Java Scanner

“The only way of discovering the limits of the possible is to venture a little way past them into the impossible.”
― Arthur C. Clarke

Introduction

Java provides a Scanner class that can be used as a text parser. It accepts a regular expression as a delimiter and returns tokens separated by the delimiter. Let us look at some usage scenarios of the Scanner class.

Continue reading “Java Scanner”

Java HashMap Examples

1. Introduction

A HashMap is a map of keys to values which uses a hash table for implementation. The HashMap organizes the keys into buckets based on the value of the hashCode() of the key. It provides constant-time performance for get and put operations. What this means is that the time taken by get and put operations does not depend on the size of the map.

Continue reading “Java HashMap Examples”

Java Math.random Examples

Math.Random

1. Introduction

Let us learn how to generate some random numbers in Java. Random numbers are needed for various purposes; maybe you want to generate a password or a session identifier. Whatever the purpose may be, there are a number of issues to be aware of when generating a random number.

Continue reading “Java Math.random Examples”

Java String Format Examples

1. Introduction

Have you tried to read and understand Java’s String format documentation? I have and found it hard to understand. While it does include all the information, the organization leaves something to be desired.

This guide is an attempt to bring some clarity and ease the usage of string formatting in java.

2. String Formatting

Most common way of formatting a string in java is using String.format(). If there were a “java sprintf”, this would be it.

String output = String.format("%s = %d", "joe", 35);

For formatted console output, you can use printf() or the format() method of System.out and System.err PrintStreams.

System.out.printf("My name is: %s%n", "joe");

Create a Formatter and link it to a StringBuilder. Output formatted using the format() method will be appended to the StringBuilder.

StringBuilder sbuf = new StringBuilder();
Formatter fmt = new Formatter(sbuf);
fmt.format("PI = %f%n", Math.PI);
System.out.print(sbuf.toString());
// you can continue to append data to sbuf here.

3. Format Specifiers

Here is a quick reference to all the conversion specifiers supported.

Specifier Applies to Output
%a floating point (except BigDecimal) Hex output of floating point number
%b Any type “true” if non-null, “false” if null
%c character Unicode character
%d integer (incl. byte, short, int, long, bigint) Decimal Integer
%e floating point decimal number in scientific notation
%f floating point decimal number
%g floating point decimal number, possibly in scientific notation depending on the precision and value.
%h any type Hex String of value from hashCode() method.
 %n none Platform-specific line separator.
%o integer (incl. byte, short, int, long, bigint) Octal number
%s any type String value
%t Date/Time (incl. long, Calendar, Date and TemporalAccessor) %t is the prefix for Date/Time conversions. More formatting flags are needed after this. See Date/Time conversion below.
%x integer (incl. byte, short, int, long, bigint) Hex string.

3.1. Date and Time Formatting

Note: Using the formatting characters with “%T” instead of “%t” in the table below makes the output uppercase.

