albert's blog

2016/10/31

2016/10/10

[Java 8] Using lambda to convert a List of object to comma-separated string

If I java an object which named Group

    import lombok.Data;
    
    @Data
    public class Group {    
        private Integer id;
        private String name;
    }

The data in List of Group looks like:

1
2
3

    Group(id=1, name=Group A)
    Group(id=2, name=Group B)
    Group(id=3, name=Group C)

The expected result is to convert to comma-separated string:
Group A, Group B, Group c

If we does not utilize lambda expression, the code snippet is as following:

    List<Group> groups = dto.getUser().getGroups();
    StringBuilder groupStringBuilder = new StringBuilder();
    for (int i = 0; i < groups.size(); i++) {
        if (i == groups.size() - 1) {
            groupStringBuilder = groupStringBuilder.append(groups.get(i).getName());
        } else {
            groupStringBuilder = groupStringBuilder.append(groups.get(i).getName()).append(", ");
        }
    }

If we apply lambda expression, we only need to use one line of code to meet this requirement:

    String groupStr = groups.stream().map(g -> g.getName()).collect(Collectors.joining(", "));

2016/10/09

[Java 8] Convert a List of String to a Comma-Separated String

If I have a List of String looks like:

1
2
3

private List<String> getDummyPlayers() {
    return Arrays.asList("王 柏 融", "林 智 勝", "林 泓 育", "蔣 智 賢", "高 國 慶");
}

If I would like to convert it to String, and each player should separate with comma, i.e. 王柏融, 林智勝, 林泓育, 蔣智賢, 高國慶

Before Java 8, we may do this way:

private String separatePlayersWithComma(){
    List<String> players =getDummyPlayers();
    String playersWithComma = "";
    for (int i = 0; i < players.size(); i++) {
        if (i == players.size() - 1) {
            playersWithComma = playersWithComma + players.get(i);
        } else {
            playersWithComma = playersWithComma + players.get(i) +", "; 
        }
    }
    return playersWithComma;
}

In Java 8, it is very easy and straightforward:

private String separatePlayersWithComma_usingJDK8(){
    List<String> players =getDummyPlayers();
    String playersWithComma = players.stream().collect(Collectors.joining(", "));
    return playersWithComma;
}

2016/10/08

Project Lombok provide an easy way to ensure a given resource is automatically cleaned up before the code execution path exits your current scope. All you need to do is to annotate any local variable declaration with the @Cleanup annotation.

As-Is (Do not annotate @Cleanup)

private void readExcelTest1(String excelFile) throws IOException {
     List<Issue> issues = new ArrayList<Issue>();

     InputStream inputStream = null;
     Workbook workbook = null;

     try {
         // 1. Create a Workbook.
         inputStream = new FileInputStream(new File(excelFile));

         // 2. Get first sheet
         workbook = new HSSFWorkbook(inputStream);

         Sheet sheet = workbook.getSheetAt(0);

         // 3. Iterate through each rows from first sheet
         Iterator<Row> rowIterator = sheet.iterator();
         int rowCount = 1;
         while (rowIterator.hasNext()) {
            // (1) ignore header row
            if (rowCount == 1) {
              rowIterator.next();
              rowCount++;
             }
             // (2) start to read each row from second row
             else {
                 Row row = rowIterator.next();
                 Integer id = Double.valueOf(row.getCell(0).getNumericCellValue()).intValue();
                 String subject = row.getCell(1).getStringCellValue();
                 String status = row.getCell(2).getStringCellValue();
                 String priority = row.getCell(3).getStringCellValue();
                 String notes = row.getCell(4).getStringCellValue();

