Apache POI for creating , reading , writing excel (xls,xlsx)

I compared different API's and found that Apache POI is best api for creating , reading , writing excel (xls,xlsx) from available open source apis for excel.

Here is Quick start at Apaceh POI.

Maven Dependencies

<!-- Apache POI for excel export -->
         <dependencies>
          <dependency>
            <groupId>org.apache.poi</groupId>
            <artifactId>poi</artifactId>
            <version>3.9</version>
        </dependency>
        <dependency>
    <groupId>org.apache.poi</groupId>
    <artifactId>poi-ooxml</artifactId>
    <version>3.9</version>
</dependency>
<!-- Apache POI for excel export -->

Sample Code

XSSFWorkbook class for creating,reading,updating .xlsx file and HSSFWorkbook  for creating,reading, writing .xls file

 public class CalendarDemo {

    private static final String[] days = {
            "Sunday", "Monday", "Tuesday",
            "Wednesday", "Thursday", "Friday", "Saturday"};

    private static final String[]  months = {
            "January", "February", "March","April", "May", "June","July", "August",
            "September","October", "November", "December"};

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

        Calendar calendar = Calendar.getInstance();
        boolean xlsx = true;
        for (int i = 0; i < args.length; i++) {
            if(args[i].charAt(0) == '-'){
                xlsx = args[i].equals("-xlsx");
            } else {
              calendar.set(Calendar.YEAR, Integer.parseInt(args[i]));
            }
        }
        int year = calendar.get(Calendar.YEAR);
      
          /*create xlsx or xls file . XSSFWorkbook class for creating .xlsx file and
HSSFWorkbook  for creating .xls file*/

        Workbook wb = xlsx ? new XSSFWorkbook() : new HSSFWorkbook();
     
        Map<String, CellStyle> styles = createStyles(wb);

        for (int month = 0; month < 12; month++) {
            calendar.set(Calendar.MONTH, month);
            calendar.set(Calendar.DAY_OF_MONTH, 1);
            //create a sheet for each month
            Sheet sheet = wb.createSheet(months[month]);

            //turn off gridlines
            sheet.setDisplayGridlines(false);
            sheet.setPrintGridlines(false);
            sheet.setFitToPage(true);
            sheet.setHorizontallyCenter(true);
            PrintSetup printSetup = sheet.getPrintSetup();
            printSetup.setLandscape(true);

            //the following three statements are required only for HSSF
            sheet.setAutobreaks(true);
            printSetup.setFitHeight((short)1);
            printSetup.setFitWidth((short)1);

            //the header row: centered text in 48pt font
            Row headerRow = sheet.createRow(0);
            headerRow.setHeightInPoints(80);
            Cell titleCell = headerRow.createCell(0);
            titleCell.setCellValue(months[month] + " " + year);
            titleCell.setCellStyle(styles.get("title"));
            sheet.addMergedRegion(CellRangeAddress.valueOf("$A$1:$N$1"));

            //header with month titles
            Row monthRow = sheet.createRow(1);
            for (int i = 0; i < days.length; i++) {
                //set column widths, the width is measured in units of 1/256th of a character width
                sheet.setColumnWidth(i*2, 5*256); //the column is 5 characters wide
                sheet.setColumnWidth(i*2 + 1, 13*256); //the column is 13 characters wide
                sheet.addMergedRegion(new CellRangeAddress(1, 1, i*2, i*2+1));
                Cell monthCell = monthRow.createCell(i*2);
                monthCell.setCellValue(days[i]);
                monthCell.setCellStyle(styles.get("month"));
            }

            int cnt = 1, day=1;
            int rownum = 2;
            for (int j = 0; j < 6; j++) {
                Row row = sheet.createRow(rownum++);
                row.setHeightInPoints(100);
                for (int i = 0; i < days.length; i++) {
                    Cell dayCell_1 = row.createCell(i*2);
                    Cell dayCell_2 = row.createCell(i*2 + 1);

                    int day_of_week = calendar.get(Calendar.DAY_OF_WEEK);
                    if(cnt >= day_of_week && calendar.get(Calendar.MONTH) == month) {
                        dayCell_1.setCellValue(day);
                        calendar.set(Calendar.DAY_OF_MONTH, ++day);