 Flag Notes
 %tA Full name of the day of the week, e.g. “Sunday“, “Monday
 %ta Abbreviated name of the week day e.g. “Sun“, “Mon“, etc.
 %tB Full name of the month e.g. “January“, “February“, etc.
 %tb Abbreviated month name e.g. “Jan“, “Feb“, etc.
 %tC Century part of year formatted with two digits e.g. “00” through “99”.
 %tc Date and time formatted with “%ta %tb %td %tT %tZ %tY” e.g. “Fri Feb 17 07:45:42 PST 2017
 %tD Date formatted as “%tm/%td/%ty
 %td Day of the month formatted with two digits. e.g. “01” to “31“.
 %te Day of the month formatted without a leading 0 e.g. “1” to “31”.
%tF ISO 8601 formatted date with “%tY-%tm-%td“.
%tH Hour of the day for the 24-hour clock e.g. “00” to “23“.
%th Same as %tb.
%tI Hour of the day for the 12-hour clock e.g. “01” – “12“.
%tj Day of the year formatted with leading 0s e.g. “001” to “366“.
%tk Hour of the day for the 24 hour clock without a leading 0 e.g. “0” to “23“.
%tl Hour of the day for the 12-hour click without a leading 0 e.g. “1” to “12“.
%tM Minute within the hour formatted a leading 0 e.g. “00” to “59“.
%tm Month formatted with a leading 0 e.g. “01” to “12“.
%tN Nanosecond formatted with 9 digits and leading 0s e.g. “000000000” to “999999999”.
%tp Locale specific “am” or “pm” marker.
%tQ Milliseconds since epoch Jan 1 , 1970 00:00:00 UTC.
%tR Time formatted as 24-hours e.g. “%tH:%tM“.
%tr Time formatted as 12-hours e.g. “%tI:%tM:%tS %Tp“.
%tS Seconds within the minute formatted with 2 digits e.g. “00” to “60”. “60” is required to support leap seconds.
%ts Seconds since the epoch Jan 1, 1970 00:00:00 UTC.
%tT Time formatted as 24-hours e.g. “%tH:%tM:%tS“.
%tY Year formatted with 4 digits e.g. “0000” to “9999“.
%ty Year formatted with 2 digits e.g. “00” to “99“.
%tZ Time zone abbreviation. e.g. “UTC“, “PST“, etc.
%tz Time Zone Offset from GMT e.g. “-0800“.

4. Argument Index

An argument index is specified as a number ending with a “$” after the “%” and selects the specified argument in the argument list.

String.format("%2$s", 32, "Hello");
// prints: "Hello"

5. Formatting an Integer

With the %d format specifier, you can use an argument of all integral types including byte, short, int, long and BigInteger.

Default formatting:
String.format("%d", 93);
// prints 93
Specifying a width:
String.format("|%20d|", 93);
// prints: |                  93|
Left-justifying within the specified width:
String.format("|%-20d|", 93);
// prints: |93                  |
Pad with zeros:
String.format("|%020d|", 93);
// prints: |00000000000000000093|
Print positive numbers with a “+”:

(Negative numbers always have the “-” included):

String.format("|%+20d|', 93);
// prints: |                 +93|
A space before positive numbers.

A “-” is included for negative numbers as per normal.

String.format("|% d|", 93);
// prints: | 93|

String.format("|% d|", -36);
// prints: |-36|
Use locale-specific thousands separator.

For the US locale, it is “,”:

String.format("|%,d|", 10000000);
// prints: |10,000,000|
Enclose negative numbers within parantheses (“()”) and skip the “-“:
String.format("|%(d|", -36);
// prints: |(36)|
Octal Output
String.format("|%o|"), 93);
// prints: 135
Hex Output
String.format("|%x|", 93);
// prints: 5d
Alternate Representation for Octal and Hex Output

Prints octal numbers with a leading “0” and hex numbers with leading “0x“.

String.format("|%#o|", 93);
// prints: 0135

String.format("|%#x|", 93);
// prints: 0x5d

String.format("|%#X|", 93);
// prints: 0X5D

6. String and Character Conversion

Default formatting:

Prints the whole string.

String.format("|%s|", "Hello World");
// prints: "Hello World"
Specify Field Length
String.format("|%30s|", "Hello World");
// prints: |                   Hello World|
Left Justify Text
String.format("|%-30s|", "Hello World");
// prints: |Hello World                   |
Specify Maximum Number of Characters
String.format("|%.5s|", "Hello World");
// prints: |Hello|
Field Width and Maximum Number of Characters
String.format("|%30.5s|", "Hello World");
|                         Hello|

Summary

This guide explained String formatting in Java. We covered the supported format specifiers. Both numeric and string formatting support a variety of flags for alternative formats.

How to Read CSV File in Java

Reading a CSV file in Java including handling BOM (Byte-Order-Marker), quoted fields, multi-line fields and more.

