed in).

We can pass this function to other methods as an argument. The method below takes in a function and an integer, then calls function's apply method with the passed in integer. That applies function to the integer.

public int myMethod(Function func, Integer number) {
    return func.apply(number)
}

If we call above method with number 5 like below, we'll get 25 in return.

int answer = myMethod(myFunction, 5);

Also we are able to use already defined methods as functions. As an example, if we have following method implemented in myClass,

public class MyClass {
  public Integer calculateSquare(Integer number) {
    return number * number;
  }
}

then we can define a function like this that will act similar to myFunction.

Function func = MyClass::calculateSquare;

You can find all sorts of functions that you can define in this link. I only used one kind (Function : Represents a function that accepts one argument and produces a result) in the example.

NSIS - How to Write the Uninstaller

If you know how to create an installer using NSIS, you'll probably want to know how to create the uninstaller too :)

So you have to write a section named "Uninstall" inside your .nsi script file.

section "Uninstall"  
  SetShellVarContext all
  Delete "$DESKTOP\Your Application.lnk"
  RMDir /r "$SMPROGRAMS\YourApp"
  RMDir /r "$INSTDIR"
  Delete $INSTDIR\uninstall.exe
  DeleteRegKey HKLM "Software\Microsoft\Windows\CurrentVersion\Uninstall\YourApp"
  
sectionEnd

In line 2 it sets ShellVarContext to all (all user's shell folder will be used). If not the default is current user's shell folder.  Next line deletes the desktop shortcut. Next 3 lines delete the installation folders. If you entered any registry keys during the installation then you can delete them here (line 7).

While having this "Uninstall" section in  your nsis script, you have to invoke it from your "Install" section. Then only the installation will write the uninstaller.   Here is a possible association of a installer section to the above uninstaller section.

Section "GENERAL"

  SetShellVarContext all
  #Installer body---------(creating desktop/start menu short cuts, granting permissions, etc)
  
  #Writing registry keys
  WriteRegStr HKLM "Software\Microsoft\Windows\CurrentVersion\Uninstall\YourApp" "DisplayName" "Your Application"
  WriteRegStr HKLM "Software\Microsoft\Windows\CurrentVersion\Uninstall\YourApp" "UninstallString" "$INSTDIR\uninstall.exe"

  # create the uninstaller
  writeUninstaller "$INSTDIR\uninstall.exe"

SectionEnd

NSIS - How to Embed Other Installers in Your Installer

Sometimes you may want to embed some other installer in your installer. As an example if you need Java as a prerequisite then you can embed Java installation inside your installer. Then it will run the Java installation before installing your application, when you run your installer. 

So let's write a section to embed JRE installer in your installer

Section "jre" SECJRE
  File "jre-6u14-windows-i586.exe"

  DetailPrint "Starting the JRE installation"   
  ExecWait "$TEMP\jre-6u14-windows-i586.exe" 
SectionEnd

In line 2 it is checking the existence of the file "jre-6u14-windows-i586.exe". At the point of compiling NSIS script, this exe should exist in the stated relative path (in this case it should be inside the same folder as the nsis script file). In line 5 it will execute the jre installer and your installation program will be suspended until JRE installation is completed. After the JRE setup is complete it will resume the installation of your application. In this way, you can embed any number of installers inside your installer.

This can be improved in many ways. Well, before installing JRE, you can check whether JRE is already installed in the system. In that case you can consider the version too. If it is not installed then you can use the above script to install JRE. Another improvement is, instead of installing a precise JRE you can download the latest version from Internet and then install. This nsis example shows a similar thing.

NSIS - How to Detect Whether Java is Installed in the System

While writing my installer I wanted to install JRE (embed JRE installer in my installer) only if Java is not installed already. So I had to detect whether Java is already installed.

My previous post on NSIS points to a quick start guide (in Harshani's blog) for NSIS, including how to install NSIS and how to write a simple installer. So at this point you should have installed NSIS and been somewhat familiar with NSIS scripting (sections, etc..). So here I am posting the piece of script I used to detect whether Java is installed in the system.