                 Issue issue = new Issue();
                 issue.setId(id);
                 issue.setSubject(subject);
                 issue.setStatus(status);
                 issue.setPriority(priority);
                 issue.setNotes(notes);

                 issues.add(issue);
              }
           }

          // print collection
          issues.stream().forEach(issue -> log.debug(issue.toString()));
     } catch (FileNotFoundException e) {
         throw e;
     } catch (IOException e) {
         throw e;
     } finally {
         if (workbook != null) {
           workbook.close();
         }

         if (inputStream != null) {
           inputStream.close();
         }
     }
}

To-Be(Annotte with @Cleanup)
It will ensure the variable declaration that you annotate will be cleaned up by calling its close method, regardless of what happens. Implemented by wrapping all statements following the local variable declaration to the end of your scope into a try block that, as a finally action, closes the resource.

 private void readExcelTest2(String excelFile) throws Exception {
     try {
         // 1. Create a Workbook.
         @Cleanup
         InputStream inputStream = new FileInputStream(new File(excelFile));

         // 2. Get first sheet
         @Cleanup
         Workbook workbook = new HSSFWorkbook(inputStream);

         List<Issue> issues = new ArrayList<Issue>();

         Sheet sheet = workbook.getSheetAt(0);
 
         // 3. Iterate through each rows from first sheet
         Iterator<Row> rowIterator = sheet.iterator();
         int rowCount = 1;
         while (rowIterator.hasNext()) {
              // (1) ignore header row
              if (rowCount == 1) {
              rowIterator.next();
              rowCount++;
              }
              // (2) start to read each row from second row
              else {
                  Row row = rowIterator.next();
                  Integer id = Double.valueOf(row.getCell(0).getNumericCellValue()).intValue();
                  String subject = row.getCell(1).getStringCellValue();
                  String status = row.getCell(2).getStringCellValue();
                  String priority = row.getCell(3).getStringCellValue();
                  String notes = row.getCell(4).getStringCellValue();

                  Issue issue = new Issue();
                  issue.setId(id);
                  issue.setSubject(subject);
                  issue.setStatus(status);
                  issue.setPriority(priority);
                  issue.setNotes(notes);
    
                  issues.add(issue);
              }
         }

         // print collection
         issues.stream().forEach(issue -> log.debug(issue.toString()));
    
     } catch (FileNotFoundException e) {
         throw e;
     } catch (Exception e) {
         throw e;
     }
 }

Reference
[1] https://projectlombok.org/features/Cleanup.html

2016/10/07

[PDFBox] Generate TIF with 0-kb

Problem
I am using Apache PDFBox to convert PDF to TIF file.

The scenario is to generate TIF file to a specific path, then another application will read this TIF file and move to another directory.

But it will generate TIF file with 0-kb at times and fail to open this TIF file, and does not have any abnormal message in application log.

How-To
Here is the code snippet:

    private List<String> writeImage(PDDocument document, String outputPrefix, ImageType imageType,
            float dpi, String fileName) throws IOException {
        ImageIO.scanForPlugins();

        List<String> tifNames = new ArrayList<String>();

        PDFRenderer renderer = new PDFRenderer(document);

        for (int i = 0; i < document.getNumberOfPages(); i++) {

            BufferedImage image = renderer.renderImageWithDPI(i, dpi, imageType);

            String outputPostfix = "_" + String.format("%02d", i + 1); // 01開始
            String outputFileName = outputPrefix + outputPostfix;// 檔名: 保單號碼_0X

            tifNames.add(fileName + outputPostfix); // 回傳產了那些檔名

            ImageIOUtil.writeImage(image, outputFileName + "." + "tif", Math.round(dpi));
            image.flush();
        }
        return tifNames;
    }

We can see implementation details in ImageIOUtil.writeImage
https://github.com/apache/pdfbox/blob/04292e421e9531616aa2856b908fbdb05b381af7/tools/src/main/java/org/apache/pdfbox/tools/imageio/ImageIOUtil.java#L65-L79

We can see it will create a File instance at first, then write image to TIF file.

As it create a File instance, this file will be created. It another application had read this file, then this file may be locked and failed to write.

Therefore, we should create TIF file to a temporary directory first, then move the finished TIF file to the destination.