“The reason I talk to myself is because I’m the only one whose answers I accept.”
― George Carlin

1. Introduction

CSV files are extensively used in data interchange between applications. Especially useful when the only structure to the data being exchanged is rows and columns. This format is particularly popular as the data can be imported into Microsoft Excel and used for charts and visualization.

Continue reading “How to Read CSV File in Java”

Load XML into Mysql Using Java

Load XML into MySQL by using Java DOM Parser.

“Never memorize something that you can look up.”
― Albert Einstein

1. Introduction

XML provides the ability to represent hierarchical structure with its parent-child relationships. This enables applications to store structured data in XML for export. Importing this XML data into a database is a bit involved as we shall see in this article. You need to write code to manage the database connection. In addition you need parse the XML and isolate the data that needs to be imported.

Continue reading “Load XML into Mysql Using Java”

Java ArrayList Examples

1. Introduction

The ArrayList in Java is one of the most used classes in the JDK. It is an implementation of a resizable array with a range of useful methods for manipulating the array. In this article, we present some of the usage patterns of the ArrayList.

The ArrayList is a concrete implementation of the List interface. As such, it is fine to refer to List instead of ArrayList when the implementation detail does not matter. The examples below use List instead of ArrayList when this difference is not relevant.

2. Array vs ArrayList

A regular array in java is a container which holds a fixed number of elements of a specific type. The following (stringArray) is a declaration of an array of String objects and is initialized to an array enough to store 20 strings.

String[] stringArray = new String[20];

An ArrayList for storing Strings is defined as shown below. The number 20 is an indication to the ArrayList class to reserve enough space for 20 strings.

ArrayList<String> stringList = new ArrayList<String>(20);

Some of the differences between the two:

  • Addition: The stringArray can store upto 20 strings. If you need to store more later on, you will have to re-initialize the array and copy the contents from the old array. With the ArrayList, you can add more than 20 strings without worrying about the capacity.
  • Removal: With the ArrayList it is easy to remove entries, either at a particular index or a range or a particular object. Not so easy with the array. You will need loops and other boiler-plate code to manage the removal in your code.

3. Creating an ArrayList

As already shown above you can create an ArrayList using the constructor. To create an empty ArrayList with a capacity for 10 Objects:

ArrayList<Object> objArray = new ArrayList<>();

To create an ArrayList from another Collection such as a Set or a List, you can use:

Set<String> stringSet = ..;
List<String> list = new ArrayList<String>(stringSet);

What if you need to create a List from a bunch of objects already lying around in your code? You can use Arrays.asList() as follows:

List<String> list = Arrays.asList("Apple", "Banana", "Orange");

The following selects lines from a text file using Java 8 streams.

List<String> lines = Files
    .lines(Paths.get(textFile))
    .filter(x -> x.length() > 50)
    .collect(Collectors.toList());

4. Convert Array to ArrayList

As presented above, it is simple to convert to a List if you have a plain array of objects.

List<String> a = Arrays.asList(args);

The above is suitable if you have an array of objects. What if you have an array of a primitive type, say an int? Here is how you can handle that case.

int[] array = { 2, 4, 5 };
List<Integer> list = Arrays
    .stream(array)
    .boxed()
    .collect(Collectors.toList());

5. Convert ArrayList to Array

The List interface provides a method toArray() which creates and returns a new array containing all the elements in the ArrayList.

List<String> strList = ...;
String[] strArray = strList.toArray();

6. Add to ArrayList

Use the method add() to add items to a List. This invocation adds elements at the end of the list.

List<String> strList = new ArrayList<>();
strList.add("Apple");
strList.add("Banana");
strList.add("Orange");
strList.stream().forEach(System.out::println);

// prints
Apple
Banana
Orange

To add all elements of a collection, use addAll().

List<String> modList = new ArrayList<>();
modList.addAll(strList);
modList.add("Melon");

7. UnsupportedOperationException

The following code results in an UnsupportedOperationException. The reason is than Arrays.asList() returns a fixed sized list backed by the input array. So you cannot add elements to the List returned by Arrays.asList().