Section "find java" FINDJAVA

  StrCpy $1 "SOFTWARE\JavaSoft\Java Runtime Environment"
  StrCpy $2 0
  ReadRegStr $2 HKLM "$1" "CurrentVersion"
  StrCmp $2 "" DetectTry2
  ReadRegStr $5 HKLM "$1\$2" "JavaHome"
  StrCmp $5 "" DetectTry2
  goto done

  DetectTry2:
  ReadRegStr $2 HKLM "SOFTWARE\JavaSoft\Java Development Kit" "CurrentVersion"
  StrCmp $2 "" NoJava
  ReadRegStr $5 HKLM "SOFTWARE\JavaSoft\Java Development Kit\$2" "JavaHome"
  StrCmp $5 "" NoJava done

  done:
  ;All done. 

  NoJava:
  ;Write the script to install Java here

SectionEnd

In 3rd line of the script, it copies the path "Software\JavaSoft\Java Runtime Environment" to the variable 1. It is the Windows registry entry in which we should look for Java. In line 5 it reads the value of registry attribute "Current Version" which belongs to "SOFRWARE\JavaSoft\Java Runtime Environment", to the variable 2. ReadRegStr is the command used to read the registry. Here, "HKLM" means that it should look for this key under "HKEY_LOCAL_MACHINE" registry main key. 

In line 6 the command StrCmp compares the value of var 2 with "empty". If it is empty then that means there is no JRE installed in the system. So it passes the control to label "DetectTry2" which tries to detect if there is any Jdk installed. Line 7 is executed only if there is a value for JRE version. If so it tries to find the value for JavaHome attribute in key "Software\JavaSoft\Java Runtime Environment\1.x.x". In line 8 it checks whether JavaHome is null and if so, then again it jumps to label "DetectTry2". If it is not null that means there is a JRE installed and line 9 passes it to label "done".

Now let's see what "DetectTry2" label does. It is executed only if there is no JRE installed. In line 12 it reads the value of  "CurrentVersion" attribute of key "SOFTWARE\JavaSoft\Java Development Kit". In line 13 it checks whether the value it got is empty. If so it will hit the label "NoJava" means that neither JRE nor JDK is installed. If it is not empty then line 14 will check whether JavaHome is empty like before. Again, if JavaHome is empty it jumps to "NoJava". If not it jumps to "done".

If we ever hit "done" then that means either JRE or JDK is installed in the system. So there is nothing to do there. If we hit NoJava then we are going to install Java. I will post the script for that in a later post, NSIS - How to embed some other installer in your own installer.

This is valid for any other software like .NET framework, MS Office, etc as long as you know the correct registry entries to look in. 

Nullsoft Scriptable Install System (NSIS) - A Really Good Windows Installation System

I wanted to write an installation for IFRI data entry application which is currently being used to enter forest data to their database. The existing installation had some inconsistencies like users had to install the other required software themselves, some permission issues etc. So I had to search for a pretty stable installation system with all those features I wanted.

While searching I found this Harshani's blog about NSIS and thought to give it a try and  decided to write IFRI installer using NSIS. First it was not pretty simple and straight forward. I had to learn NSIS scripting language , tested examples in NSIS site. NSIS documentation was pretty helpful too. So finally I was able to write an installer for IFRI with all the features they expected and NSIS helped me a lot on that. I think it wouldn't be this easy and stable if I used another installation system other than NSIS.

So I'll post some tips and tricks I found  in NSIS scripting later. However Harshani's blog post about NSIS (stated above) is a nice post and shows how to build a simple installer. It will be a good first step on NSIS.

How to Print from Java (JPS) - javax.print Package

javax.print package is available in JDKs 1.4 and above. Formerly Java AWT printing API was used to perform basic printing and now it is enhanced with advanced options like printer service discovery, in javax.print API.

I am posting a simple example showing how to perform a basic print using javax.print API.

import java.io.*;
import javax.print.*;

public class PrintTest {
 public static void main(String[] args) throws IOException {
  //we are going to print "printtest.txt" file which is inside working directory
  File file = new File("printtest.txt");
  InputStream is = new BufferedInputStream(new FileInputStream(file));
  
  //Discover the default print service. If you call PrintServiceLookup.lookupPrintServices
  //then it will return an array of print services available and you can choose a
  //printer from them
  PrintService service = PrintServiceLookup.lookupDefaultPrintService();
  
  //Doc flavor specifies the output format of the file (Mime type + Char encoding)
  //You can retrieve doc flavors supported by the printer, like this
  //DocFlavor [] supportedFlavors = service.getSupportedDocFlavors();
  DocFlavor flavor = DocFlavor.INPUT_STREAM.TEXT_PLAIN_US_ASCII;  
  