2016/10/06

[Mac] Yahoo 輸入法的全型與半型的切換

當我在 Mac 平台使用 Yahoo 輸入法時，當我切換為英數字的輸入時，若為全型時，該如何轉換成半型

此時你只要 shift + command + space ，即可將全型轉成半型

輸入結果如下

若要從半形再轉回全形，你只要再按一次 shift + command + space

2016/10/05

[Mac] Yahoo 輸入法如何中英切換

我在mac平台，使用yahoo輸入法作為我的中文輸入的工具

若我要從中文切換為英文的話，按下caps lock，即可輸入英文

但是此時會是小寫的英文，若要輸入大寫英文，只要按 shfit + 你要打的英文即可（前提是你已經按下了 caps lock）

2016/10/04

[Java Mail] com.sun.mail.smtp.SMTPSendFailedException: 452 message processing failed: connection timeout

Problem
I am using JavaMail to send email. But I got this error message as bellows:

DEBUG SMTP: MessagingException while sending, THROW: 
com.sun.mail.smtp.SMTPSendFailedException: 452 message processing failed: connection timeout

 at com.sun.mail.smtp.SMTPTransport.issueSendCommand(SMTPTransport.java:2203)
 at com.sun.mail.smtp.SMTPTransport.finishData(SMTPTransport.java:1981)
 at com.sun.mail.smtp.SMTPTransport.sendMessage(SMTPTransport.java:1197)
 at org.springframework.mail.javamail.JavaMailSenderImpl.doSend(JavaMailSenderImpl.java:448)
 at org.springframework.mail.javamail.JavaMailSenderImpl.send(JavaMailSenderImpl.java:345)
 at org.springframework.mail.javamail.JavaMailSenderImpl.send(JavaMailSenderImpl.java:340)
 at com.yuantalife.ecp.commons.service.EmailService.send(EmailService.java:260)

How-to
This is not program's problem. This error may resulted from SMTP server.
It may :

The ISP server’s disk system has run out of storage space, so the action had to be cancelled.
Most ISPs mail servers impose a maximum number of concurrent connections that client’s mail servers can attempt to make, and they usually also have a limit on the number of messages that are sent per connection.
This error can also be indicative of a problem on your own mail server. ex. out of memory.

Reference
[1] http://www.answersthatwork.com/Download_Area/ATW_Library/Networking/Network__3-SMTP_Server_Status_Codes_and_SMTP_Error_Codes.pdf

2016/10/03

[Java] Lambda Expressions Example

If I have POJO, which named Person

package albert.practice.lambda;

import java.io.Serializable;

import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;
import lombok.ToString;

@Data
@ToString
@AllArgsConstructor
@NoArgsConstructor
public class Person implements Serializable{

    /**
     * 
     */
    private static final long serialVersionUID = 1L;
    
    private String name;
    private Gender gender;
    private int age;
    private String email;
    
}

If I have a List of Person, I would like to do search based on some criteria.

Example 1: I would like to find the person who is male and age is equal or greater than 25
Using Apache Commons Collection Utils:

private List<Person> findByApacheCommonsEx1(List<Person> dummyData) {
     return (List<Person>) CollectionUtils.select(dummyData, new Predicate() {
         @Override
         public boolean evaluate(Object object) {
              Person person = (Person) object;
              Boolean condition = person.getAge() >= 25 && Gender.MALE.equals(person.getGender());
              return condition;
         }
     });
}

Using Lambda Expression:

private List<Person> findByLambdaEx1(List<Person> dummyData) {
     return dummyData.stream()
                      .filter(person -> person.getAge() >= 25 && Gender.MALE.equals(person.getGender()))
                      .collect(Collectors.toList());
}

Example 2: I would like to find the person who is equal or greater than 25, and find the first person from the collection.
Using Apache Commons Collection Utils:

private Person findByApacheCommonsEx2(List<Person> dummyData) {
     List<Person> result = (List<Person>) CollectionUtils.select(dummyData, new Predicate() {
         @Override
         public boolean evaluate(Object object) {
              Person person = (Person) object;
              return person.getAge() >= 25;
         }
     });

     Person person = null;
     if (!result.isEmpty()) {
         person = result.get(0);
     }
     return person;
}