A simple solution is to create a new ArrayList from the original list as follows:

List<String> strList = Arrays.asList("Apple", "Banana", "Orange");
List<String> modList = new ArrayList<>(strList);
modList.add("Melon");

Or you could use Java 8 streams:

List<String> modList = strList
	.stream()
	.collect(Collectors.toList());
modList.add("Melon");
modList.stream().forEach(System.out::println);

// prints:
Apple
Banana
Orange
Melon

8. Remove from ArrayList

Removal of items from a List is also easy. Remove an element at a particular index.

List<String> modList = new ArrayList<>();
modList.addAll(strList);
modList.add("Melon");
modList.remove(2);

// contains:
Apple
Banana
Melon

Remove a particular object as shown below. Note: The equals() method is used for checking whether to remove the object. It works as shown for String. For another class, equals() might be defined in terms of memory addresses, in which case the same object instance must be passed in to remove it.

List<String> modList = new ArrayList<>();
modList.addAll(strList);
modList.add("Melon");
modList.remove("Orange");

// contains
Apple
Banana
Melon

What if you want to remove an object with some specific characteristic. Use the removeIf() method with the predicate. The methods removes all elements that match the predicate.

List<String> modList = new ArrayList<>();
modList.addAll(strList);
modList.add("Melon");
modList.removeIf(x -> x.endsWith("e"));

// contains
Banana
Melon

Remove all objects from one List that are found in another List using removeAll().

List<String> modList = new ArrayList<>();
modList.addAll(strList);
modList.add("Melon");
modList.removeAll(Arrays.asList("Banana", "Kiwi"));

// contains
Apple
Orange
Melon

Summary

This article explained Java arrays and ArrayList from the perspective of a beginner. We showed creation and initialization of ArrayList. The ArrayList provides a bunch of methods to add and remove elements from the list. In some cases, the List created may not be modifiable in which case an exception is thrown when you try to modify it.

Java File Examples

1. Introduction

Java provides the File class as an abstraction of file and directory path names. A path is presented as a hierarchical operating system independent view of files.

The File class supports a bunch of operations on files and directories which work on both Unix-like and Windows systems. Let us look at some of these operations in more detail.

2. Creating a New File

To create a new file, use the method createNewFile(). The following creates a new file “joe” in the current directory.

File file = new File("joe");
if ( file.createNewFile() ) System.out.println("created");

If a file specified already exists, the method returns false. In this case, the existing file is not touched or modified.

2.1. Hierarchy Not Created

The method does not automatically create the directory hierarchy if any of the parent directories do not exist. The following throws an exception if directory “joe” does not exist in the current directory.

File file = new File("joe/jack");
if ( file.createNewFile() ) System.out.println("created.");

// throws: java.io.IOException: Not a directory

However, if you run the above code after creating the parent directory, it works normally.

2.2. Creating Absolute Path File

In addition to specifying relative paths as above, you can also specify an absolute path as shown below:

File file = new File("/tmp/joe");
if ( file.createNewFile() ) System.out.println("created.");

// prints "created."

2.3. Permission Denied

When attempting to create a file where you don’t have permission, you get an exception:

File file = new File("/joe");
if ( file.createNewFile() ) System.out.println("created.");

// throws java.io.IOException: Permission denied

3. Delete a File

The File class provides a delete() method to delete a file. It is used as follows:

File file = new File("joe");
if ( file.delete() ) System.out.println("deleted.");

The method does not throw an exception if the file does not exists. It just returns false.

4. List Files

When a File object is created with a directory, you can use the list() method to list the files and directories in that directory. Here is an example of listing the files in the current directory using Java 8 streams.

Arrays.stream(file.list()).forEach(System.out::println);

You can also specify a FilenameFilter to select the required files. FilenameFilter is a functional interface so let us use a lambda expression to select and print the “.class” in the directory.

