Chapter 13

The Utilities Package

by Michael Girdley and Richard Lesh

• Linked Lists, Queues, Search Trees, and Other Dynamic Data Structures
• Using the Utilities Package
  • The Enumeration Interface
  • The Observer Interface
• Classes
  • The BitSet Class
  • The Date Class
  • The Random Class
  • The StringTokenizer Class
  • The Vector Class
  • The Stack Class
  • The Dictionary Class
  • The Hashtable Class
  • The Properties Class
  • The Observable Class
• Summary

This chapter describes the classes in the java.util package in the Java class library. These classes implement many of those features or functions usually left for the programmer or someone else to implement. In programming experiences, I regularly find myself saying, "It would be so much easier if there were a built-in object that would do some common but complicated task." The java.util package is a well-designed and effective attempt to satisfy many of these specialized needs.

In many languages, you find yourself implementing a stack or a hash table class and all the corresponding methods. Java has a built-in stack type that enables you to quickly and efficiently include your own stack data structures in your Java programs. This frees you to deal with more important design and implementation issues. These classes are also useful in a variety of other ways and are the fundamental building blocks of the more complicated data structures used in other Java packages and in your own applications.

This chapter covers the following topics:

• Each of the features of the utilities package
• The implementation of each of the different classes in the utilities package

Table 13.1 shows the classes that are part of the utilities package and that are discussed in this chapter.

Linked Lists, Queues, Search Trees, and Other Dynamic Data Structures

One would expect that the Vector class would eliminate the necessity for creating your own data structures. But there may be times when you want to conserve space to the maximum or access your data in a specialized way. In these cases, there is a technique to implement such data structures in Java.

As you know, Java has no pointers. Because dynamically linked lists and queues are implemented using pointers, is it then impossible to create these two data structures in Java? Not quite. Just as with many other tasks in Java, you have to do a little "funky stepping" to get it right because the implementation of lists, queues, and other dynamic data structures is not intuitive.

To define your own dynamic data structures, you will want to make use of the fact that references to objects in Java are already dynamic. This is demonstrated and necessitated by the practices Java uses, such as interfaces and abstract implementations.

If you are accustomed to implementing dynamically linked lists or queues in C++, the format you use in Java to create your own version of these structures should seem very familiar to you. For example, the following code creates a Node class for the list that contains a string:

class Node {
    String Name;
    Node Prev;
    Node Next;
}

Of course, this code creates a doubly linked list, which has links backward and forward to other nodes containing strings. You could easily convert this type to link objects in just about any way to exhibit just about any behavior you want: queues, stacks (remember, there is already a Stack object in the class library), doubly linked lists, circular lists, binary search trees, and so on.

To implement such a list, you create a DoubleList class that contains one such Node object and links strung out from there. You can use the keyword null to represent an empty object. Here is an example of the DoubleList declaration:

class DoubleList {
    // Declare the listhead to be of the Node type we created earlier.
    // Also, set it to be an empty object.
    Node ListHead = null;
.
.
}

You then create methods to act on the list, such as InsertNode() or ClearMyListJerk()--whatever you want.

You may also want to create a constructor method for the Node class that accepts parameters to set the previous and next nodes at construction time; or you may want to create a method such as SetNext() or SetNextToNull(). Either choice would work just fine.

Using the Utilities Package

The utilities package has two interfaces you can use in classes of your own design: Enumeration and Observer. An interface is a set of methods that must be written for any class that claims to implement the interface. This arrangement provides a way to consistently use all classes that implement the interface. Following is a summary of the Enumeration and Observer interfaces:

The Enumeration interface is used for classes that can retrieve data from a list, element by element. For example, there is an Enumeration class in the utilities package that implements the Enumeration interface for use with the Vector class. This frees you from hard-core traversal of the different classes of data structures.

The Observer interface is useful in designing classes that can watch for changes that occur in other classes.

The Enumeration Interface

The Enumeration interface specifies a set of methods used to enumerate--that is, iterate through--a list. An object that implements this interface can be used to iterate through a list only once because the Enumeration object is consumed through its use.

For example, an Enumeration object can be used to print all the elements of a Vector object, v, as follows:

for (Enumeration e=v.elements();e.hasMoreElements();)
    System.out.print(e.nextElement()+" ");

The Enumeration interface specifies only two methods: hasMoreElements() and nextElement(). The hasMoreElements() method must return true if there are elements remaining in the enumeration. The nextElement() method must return an object representing the next element within the object that is being enumerated. The details of how the Enumeration interface is implemented and how the data is represented internally are left up to the implementation of the specific class.