  
  // Create the print job
  DocPrintJob job = service.createPrintJob();
  //Create the Doc. You can pass set of attributes(type of PrintRequestAttributeSet) as the 
  //3rd parameter specifying the page setup, orientation, no. of copies, etc instead of null. 
  Doc doc = new SimpleDoc(is, flavor, null);

  //Order to print, (can pass attributes instead of null)
  try {
   job.print(doc, null);
  } catch (PrintException e) {
   e.printStackTrace();
  }  

  //DocPrintJob.print() is not guaranteed to be synchronous. So it's better to wait on completion
  //of the print job before closing the stream. (See the link below)
  is.close();
  System.out.println("Printing done....");
 }

}

This example shows how to create a Print job watcher. You can create an object from PrintJobWatcher in your PrintTest class and call waitForDone() before closing the stream.

You can find documentation on javax.print API here.

The DocFlavor I have used in this example (INPUT_STREAM.TEXT_PLAIN_US_ASCII) wont work on Windows. The PrintService doesn't discover this flavor as a supported one for some reason. Yes, this Java Print Service facility is still buggy (in JDK 1.6). It is weird that we can't print ASCII formatted files while we can print any JPEG or PNG.  I used INPUT_STREAM.AUTOSENSE flavor which uses mime type "application/octet-stream", to print on Windows. Then the printer will print just the octets coming from the stream.However it works fine.

How to Fetch MP3 from YouTube Videos

I found a nice site to fetch mp3s from YouTube videos. The process is pretty simple and fast. Check it out, you'd love it.

How to Find which Process Runs on which Port in Windows

netstat -ab

shows which process runs on which port. If you need to find a process which runs on a particular port, the command can be used as follows

netstat -ab | find ":8080 "

How to Remove Duplicate Records from a Table, Considering a Subset of Columns - Sql

I wanted to delete some duplicate records from a table. "Duplicate" doesn't mean identical records in this scenario. I wanted to check whether the values in a subset of columns are identical and if so, to remove additional records keeping just one record in the table. 

Let's assume we have a table called Person like below

Id	Name	Age	City
10	Sunil	24	Matara
11	Sandun	25	Colombo
12	Nimali	23	Matara
13	Dilani	25	Matara
14	Savini	25	Galle

Say we need only one person from one city. Then we have to remove two records having Matara as the city.

To do that we can write a query like below

DELETE FROM Person WHERE Id > (SELECT MIN(Id) FROM Person p where p.City = Person.City)

We used Id as a controller to decide which records to delete and to keep just one record (by checking min Id).

After executing the query, the table will look like this

Id	Name	Age	City
10	Sunil	24	Matara
11	Sandun	25	Colombo
14	Savini	25	Galle

How to merge multiple images into one image - Java ImageIO

My previous post shows how to split an image into chunks. Now let's see how to merge multiple images into one image. Say we need to concatenate following four image chunks. I got these chunks by splitting the image in the right hand side, using the image splitter.

 

Following code shows how to concatenate the image chunks above into one image.

int rows = 2;   //we assume the no. of rows and cols are known and each chunk has equal width and height
        int cols = 2;
        int chunks = rows * cols;

        int chunkWidth, chunkHeight;
        int type;
        //fetching image files
        File[] imgFiles = new File[chunks];
        for (int i = 0; i < chunks; i++) {
            imgFiles[i] = new File("archi" + i + ".jpg");
        }

       //creating a bufferd image array from image files
        BufferedImage[] buffImages = new BufferedImage[chunks];
        for (int i = 0; i < chunks; i++) {
            buffImages[i] = ImageIO.read(imgFiles[i]);
        }
        type = buffImages[0].getType();
        chunkWidth = buffImages[0].getWidth();
        chunkHeight = buffImages[0].getHeight();

        //Initializing the final image
        BufferedImage finalImg = new BufferedImage(chunkWidth*cols, chunkHeight*rows, type);

        int num = 0;
        for (int i = 0; i < rows; i++) {
            for (int j = 0; j < cols; j++) {
                finalImg.createGraphics().drawImage(buffImages[num], chunkWidth * j, chunkHeight * i, null);
                num++;
            }
        }
        System.out.println("Image concatenated.....");
        ImageIO.write(finalImg, "jpeg", new File("finalImg.jpg"));