To get sensitive user input without echoing it to the console/terminal, the Java SDK has introduced the java.io.Console class. The Console class is quite unique in the way it handles its data. It uses the native encoding of the system instead of the using the JVM’s default encoding.

The Console class provides methods to access the console/terminal, if any is associated with the JVM.

Using the Console class:
So how do you use the Console class ? Its pretty simple really. First get a reference to the Console class from the System class

Console con = System.console();

It is not always possible to get a reference to the console. Some scenarios prevent the developer from gaining access to the console. This method may return null in these cases. For example a batch process running on a server will not expect user input and usually does have a console. Your code must be capable of handling such situations.

Consider that you have a program that requires the user to secretly type in his/her password.

Entering sensitive data through the console:

Console con = System.console();
String user = con.readLine("Username: ");
char[] password = con.readPassword("Password: ");

If you were to run this code using the eclipse IDE it would not work (At least in eclipse 3.2).
This is because eclipse does not handle this code properly as yet. Run this code from the command prompt for now. Here is the output of the program

Output:

java com.Tests.Cert
Username: hello
Password:

Even though the output does not show a password being typed in, the user has typed it in on the console. The console’s feature allows the user to type in the password without it being echoed to the screen.

The line of code con.readLine(“Username: “); reads data from the console normally and stores it into a string. It also echoes whatever the user types onto the screen (you can see “hello” on the output). The line of code con.readPassword(“Password: “); prompts the user for a password but does not echo the contents of the password to the screen. This allows the user to enter his/her password safely using the Console class. To format the output to the screen the following code can be used.

Formatting data:

Console con = System.console();
String user = con.readLine("Welcome %s. Please login \nUsername:","Guest");
char[] password = con.readPassword("What is your password %s:",user);
System.out.println("User: " + user );
System.out.println("Password: " + new String (password) );

Output:

java com.Tests.Cert
Welcome Guest. Please login
Username: hello
What is your password hello:
User: hello
Password: bla

Notice that although the password is not visible in the console screen, the program was able to retrieve it. Also the line of code con.readPassword(“What is your password %s: “,user); will format the output such that the username is present when asking for the password. %s represents the String that was passed to the method. Now let’s take a look at using the format() method.

Formatting output only:

Console con = System.console();
con.format("Welcome %s \n", "Mr bond");
String user = con.readLine("Username: ");
char[] password = con.readPassword("What is your password %s:",user);
System.out.println("User: " + user );
System.out.println("Password: " + new String (password) );

Output:

Welcome Mr bond
Username: 007
What is your password 007:
User: 007
Password: goldeneye

In this case the formatting that was present in the readLine() method has been transferred to the format() method. Console also has a printf() method that works similar to the format() method. This method however has been made available only for convenience. Finally you can make use of the reader and writer classes to read and write to the Console.

Reader and Writer:
Note: This code throws IOException since we need read/write access to the console

char buffer[]= new char[1024];
Console con = System.console();
Reader reader = con.reader();
int numRead = reader.read(buffer); // #1
System.out.println(new String (buffer).substring(0, numRead) );
Writer writer = con.writer(); // #2
writer.write(buffer);
writer.flush();
writer.close();

Output:

reader -- #a. Input provided by a user
reader -- #b. Written by System.out
reader -- #c. Written by console writer

The #a reader is the input that was provided to the program. The program reads this data using the Reader class. It reads the data into a character buffer in line #1 and prints the data to the console . At line #2 the console’s writer is obtained by the program and the contents of the character buffer are sent to the console output again (this time using System.out). Be wary however, when you use the Reader and Writer classes. They throw the IOException and since this is a checked Exception it must be handled. The code above throws this exception from a method (not shown) and thus need not handle it.

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Sets:

A Set is a collection that contains no duplicate elements. Sets are collections where no two elements are the same, which means that element1.equals(element2) should never return true. Also at most one element in a set can be null and no more.

A Set. Duplicates are not allowed and at most one null element is allowed

Sorted Sets:

A SortedSet is a set which sorts and maintains the ordering of its elements. Elements will be ordered by their natural ordering or by the use of a Comparator (if the developer provides one). A Comparable interface can help define the ordering for elements. For example a String’s natural ordering will dictate that elements be ordered this way “a”, “b”, “c” etc. The way in which elements are ordered in a Sorted data structure can be redefined by using the Comparator interface.

Sorted set where elements are sorted by natural order.