The Observer Interface

The Observer interface, if implemented by a class, allows an object of the class to observe other objects of the class Observable. The Observer interface is notified whenever the Observable object that it is watching changes.

The interface specifies only one method, update(Observable, Object). This method is called by the observed object to notify the Observer of changes. A reference to the observed object is passed along with any additional object that the observed object wants to pass to the Observer. The first argument enables the Observer to operate on the observed object; the second argument is used to pass information from the observed to the Observer.

Classes

The utilities package supplies 10 different classes that provide a wide variety of functionality. Although these classes don't generally have much in common, they all provide support for the most common data structures used by programmers. The techniques described in the next sections enable you to create your own specialized classes to supplement those missing from the package.

The classes supplied in the java.util package, however limited they are, do provide a great advantage over other languages. The main advantage is that these classes simplify some things and eliminate a lot of the garbage you were stuck with in the past, in terms of freeing memory and doing mundane programming tasks.

However, there are a number of limitations. For example, you have to "dance a little bit" to implement some of the more complicated data structures. And if you want speed, there are much faster languages to choose from. Java provides a combination of power and simplicity while sacrificing speed. However, don't worry that your programs will be slugs. Although Java is not nearly as efficient as C++ and C, it still beats Visual Basic in terms of size and speed.

The BitSet Class

The BitSet class implements a data type that represents a collection of bits. The collection grows dynamically as more bits are required. The class is useful for representing a set of true and false values. Specific bits are identified using nonnegative integers. The first bit is bit 0.

The BitSet class is most useful for storing a group of related true/false values, such as user responses to Yes and No questions. For example, if the applet has a number of radio buttons, you can slap those values into an instance of the BitSet class.

The class is also useful in terms of bitmapping your own graphics. You can create bitsets that represent a pixel at a time (of course, it would be much easier to use the Graphics class for this purpose instead).

Individual bits in the set are turned on or off with the set() and clear()methods. Individual bits are queried with the get() method. These methods all take the specific bit number as their only argument. The basic boolean operations AND, OR, and XOR can be performed on two bitsets using the and(), or(), and xor() methods. Because these methods modify one of the bitsets, you generally use the clone() method to create a duplicate of one bitset, and then use the AND, OR, or XOR operation on the clone with the second bitset. The result of the operation then ends up in the cloned bitset. The BitSet1 program in Listing 13.1 shows the basic BitSet operations.

Listing 13.1. BitSet1.java: a sample BitSet program.

import java.io.DataInputStream;
import java.util.BitSet;
class BitSet1 {
    public static void main(String args[])
        throws java.io.IOException
    {
        DataInputStream dis=new DataInputStream(System.in);
        String bitstring;
        BitSet set1,set2,set3;
        set1=new BitSet();
        set2=new BitSet();

        // Get the first bit sequence and store it
        System.out.println("Bit sequence #1:");
        bitstring=dis.readLine();
        for (short i=0;i<bitstring.length();i++){
            if (bitstring.charAt(i)=='1')
                set1.set(i);
            else
                set1.clear(i);
        }
        // Get the second bit sequence and store it
        System.out.println("Bit sequence #2:");
        bitstring=dis.readLine();
        for (short i=0;i<bitstring.length();i++){
            if (bitstring.charAt(i)=='1')
                set2.set(i);
            else
                set2.clear(i);
        }
        System.out.println("BitSet #1: "+set1);
        System.out.println("BitSet #2: "+set2);

        // Test the AND operation
        set3=(BitSet)set1.clone();
        set3.and(set2);
        System.out.println("set1 AND set2: "+set3);

        // Test the OR operation
        set3=(BitSet)set1.clone();
        set3.or(set2);
        System.out.println("set1 OR set2: "+set3);

        // Test the XOR operation
        set3=(BitSet)set1.clone();
        set3.xor(set2);
        System.out.println("set1 XOR set2: "+set3);
    }
}

The output from this program looks like this:

 Bit sequence #1:
1010
Bit sequence #2:
1100
BitSet #1: {0, 2}
BitSet #2: {0, 1}
set1 AND set2: {0}
set1 OR set2: {0, 1, 2}
set1 XOR set2: {1, 2}

Table 13.2 summarizes all the methods available in the BitSet class.