                        if(i == 0 || i == days.length-1) {
                            dayCell_1.setCellStyle(styles.get("weekend_left"));
                            dayCell_2.setCellStyle(styles.get("weekend_right"));
                        } else {
                            dayCell_1.setCellStyle(styles.get("workday_left"));
                            dayCell_2.setCellStyle(styles.get("workday_right"));
                        }
                    } else {
                        dayCell_1.setCellStyle(styles.get("grey_left"));
                        dayCell_2.setCellStyle(styles.get("grey_right"));
                    }
                    cnt++;
                }
                if(calendar.get(Calendar.MONTH) > month) break;
            }
        }

        // Write the output to a file
        String file = "calendar.xls";
        if(wb instanceof XSSFWorkbook) file += "x";
        FileOutputStream out = new FileOutputStream(file);
        wb.write(out);
        out.close();
    }

    /**
     * cell styles used for formatting calendar sheets
     */
    private static Map<String, CellStyle> createStyles(Workbook wb){
        Map<String, CellStyle> styles = new HashMap<String, CellStyle>();

        short borderColor = IndexedColors.GREY_50_PERCENT.getIndex();

        CellStyle style;
        Font titleFont = wb.createFont();
        titleFont.setFontHeightInPoints((short)48);
        titleFont.setColor(IndexedColors.DARK_BLUE.getIndex());
        style = wb.createCellStyle();
        style.setAlignment(CellStyle.ALIGN_CENTER);
        style.setVerticalAlignment(CellStyle.VERTICAL_CENTER);
        style.setFont(titleFont);
        styles.put("title", style);

        Font monthFont = wb.createFont();
        monthFont.setFontHeightInPoints((short)12);
        monthFont.setColor(IndexedColors.WHITE.getIndex());
        monthFont.setBoldweight(Font.BOLDWEIGHT_BOLD);
        style = wb.createCellStyle();
        style.setAlignment(CellStyle.ALIGN_CENTER);
        style.setVerticalAlignment(CellStyle.VERTICAL_CENTER);
        style.setFillForegroundColor(IndexedColors.DARK_BLUE.getIndex());
        style.setFillPattern(CellStyle.SOLID_FOREGROUND);
        style.setFont(monthFont);
        styles.put("month", style);

        Font dayFont = wb.createFont();
        dayFont.setFontHeightInPoints((short)14);
        dayFont.setBoldweight(Font.BOLDWEIGHT_BOLD);
        style = wb.createCellStyle();
        style.setAlignment(CellStyle.ALIGN_LEFT);
        style.setVerticalAlignment(CellStyle.VERTICAL_TOP);
        style.setFillForegroundColor(IndexedColors.LIGHT_CORNFLOWER_BLUE.getIndex());
        style.setFillPattern(CellStyle.SOLID_FOREGROUND);
        style.setBorderLeft(CellStyle.BORDER_THIN);
        style.setLeftBorderColor(borderColor);
        style.setBorderBottom(CellStyle.BORDER_THIN);
        style.setBottomBorderColor(borderColor);
        style.setFont(dayFont);
        styles.put("weekend_left", style);

        style = wb.createCellStyle();
        style.setAlignment(CellStyle.ALIGN_CENTER);
        style.setVerticalAlignment(CellStyle.VERTICAL_TOP);
        style.setFillForegroundColor(IndexedColors.LIGHT_CORNFLOWER_BLUE.getIndex());
        style.setFillPattern(CellStyle.SOLID_FOREGROUND);
        style.setBorderRight(CellStyle.BORDER_THIN);
        style.setRightBorderColor(borderColor);
        style.setBorderBottom(CellStyle.BORDER_THIN);
        style.setBottomBorderColor(borderColor);
        styles.put("weekend_right", style);

        style = wb.createCellStyle();
        style.setAlignment(CellStyle.ALIGN_LEFT);
        style.setVerticalAlignment(CellStyle.VERTICAL_TOP);
        style.setBorderLeft(CellStyle.BORDER_THIN);
        style.setFillForegroundColor(IndexedColors.WHITE.getIndex());
        style.setFillPattern(CellStyle.SOLID_FOREGROUND);
        style.setLeftBorderColor(borderColor);
        style.setBorderBottom(CellStyle.BORDER_THIN);
        style.setBottomBorderColor(borderColor);
        style.setFont(dayFont);
        styles.put("workday_left", style);

        style = wb.createCellStyle();
        style.setAlignment(CellStyle.ALIGN_CENTER);
        style.setVerticalAlignment(CellStyle.VERTICAL_TOP);
        style.setFillForegroundColor(IndexedColors.WHITE.getIndex());
        style.setFillPattern(CellStyle.SOLID_FOREGROUND);
        style.setBorderRight(CellStyle.BORDER_THIN);
        style.setRightBorderColor(borderColor);
        style.setBorderBottom(CellStyle.BORDER_THIN);
        style.setBottomBorderColor(borderColor);
        styles.put("workday_right", style);