File file = new File(dirPath);
Arrays
    .stream(file.list((d, f) -> f.endsWith(".class")))
    .forEach(System.out::println);

In the above example, we use the FilenameFilter for filtering files by name. However, FileFilter is another functional interface provided by java that can be used to select files by some file property. In the following example, we select empty files.

File file = new File(path);
File[] emptyFiles = Arrays
    .stream(file.listFiles(f -> f.length() == 0));

Or select hidden files for processing.

File file = new File(path);
List<File> hiddenFiles = Arrays
    .stream(file.listFiles(f -> f.isHidden()))
    .collect(Collectors.toList());

5. Creating a Directory

The File class provides the method mkdir() to create a directory. It returns false if the directory cannot be created which could be for a variety of reasons. In other words, exceptions are not thrown including for the following reasons:

  • File or directory already exists.
  • No permission.
  • Hierarchy does not exist.
File file = new File(path);
if ( file.mkdir() ) System.out.println("created.");

If you want to create all non-existent directories of the hierarchy, use mkdirs() instead.

6. Renaming a File

To rename a file, use renameTo(). You must pass in a target File object to rename the file to.

File source = new File(pathA);
File target = new File(pathB);
if ( source.renameTo(target) ) System.out.println("renamed.");

Note that this method comes with a bunch of caveats including:

  • Might not be able to move a file from one file system to another.
  • Might not work if the target path exists. (In other words, might not overwrite the target file.)
  • It might not be atomic. In other words, removing the old file and creating the new file are two separate operations in which one might fail!

With all these warnings, it is better to check the return value to be sure the operation succeeded.

Summary

The Java File class is an system independent abstraction of file and directory paths. It provides a bunch of operations to create a file, remove a file, create a directory including the hierarchy and renaming files.

Java Enum Examples

1. Introduction

Java has provided native enum types from version 1.5 onwards. Some issues arise with the use of enums in java code which this article attempts to address.

2. Enum Abstract Method

An enum type can have abstract methods just like a class. Each enum constant needs to implement the abstract method. An example follows:

public enum Animal {
  Dog { String sound() { return "bark"; } },
  Cat { String sound() { return "meow"; } },
  Lion { String sound() { return "roar"; } },
  Snake { String sound() { return "hiss"; } };

  abstract String sound();
};

Use it as follows:

String str = "Dog";
Animal animal = Animal.valueOf(Animal.class, str);
System.out.println(animal + " makes sound: " + animal.sound());

// prints
Dog makes sound: bark

3. String to Enum

Use valueOf() to lookup an enum by the name.

private enum Day {
    SUNDAY, MONDAY, TUESDAY, WEDNESDAY,
    THURSDAY, FRIDAY, SATURDAY 
};
Day day = Day.valueOf(Day.class, "MONDAY");

The method throws an IllegalArgumentException if the name (with the exact case) is not found.

Day day = Day.valueOf(Day.class, "Monday");

// throws an IllegalArgumentException

4. String to Enum Ignore Case

To lookup an enum by string ignoring the case, you can add a static method to the enum class and use it as shown.

static public Day forNameIgnoreCase(String value) {
    for (Day day : Day.values()) {
	if ( day.name().equalsIgnoreCase(value) ) return day;
    }
    return null;
}

No exceptions are thrown by this code.

String args[] = { "joe", "monday", "Monday", "MONDAY" };
for (String arg : args) {
    Day day = Day.forNameIgnoreCase(arg);
    System.out.println(arg + " => " + day);
}

// prints:
joe => null
monday => MONDAY
Monday => MONDAY
MONDAY => MONDAY

5. EnumSet – Set of Enums

Java also provides a new type EnumSet which is a Set of Enums and provides a bunch of useful operations.

Select a range of enums using EnumSet.range(). Enum constants are returned in the order of declaration.

for (Animal animal : EnumSet.range(Animal.Cat, Animal.Snake)) {
    System.out.println(animal);
}

The EnumSet can also be used as a type-safe alternative to traditional bit flags.