In addition to extending the java.lang.Object class, BitSet implements the java.lang.Cloneable interface. This, of course, allows instances of the object to be cloned to create another instance of the class.

The Date Class

You will regularly run into instances in which you have to access and manipulate dates and times in your applets on the Web. For example, you may want an applet to display the current time or date during its execution. If you are programming a game, you may want to use the system clock to get your elapsed time right.

The Date class is used to represent dates and times in a platform-independent fashion. For example, the current date or a specific date can be printed as shown in Listing 13.2.

Listing 13.2. Date1.java: A sample Date program.

import java.util.Date;

public class Date1{
    public static void main (String args[]){
        Date today=new Date();
        System.out.println("Today is "+today.toLocaleString()+
            " ("+today.toGMTString()+")");

        Date birthday=new Date(89,10,14,8,30,00);
        System.out.println("My birthday is"+
            birthday.toString()+" ("+birthday.toGMTString()+")");

        Date anniversary=new Date("Jun 21, 1986");
        System.out.println("My anniversary is "+
            anniversary+" ("+anniversary.toGMTString()+")");
    }
}

The output from this program looks like this:

Today is 01/21/97 19:55:17 (22 Jan 1997 01:55:17 GMT)
My birthday is Thu Nov 14 08:30:00  1989 (14 Nov 1989 14:30:00 GMT)
My anniversary is Sat Jun 21 00:00:00  1989 (21 Jun 1986 05:00:00 GMT)

The default constructor is used when the current date and time are needed. A specific date and time can be used to initialize a Date object using the constructors that take three, five, and six integers. These constructors allow the date and time to be specified using YMD, YMDHM, or YMDHMS formats. Any parts of the time not specified by the three-integer and five-integer constructors are set to zero.

Alternatively, a Date object can be constructed using a single string that represents a date and time using a variety of different syntax. One of the most important is the international standard date syntax of the form, "Sun, 14 Aug 1995 9:00:00 GMT." Continental U.S. time zone abbreviations are understood, but time zone offsets should be considered for general use; for example, "Sun, 14 Aug 1995 9:00:00 GMT+0600" (six hours west of the Greenwich meridian). The local time zone to the computer executing the code is assumed if none is supplied.

The date can be converted to a text representation using the methods toString(), toGMTString(), and toLocaleString(), which convert the date and time to the standard UNIX, GMT, or local time formats. The toLocaleString() function is very useful because you do not have to determine what your system's date format is. This may not sound like much, but it is just another piece of the very complicated puzzle that Sun has put together to allow your applets and applications to flow seamlessly into the system on which they are running.

When a date is converted to a string by an automatic coercion, the toString() method is used. The resulting string returned by the toString() function follows UNIX time and date standards.

The Date class also has methods for setting and querying the date and time component values once the Date object is constructed. The individual parts of the date (month, date, year) and time (hours, minutes, seconds) are always specified in local time. When referring to the various parts of the date and time, the first letter of each part is typically used in an abbreviation. For example, YMDHMS indicates that all six parts (year, month, date, hour, minute, second) are present. Each of these parts of the date and time have a specific range of acceptable values, as shown in Table 13.3.

The date and time also can be specified using a single integer UTC value that represents the number of milliseconds that have elapsed since a specific starting date (which may vary from system to system). For UNIX systems, this date is January 1, 1970. The program Date2 in Listing 13.3 shows how this single value corresponds to the normal YMDHMS representation.

Listing 13.3. Date2.java: A sample Date program.

import java.util.Date;

public class Date2{
    public static void main (String args[]){
        Date beginning=new Date(0);
        Date anniversary=new Date("Jun 21, 1986");
        Date today=new Date();

        System.out.println(beginning+"="+beginning.getTime());
        System.out.println(anniversary+"="+anniversary.getTime());
        System.out.println(today+"="+today.getTime());
    }
}

The output from this program looks like this:

Wed Dec 31 18:00:00  1969=0
Sat Jun 21 00:00:00  1986=519714000000
Sun Jan 21 19:55:17  1996=822275717000

Dates can be compared to each other by using this UTC value or by using the method after(), before(), or equals().

Table 13.4 summarizes all the methods available in the Date class.