        style = wb.createCellStyle();
        style.setBorderLeft(CellStyle.BORDER_THIN);
        style.setFillForegroundColor(IndexedColors.GREY_25_PERCENT.getIndex());
        style.setFillPattern(CellStyle.SOLID_FOREGROUND);
        style.setBorderBottom(CellStyle.BORDER_THIN);
        style.setBottomBorderColor(borderColor);
        styles.put("grey_left", style);

        style = wb.createCellStyle();
        style.setFillForegroundColor(IndexedColors.GREY_25_PERCENT.getIndex());
        style.setFillPattern(CellStyle.SOLID_FOREGROUND);
        style.setBorderRight(CellStyle.BORDER_THIN);
        style.setRightBorderColor(borderColor);
        style.setBorderBottom(CellStyle.BORDER_THIN);
        style.setBottomBorderColor(borderColor);
        styles.put("grey_right", style);

        return styles;
    }
}

Good Core Java interview Questions

Good Core Java interview Questions

Here are some of few Good questions to be asked to Software Engineer in Java Technology.

Q:	If I do not provide any arguments on the command line, then the String array of Main method will be empty or null?
A:	It is empty. But not null.

Q:

What is an Iterator?

A:

Some of the collection classes provide traversal of their contents via a java.util.Iterator interface. This interface allows you to walk through a collection of objects, operating on each object in turn. Remember when using Iterators that they contain a snapshot of the collection at the time the Iterator was obtained; generally it is not advisable to modify the collection itself while traversing an Iterator.

Q:	Difference between HashMap and HashTable?
A:	The HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls. (HashMap allows null values as key and value whereas Hashtable doesnt allow). HashMap does not guarantee that the order of the map will remain constant over time. HashMap is unsynchronized and Hashtable is synchronized.

Q:

What is the difference between an Interface and an Abstract class?

A:

An abstract class can have instance methods that implement a default behavior. An Interface can only declare constants and instance methods, but cannot implement default behavior and all methods are implicitly abstract. An interface has all public members and no implementation. An abstract class is a class which may have the usual flavors of class members (private, protected, etc.), but has some abstract methods.

Q:

What are Checked and UnChecked Exception?

A:

A checked exception is some subclass of Exception (or Exception itself), excluding class RuntimeException and its subclasses. Making an exception checked forces client programmers to deal with the possibility that the exception will be thrown. eg, IOException thrown by java.io.FileInputStream's read() method· Unchecked exceptions are RuntimeException and any of its subclasses. Class Error and its subclasses also are unchecked. With an unchecked exception, however, the compiler doesn't force client programmers either to catch the exception or declare it in a throws clause. In fact, client programmers may not even know that the exception could be thrown. eg, StringIndexOutOfBoundsException thrown by String's charAt() method· Checked exceptions must be caught at compile time. Runtime exceptions do not need to be. Errors often cannot be.

Q:	Objects are passed by value or by reference?
A:	Java only supports pass by value. With objects, the object reference itself is passed by value and so both the original reference and parameter copy both refer to the same object .
	[ Update from Eki and Jyothish Ven

Q:	What is the common usage of serialization?
A:	Whenever an object is to be sent over the network, objects need to be serialized. Moreover if the state of an object is to be saved, objects need to be serilazed.
	[ Received from Sandesh Sadhale]	TOP

Q:	What is Externalizable interface?
A:	Externalizable is an interface which contains two methods readExternal and writeExternal. These methods give you a control over the serialization mechanism. Thus if your class implements this interface, you can customize the serialization process by implementing these methods.
	[ Received from Sandesh Sadhale]

Q:	When you serialize an object, what happens to the object references included in the object?
A:	The serialization mechanism generates an object graph for serialization. Thus it determines whether the included object references are serializable or not. This is a recursive process. Thus when an object is serialized, all the included objects are also serialized alongwith the original obect.
	[ Received from Sandesh Sadhale]

Q:	What one should take care of while serializing the object?
A:	One should make sure that all the included objects are also serializable. If any of the objects is not serializable then it throws a NotSerializableException.
	[ Received from Sandesh Sadhale]

Q:	What happens to the static fields of a class during serialization?
A:	There are three exceptions in which serialization doesnot necessarily read and write to the stream. These are 1. Serialization ignores static fields, because they are not part of ay particular state state. 2. Base class fields are only hendled if the base class itself is serializable. 3. Transient fields.
	[ Received from Sandesh Sadhale Modified after P.John David comments.]