Navigable set:

A navigable set is a sorted set. Technically this means that a NavigableSet extends from a SortedSet. The navigable set has a couple of methods that will return the closest matching element given a search target. A NavigableSet can be accessed in the ascending or descending order. To get an ascending view of this structure the iterator() method can be used like in all Collection classes. To get a descending view of this structure use the descendingIterator() method. This method returns an Iterator whose elements are in the reverse natural order. That.s a fancy way of saying .in descending order.

The implementations of NavigableSet are the TreeSet and the ConcurrentSkipListSet. Let us take an example of the Iteration orders of the navigable set

Code:

public static void main(String[] args) 
{
    NavigableSet <Integer> navigableSet  = new TreeSet<Integer>();
    navigableSet.add(10);
    navigableSet.add(20);
    navigableSet.add(-10);
    navigableSet.add(40);
    navigableSet.add(90);
    Iterator< Integer> iterator = navigableSet.iterator();
    Iterator< Integer> reverseIterator = navigableSet.descendingIterator();
    System.out.println("Forward ahoy !");
    while (iterator.hasNext())
    {
        System.out.println(iterator.next());
    }
    System.out.println();
    System.out.println("Reverse yoha !");
    while (reverseIterator.hasNext())
    {
        System.out.println(reverseIterator.next());
    }
}

Output:

The output of this program would be:
Forward ahoy !
-10
10
20
40
90
 
Reverse yoha !
90
40
20
10
-10

As expected the Iterators iterate through the set in the natural and reverse natural order. Tree sets also automatically sort their elements. There are some key methods that make the NavigableSet useful in certain scenarios. We will have a look at them now.

Key methods in the NavigableSet:

Don’t worry if some of these methods do not make sense. We will take a look at them with an example

A small program illustrates each method. Assume that a NavigableSet is inserted with the following data just like before.

Values added into the set in this order:

Let’s say the following lines were executed in the program

System.out.println(navigableSet.floor(5));
System.out.println(navigableSet.ceiling(5));
System.out.println(navigableSet.lower(30));
System.out.println(navigableSet.higher(30));
System.out.println(navigableSet.higher(90));

The output would be as follows:

-10
10
20
40
Null

To explain the output, let’s arrange the elements first, the way a sorted set would see them

When the Set encounters methods like floor(),ceiling() etc, this is what it translates to.

floor(5) – Give me the element that is less than or equal to 5. Result is -10

ceiling(5) - Give me the element that is greater than or equal to 5. Result is 10

lower(30) - Give me the element that is less than 30. Result is 20

higher(30) - Give me the element that is greater than 30. Result is 40

Any request for an element that does not match the criteria will result in a null return. For example a request to the set to return something higher than 90 gives null since 90 is the highest value in the set.

More examples:

System.out.println(navigableSet.subSet(10,true,40,true));
System.out.println(navigableSet.tailSet(-10, false));
System.out.println(navigableSet.headSet(40, true));
System.out.println(navigableSet.descendingSet());

The output:

[10, 20, 40]
[10, 20, 40, 90]
[-10, 10, 20, 40]
[90, 40, 20, 10, -10]

Again, given the sorted set this result can be explained.

subSet(10,true,40,true)- Give me all the elements between 10 and 40, inclusive of 10 and 40.

tailSet(-10, false) - Give me all the elements after -10, exclusive of -10.

headSet(40, true)- Give me all the elements before 40, inclusive of 40.

descendingSet()- Show me the Navigable set in descending order

The descending set is nothing but the set in reverse order. It is helpful if you need the entire set in descending order instead of a descending Iterator to the set. That laves us with pollFirst() and pollLast().

Polling through navigable sets:

System.out.println(navigableSet);
System.out.println(navigableSet.pollFirst());
System.out.println(navigableSet.pollLast());
System.out.println(navigableSet);

Output:

[-10, 10, 20, 40, 90]
-10
90
[10, 20, 40]

The pollFirst() method retrieves and removes the first value in the set. Which is -10. The pollLast() method retrieves and removes the last value in the set, which is 90. As a result the set.s final size is reduced by 2. After removal of -10 and 90 from the set, the final set looks like this [10, 20, 40].

The NavigableMap API is very similar to that of the NavigableSet (It is unfair to compare a Map and Set, but you get the picture). Now that you know how to use a NavigableSet, learning the NavigableMap should easy.

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