One of the most helpful methods available in the Date class is the parse() method. This void takes an instance of the String type and then parses that string. The result of that parse() is then placed in the calling instance of the class. If you have a date called ADate, you can set its value to be the date in the SomeString class with this code line:

ADate.parse(SomeString);

You will also find the before() and after() functions useful. They enable you to send in another instance of the Date class and compare that date to the value in the calling instance.

The sample applet in Listing 13.4 demonstrates the use of the Date class.

Listing 13.4. Using the Date class.

import java.awt.*;
import java.util.*;

public class MichaelSimpleClock extends java.applet.Applet {

    Date TheDate = new Date();
    Button DateButton = new Button(
        "                   Click me!                     ");

    public void init()  {
        add(DateButton);
    }

    public boolean handleEvent(Event e) {
        if (e.target == DateButton) {
            DateButton.setLabel(TheDate.toString());
        }
        return true;
    }

}

Figure 13.1 shows the MichaelSimpleClock applet. Note that the clock in the applet is wrong: it is not actually 8:00 a.m. There is no way I would write that early in the morning.

Figure 13.1 : The MichaelSimpleClock applet.

What about a real-time clock that updates as the clock changes? To accomplish this small feat, you must include in the applet a loop that has each iteration reconstructing the internal Date instance. Then, you regularly repaint that value inside the applet's paint() method. You also have to include threading to keep your system from locking up during the applet's execution. Threading is covered in Chapter 9 "Threads and Multithreading," so a real-time clock was not included in this chapter.

The Random Class

For the programming of games and many other program types, it is important to be able to generate random numbers. Java includes the capability to generate random numbers efficiently and effectively.

The Random class implements a pseudo-random number data type that generates a stream of seemingly random numbers. To create a sequence of different pseudo-random values each time the application is run, create the Random object as follows:

Random r=new Random();

This statement seeds the random generator with the current time. On the other hand, consider the following statement:

Random r=new Random(326);    // Pick any value

This statement seeds the random generator with the same value each time, resulting in the same sequence of pseudo-random numbers each time the application runs. The generator can be reseeded at any time using the setSeed() method.

Pseudo-random numbers can be generated using one of these functions: nextInt(), nextLong(), nextFloat(), nextDouble(), or nextGaussian(). The first four functions return integers, longs, floats, and doubles. For more information about the Gaussian distribution, refer to the following sidebar. The program Random1 in Listing 13.5 prints out pseudo-random uniformly distributed values using these functions.

Listing 13.5. Random1.java: A sample Random program.

import java.lang.Math;
import java.util.Date;
import java.util.Random;

class Random1 {
    public static void main(String args[])
        throws java.io.IOException
    {
        int count=6;
        Random randGen=new Random();

        System.out.println("Uniform Random Integers");
        for (int i=0;i<count;i++)
        System.out.print(randGen.nextInt()+" ");
        System.out.println("\n");
        System.out.println("Uniform Random Floats");
        for (int i=0;i<count;i++)
        System.out.print(randGen.nextFloat()+" ");
        System.out.println("\n");

        System.out.println("Gaussian Random Floats");
        for (int i=0;i<count;i++)
            System.out.print(randGen.nextGaussian()+" ");
        System.out.println("\n");

        System.out.println("Uniform Random Integers [1,6]");
        for (int i=0;i<count;i++)
            System.out.print((Math.abs(randGen.nextInt())%6+1)+" ");
        System.out.println("\n");
        }
}

The output from the preceding program looks like this:

Uniform Random Integers
1704667569 -1431446235 1024613888 438489989 710330974 -1689521238

Uniform Random Floats
0.689189 0.0579988 0.0933537 0.748228 0.400992 0.222109

Gaussian Random Floats
-0.201843 -0.0111578 1.63927 0.205938 -0.365471 0.626304

Uniform Random Integers [1,6]
4 6 1 6 3 2

If you want to generate uniformly distributed random integers within a specific range, the output from nextInt(), nextLong(), or nextDouble() can be scaled to match the required range. However, a simpler approach is to take the remainder of the result of nextInt() divided by the number of different values plus the first value of the range. For example, if the values 10 to 20 are needed, you can use the formula nextInt()%21+10. Unfortunately, although this method is much simpler than scaling the output of nextInt(), it is guaranteed to work only on truly random values. Because the pseudo-random generator may have various undesired correlations, the
modulus operator may not provide acceptable results--you might get all odd numbers, for example. In other words, don't plan on simulating the detonation of your new H-bomb in Java because you may find yourself a couple miles too close.