Q:	What are wrapper classes?
A:	Java provides specialized classes corresponding to each of the primitive data types. These are called wrapper classes. They are e.g. Integer, Character, Double etc.
	[ Received from Sandesh Sadhale]

Q:	Why do we need wrapper classes?
A:	It is sometimes easier to deal with primitives as objects. Moreover most of the collection classes store objects and not primitive data types. And also the wrapper classes provide many utility methods also. Because of these resons we need wrapper classes. And since we create instances of these classes we can store them in any of the collection classes and pass them around as a collection. Also we can pass them around as method parameters where a method expects an object.
	[ Received from Sandesh Sadhale]

Q:	If I write return at the end of the try block, will the finally block still execute?
A:	Yes even if you write return as the last statement in the try block and no exception occurs, the finally block will execute. The finally block will execute and then the control return.
	[ Received from Sandesh Sadhale]

Q:	What is the difference between preemptive scheduling and time slicing?
A:	Under preemptive scheduling, the highest priority task executes until it enters the waiting or dead states or a higher priority task comes into existence. Under time slicing, a task executes for a predefined slice of time and then reenters the pool of ready tasks. The scheduler then determines which task should execute next, based on priority and other factors.
	[ Received from Venkateswara Manam]

Q:	What is the purpose of finalization?
A:	The purpose of finalization is to give an unreachable object the opportunity to perform any cleanup processing before the object is garbage collected.
	[ Received from Venkateswara Manam]

Q:	What is daemon thread and which method is used to create the daemon thread?
A:	Daemon thread is a low priority thread which runs intermittently in the back ground doing the garbage collection operation for the java runtime system. setDaemon method is used to create a daemon thread.
	[ Received from Shipra Kamra]

Q:	Can an unreachable object become reachable again?
A:	An unreachable object may become reachable again. This can happen when the object's finalize() method is invoked and the object performs an operation which causes it to become accessible to reachable objects.
	[Received from P Rajesh]

Q:	What are some alternatives to inheritance?
A:	Delegation is an alternative to inheritance. Delegation means that you include an instance of another class as an instance variable, and forward messages to the instance. It is often safer than inheritance because it forces you to think about each message you forward, because the instance is of a known class, rather than a new class, and because it doesn't force you to accept all the methods of the super class: you can provide only the methods that really make sense. On the other hand, it makes you write more code, and it is harder to re-use (because it is not a subclass).
	[ Received from P Rajesh]

Q. Is Java a pure object oriented language?
Java uses primitive data types and hence is not a pure object oriented language.
Q. What is difference between Path and Classpath?
Path and Classpath are operating system level environment variales. Path is used define where the system can find the executables(.exe) files and classpath is used to specify the location .class files.

Q: Why Generics are used in Java?

A: Generics provide compile-time type safety that allows programmers to catch invalid types at compile time. Java Generic methods and generic classes enable programmers to specify, with a single method declaration, a set of related methods or, with a single class declaration, a set of related types.

What are the Legacy classes?

Early versions of java did not include Collections framework. Instead it defined several classes and one interface to store objects. When collection came these classes reengineered to support the Collection interfaces. These old classes are known are legacy classes.
Legacy classes-Dictionary,HashTable,Properties, Stack, Vector
Legacy interface- Enumeration.

24.What is a marker interface?

Marker interfaces are those which do not declare any required methods, but signify their compatibility with certain operations. The java.io.Serializable interface and Cloneable are typical marker interfaces. These do not contain any methods, but classes must implement this interface in order to be serialized and de-serialized.

What are the differences between the Comparable and Comparator interfaces ?

Comparable	Comparato
It uses the compareTo() method. int objectOne.compareTo(objectTwo).	t uses the compare() method. int compare(ObjOne, ObjTwo)
It is necessary to modify the class whose instance is going to be sorted.	A separate class can be created in order to sort the instances.
Only one sort sequence can be created.	Many sort sequences can be created.
It is frequently used by the API classes.	It used by third-party classes to sort instances.

Q. Does not overriding hashcode() method has any performance implication ?This is a good question and open to all , as per my knowledge a poor hashcode function will result in frequent collision in HashMap which eventually increase time for adding an object into Hash Map.