EnumSet.of(Style.BOLD, Style.ITALIC)

You can also add to the set using the normal Set.add() operation. Note that the order of looping is the declared order (and not the add order).

EnumSet<Animal> set = EnumSet.of(Animal.Cat);
set.add(Animal.Dog);
for (Animal animal : set) {
    System.out.println(animal);
}

// prints
Dog
Cat

Remove also works in a similar way. In the following example, we are adding all the enum constants to the set using EnumSet.allOf().

EnumSet<Animal> set = EnumSet.allOf(Animal.class);
set.remove(Animal.Snake);
for (Animal animal : set) {
 System.out.println(animal);
}

// prints
Dog
Cat
Lion

6. EnumMap – Enum as a Key

EnumMap is a specialized implementation of Map provided for cases where an enum is used as the key. According to the javadocs, the storage is compact and efficient. It is used similar to the way regular Maps are used with some change in construction; the enum class must be passed in to the constructor.

EnumMap<Animal,String> sounds = new EnumMap<Animal,String>(Animal.class);
sounds.put(Animal.Dog, "Bark");
sounds.put(Animal.Cat, "Meow");
for (Map.Entry<Animal,String> e : sounds.entrySet()) {
    System.out.println(e.getKey() + " => " + e.getValue());
}

// prints:
Dog => Bark
Cat => Meow

7. Enum Name Map

The implementation of the values() method creates an array every time it is invoked. To avoid invoking this method too many times, you can create a name map and use it for lookup. (Yes, that might possibly count as “premature optimization” but hopefully you are resorting to this approach only when invoking values() multiple times.)

static private enum Period {
    Day, Week, Fortnight, Month, Year;

    private static final Map<String,Period> nameMap = new HashMap<>();
    static {
	for (Period period : Period.values())
	    nameMap.put(period.name(), period);
    };

    static public Period forName(String value)
    {
	return nameMap.get(value);
    }
};

And use it like this. Note again that looking up a non-existent name does not result in an IllegalArgumentException.

String[] args = { "joe", "Day", "Week" };
for (String arg : args) {
    Period period = Period.forName(arg);
    System.out.println(arg + " => " + period);
}

// prints:
joe => null
Day => Day
Week => Week

8. Comparing Enums: == or equals()?

When comparing enum instances, what should you use?

Day day = ...;
if ( day == Day.MONDAY ) {
  // code here
}

if ( day.equals(Day.MONDAY) ) {
  // code here
}

Both are correct. In fact, equals() is implemented using ==. Since == never throws a NullPointerException, one might prefer using that.

9. Should I Use Enum Ordinals?

Enum ordinal is the index of the enum in the list returned by values().

Day[] days = Day.values();
for (int i = 0 ; i < days.length ; i++ ) {
    System.out.println(i + " => " + days[i]);
}

// prints:
0 => SUNDAY
1 => MONDAY
2 => TUESDAY
3 => WEDNESDAY
4 => THURSDAY
5 => FRIDAY
6 => SATURDAY

Sometimes you may want to store or transmit the ordinal as a part of storing the state. Should you use the ordinal in such cases? For instance:

System.out.println("4 => " + days[4]);

// prints
4 => THURSDAY

The answer is no, it is not a good idea to store or use the ordinal. It is a much better idea to store and transmit the name. Since the values() method returns the array in the order of declaration, using the ordinal might return wrong values if the enum definition is modified to add or remove entries.

Store and use the name. If the enum entry is removed later, valueOf() will throw an exception. Which is much better than using wrong values and not knowing about it.

Summary

We have now learnt some basics about enums in java. Enums in java are more powerful than in most other languages. Abstract methods can be declared for the enum and specialized implementation can be defined for each enum constant. Looking up enum constants in a case-insensitive operation is another area arising frequently.