Table 13.5 summarizes the complete interface of the Random class.

The applet shown in Listing 13.6 demonstrates a bit of what you can do with the Random class.

Listing 13.6. Using the Random class.

import java.awt.*;
import java.util.*;

public class TheWanderer extends java.applet.Applet {

        int xpos = 100;
        int ypos = 100;


    // Our current date
    Date D = new Date();

        // The movement button
        Button theButton = new Button("Click Me");

        // Our random number generator
        Random R;

        public void init() {
                add(theButton);
                setBackground(Color.white);

        // Our random number generator seeded with the current seconds
        int seed = D.getSeconds();
                R = new Random(seed);
        }

        public boolean handleEvent (Event e) {
                if (e.target == theButton) {
                        // Move our thing.
                        xpos = xpos + (Math.abs(R.nextInt())%10-7);
                        ypos = ypos + (Math.abs(R.nextInt())%10-7);

            // repaint the sucker
                        repaint();
        }
                return super.handleEvent(e);
    }

    public void paint(Graphics g) {
                g.setColor(Color.black);
                g.fillOval(xpos,ypos, 50, 50);
    }
}

Figure 13.2 shows TheWanderer applet during its execution.

Figure 13.2 : TheWanderer applet.

The StringTokenizer Class

This section describes the function of the StringTokenizer class, which also could have been appropriately grouped with other classes in Chapter 14, "The I/O Package," because it is so vital to the input and output functions demonstrated in that chapter. The StringTokenizer class enables you to parse a string into a number of smaller strings called tokens. This class works specifically for what is called "delimited text," which means that each individual substring of the string is separated by a delimiter. The delimiter can be anything ranging from a * to YabaDaba. You simply specify what you want the class to look for when tokenizing the string.

This class is included in this chapter because it has uses that prove helpful in everything from a spreadsheet applet to an arcade game applet.

The delimiter set can be specified when the StringTokenizer object is created, or it can be specified on a per-token basis. The default delimiter set is the set of whitespace characters. With this delimiter set, the class would find all the separate words in a string and tokenize them. For example, the StringTokenizer1 code in Listing 13.7 prints out each word of the string on a separate line.

Listing 13.7. StringTokenizer1.java: A sample StringTokenizer program.

import java.io.DataInputStream;
import java.util.StringTokenizer;

class StringTokenizer1 {
    public static void main(String args[])
        throws java.io.IOException
    {
        DataInputStream dis=new DataInputStream(System.in);

        System.out.println("Enter a sentence: ");
        String s=dis.readLine();
        StringTokenizer st=new StringTokenizer(s);
        while (st.hasMoreTokens())
            System.out.println(st.nextToken());
    }
}

Here is the output from this listing:

Enter a sentence:
Four score and seven
Four
score
and
seven

Pure excitement. The method countTokens() returns the number of tokens remaining in the string using the current delimiter set--that is, the number of times nextToken() can be called before generating an exception. This is an efficient method because it does not actually construct the substrings that nextToken() must generate.

In addition to extending the java.lang.object class, the StringTokenizer class implements the java.util.Enumeration interface.

Table 13.6 summarizes the methods of the StringTokenizer class.

The Vector Class

As stated earlier in this chapter, Java doesn't include dynamically linked list, queue, or other data structures of that type. Instead, the designers of Java envisioned the Vector class, which handles the occasions when you need to dynamically store objects. Of course, there are positive and negative consequences of this decision by the designers at Sun. On the positive side, the Vector class contributes to the simplicity of the language. The major negative point is that, at face value, the Vector class severely limits programmers from using more sophisticated programs.

In any case, the Vector class implements a dynamically allocated list of objects. It attempts to optimize storage by increasing the storage capacity of the list when needed by increments larger than just one object. Typically, with this mechanism, there is some excess capacity in the list. When this capacity is exhausted, the list is reallocated to add another block of objects at the end of the list. Setting the capacity of the Vector object to the needed size before inserting a large number of objects reduces the need for incremental reallocation. Because of this mechanism, it is important to remember that the capacity (the available elements in the Vector object) and the size (the number of elements currently stored in the Vector object) usually are not the same.

Suppose that a Vector with capacityIncrement equal to 3 has been created. As objects are added to the Vector, new space is allocated in chunks of three objects. After five elements have been added, there is still room for one more element without the need for any additional memory allocation.

After the sixth element has been added, there is no more excess capacity. When the seventh element is added, a new allocation is made to add three additional elements, giving a total capacity of nine. After the seventh element is added, there are two remaining unused elements.

The initial storage capacity and the capacity increment can both be specified in the constructor. Even though the capacity is automatically increased as needed, the ensureCapacity() method can be used to increase the capacity to a specific minimum number of elements; the trimToSize() method can be used to reduce the capacity to the minimum number of elements needed to store the current amount. New elements can be added to the Vector using the addElement() and insertElementAt() methods. The elements passed to be stored in the Vector must be derived from type Object. Elements can be changed using the setElementAt() method. Removal of elements is accomplished with the removeElement(), removeElementAt(), and removeAllElements() methods. Elements can be accessed directly using the elementAt(), firstElement(), and lastElement() methods; elements can be located using the indexOf() and lastIndexOf() methods. Information about the size and the capacity of the Vector are returned by the size() and capacity() methods. The setSize() method can be used to directly change the size of the Vector.

For example, the Vector1 code in Listing 13.8 creates a Vector of integers by adding new elements to the end. Then, using a variety of techniques, it prints the Vector.

Listing 13.8. Vector1.java: A sample Vector program.

import java.lang.Integer;
import java.util.Enumeration;
import java.util.Vector;

class Vector1 {
    public static void main(String args[]){
        Vector v=new Vector(10,10);
        for (int i=0;i<20;i++)
            v.addElement(new Integer(i));

        System.out.println("Vector in original order using an Enumeration");
        for (Enumeration e=v.elements();e.hasMoreElements();)
            System.out.print(e.nextElement()+" ");
        System.out.println();

        System.out.println("Vector in original order using elementAt");
        for (int i=0;i<v.size();i++)
            System.out.print(v.elementAt(i)+" ");
        System.out.println();

        // Print out the original vector
        System.out.println("\nVector in reverse order using elementAt");
        for (int i=v.size()-1;i>=0;i++)
            System.out.print(v.elementAt(i)+" ");
        System.out.println();

        // Print out the original vector
        System.out.println("\nVector as a String");
        System.out.println(v.toString());
    }
}

The output from this program looks like this:

Vector in original order using an Enumeration
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
Vector in original order using elementAt
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

Vector in reverse order using elementAt
19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

Vector as a String
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19]

Notice the use of the Enumeration object as one way to access the elements of a Vector. Look at the following lines:

for (Enumeration e=v.elements();e.hasMoreElements();)
    System.out.print(e.nextElement()+" ");

You can see that an Enumeration object, which represents all the elements in the Vector, is created and returned by the Vector method elements(). With this Enumeration object, the loop can check to see whether there are more elements to process using the Enumeration method hasMoreElements(); the loop can get the next element in the Vector using the Enumeration method nextElement().

The Vector2 program in Listing 13.9 shows some of the vector-accessing techniques. It first generates a vector of random integers; then it allows the user to search for a specific value. The locations of the first and last occurrences of the value are printed by the program using the indexOf() and lastIndexOf() methods.

Listing 13.9. Vector2.java: Another sample Vector program.

import java.io.DataInputStream;
import java.lang.Integer;
import java.lang.Math;
import java.util.Enumeration;
import java.util.Random;
import java.util.Vector;

class Vector2 {
    public static void main(String args[])
        throws java.io.IOException
    {
        int numElements;
        DataInputStream dis=new DataInputStream(System.in);
        Vector v=new Vector(10,10);
        Random randGen=new Random();

        System.out.println("How many random elements? ");
        numElements=Integer.valueOf(dis.readLine()).intValue();
        for (int i=0;i<numElements;i++)
            v.addElement(new Integer(Math.abs(
                randGen.nextInt())%numElements));

        System.out.println(v.toString());

        Integer searchValue;
        System.out.println("Find which value? ");
        searchValue=Integer.valueOf(dis.readLine());
        System.out.println("First occurrence is element "+
            v.indexOf(searchValue));
        System.out.println("Last occurrence is element "+
            v.lastIndexOf(searchValue));
    }
}

The output from this program looks like this:

How many random elements?
10
[0, 2, 8, 4, 9, 7, 8, 6, 3, 2]

Find which value?
8
First occurrence is element 2
Last occurrence is element 6

In addition to extending the java.lang.Object class, the Vector class implements the java.lang.Cloneable interface. Table 13.7 summarizes the methods of the Vector class.

The Stack Class

The stack data structure is key to many programming efforts, ranging from building compilers to solving mazes. The Stack class in the Java library implements a Last In, First Out (LIFO) stack of objects. Even though they are based on (that is, they extend) the Vector class, Stack objects are typically not accessed in a direct fashion. Instead, values are pushed on and popped off the top of the stack. The net effect is that the values most recently pushed are the first to pop.

The Stack1 code in Listing 13.10 pushes strings onto the stack and then retrieves them. The strings end up printed in the reverse order from which they were stored.

Listing 13.10. Stack1.java: A sample Stack program.

import java.io.DataInputStream;
import java.util.Stack;
import java.util.StringTokenizer;

class Stack1 {
    public static void main(String args[])
        throws java.io.IOException
    {
        DataInputStream dis=new DataInputStream(System.in);

        System.out.println("Enter a sentence: ");
        String s=dis.readLine();
        StringTokenizer st=new StringTokenizer(s);
        Stack stack=new Stack();
        while (st.hasMoreTokens())
            stack.push(st.nextToken());
        while (!stack.empty())
            System.out.print((String)stack.pop()+" ");
        System.out.println();
    }
}

The output from this program looks like this:

Enter a sentence:
The quick brown fox jumps over the lazy dog
dog lazy the over jumps fox brown quick The

Even though Stack objects normally are not accessed in a direct fashion, it is possible to search the Stack for a specific value using the search() method. search() accepts an object to find and returns the distance from the top of the Stack where the object was found. It returns Ð1 if the object is not found.

The peek() method returns the top object on the Stack without actually removing it from the Stack. The peek() method throws an EmptyStackException if the stack has no items.

Table 13.8 summarizes the complete interface of the Stack class.

The Dictionary Class

The Dictionary class is an abstract class used as a base for the Hashtable class. It implements a data structure that allows a collection of key and value pairs to be stored. Any type of object can be used for the keys or the values. Typically, the keys are used to find a particular corresponding value.

Because the Dictionary class is an abstract class that cannot be used directly, the code examples presented in this section cannot actually be run. They are presented only to explain the purpose and use of the methods declared by this class. The following code would, hypothetically, be used to create a Dictionary with these values:

Dictionary products = new Dictionary();
products.put(new Integer(342), "Widget");
products.put(new Integer(124), "Gadget");
products.put(new Integer(754), "FooBar");

The put() method is used to insert a key and value pair into the Dictionary. Both arguments must be derived from the class Object. The key is the first argument, and the value is the second argument.

A value can be retrieved using the get() method and a specific key to be found. get() returns the null value if the specified key is not found. Here's an example:

String name = products.get(new Integer(124));
if (name != null) {
    System.out.println("Product name for code 124 is " + name);
}

Although an individual object can be retrieved with the get() method, it is sometimes necessary to access all the keys or all the values. Two methods, keys() and elements(), return Enumerations that can be used to access the keys and the values.

Table 13.9 summarizes the complete interface of the Dictionary class.

The Hashtable Class

The Hashtable data structure is very useful when searching for and manipulating data. You should use the Hashtable class if you will be storing a large amount of data in memory and then searching it. The time needed to complete a search of a hash table is decidedly less than what it takes to search a Vector. Of course, for small amounts of data, it doesn't make much difference whether you use a hash table or a linear data structure because the overhead time is much greater than any search time would be. See the following sidebar for more information on search times in the different classes.

Hash table organization is based on keys, which are computed based on the data being stored. For example, if you want to insert a number of words into a hash table, you can base your key on the first letter of the word. When you come back later to search for a word, you can then compute the key for the item being sought. By using this key, search time is drastically reduced because the items are stored based on the value of their respective keys.

The Hashtable class implements a hash table storage mechanism for storing key and value pairs. Hash tables are designed to quickly locate and retrieve information stored by using a key. Keys and values can be of any object type, but the key object's class must implement the hashCode() and equals() methods.

The sample Hashtable1 in Listing 13.11 creates a Hashtable object and stores 10 key and value pairs using the put() method. It then uses the get() method to return the value corresponding to a key entered by the user.

Listing 13.11. Hashtable1.java: A sample Hashtable program.

import java.io.DataInputStream;
import java.lang.Integer;
import java.lang.Math;
import java.util.Random;
import java.util.Hashtable;

class Hashtable1 {
    public static void main(String args[])
        throws java.io.IOException
    {
        DataInputStream dis=new DataInputStream(System.in);
        int numElements=10;
        String keys[]={"Red","Green","Blue","Cyan","Magenta",
            "Yellow","Black","Orange","Purple","White"};
        Hashtable ht;
        Random randGen=new Random();

        ht=new Hashtable(numElements*2);
        for (int i=0;i<numElements;i++)
            ht.put(keys[i],new Integer(Math.abs(
                randGen.nextInt())%numElements));

        System.out.println(ht.toString());

        String keyValue;
        System.out.println("Which key to find? ");
        keyValue=dis.readLine();
        Integer value=(Integer)ht.get(keyValue);
        if (value!=null) System.out.println(keyValue+" = "+value);
    }
}

The output from this program looks like this:

{Cyan=4, White=0, Magenta=4, Red=5, Black=3,
Green=8, Purple=3, Orange=4, Yellow=2, _Blue=6}
Which key to find?
Red
Red = 5

In addition to the get() method, the contains() and containsKey() methods can be used to search for a particular value or key. Both return true or false depending on whether the search was successful. The contains() method must perform an exhaustive search of the table and is not as efficient as the containsKey() method, which can take advantage of the hash table's storage mechanism to find the key quickly.

Because hash tables must allocate storage for more data than actually is stored, a measurement called the load factor indicates the number of used storage spaces as a fraction of the total available storage spaces. The load factor is expressed as a value between 0 and 100 percent. Typically, the load factor should not be higher than about 50 percent for efficient retrieval of data from a hash table. When specifying the load factor in a program, use a fractional value in the range 0.0 to 1.0 to represent load factors in the range 0 to 100 percent.

Hash tables can be constructed in three different ways: By specifying the desired initial capacity and load factor; by specifying only the initial capacity; or by specifying neither the initial capacity nor the load factor. If the load factor is not specified, the Hashtable is rehashed into a larger table when it is full--otherwise, it is rehashed when it exceeds the load factor. The constructors throw an IllegalArgumentException if the initial capacity is less than or equal to zero, or if the load factor is less than or equal to zero.

The clone() method can be used to create a copy (a clone) of the Hashtable. However, it creates a shallow copy of the Hashtable, which means that the keys and values themselves are not clones. This local method overrides the inherited clone() method.

The Hashtable class extends the java.util.Dictionary class and implements the java.lang.Cloneable interface. Table 13.10 summarizes the methods of the Hashtable class.

The Properties Class

The Properties class is what enables end-users to customize their Java programs. For example, you can easily store values such as foreground colors, background colors, font defaults, and so on and then have those values available to be reloaded. This arrangement is most useful for Java applications, but you can also implement them for applets. If you have an applet that is regularly used by multiple users, you can keep a properties file on your server for each different user; the properties file is accessed each time that user loaded the applet.

The Properties class is a Hashtable, which can be repeatedly stored and restored from a stream. It is used to implement persistent properties. The Properties class also allows for an unlimited level of nesting by searching a default property list if the required property is not found. The fact that this class is an extension of the Hashtable class means that all methods available in the Hashtable class are also available in the Properties class.

The sample Properties1 program in Listing 13.12 creates two properties lists. One is the default property list and the other is the user-defined property list. When the user property list is created, the default Properties object is passed. When the user property list is searched, if the key value is not found, the default Properties list is searched.

Listing 13.12. Properties1.java: A sample Properties program.

import java.io.Data7InputStream;
import java.lang.Integer;
import java.util.Properties;

class Properties1 {
    public static void main(String args[])
        throws java.io.IOException
    {
        int numElements=4;
        String defaultNames[]={"Red","Green","Blue","Purple"};
        int defaultValues[]={1,2,3,4};
        String userNames[]={"Red","Yellow","Orange","Blue"};
        int userValues[]={100,200,300,400};
        DataInputStream dis=new DataInputStream(System.in);
        Properties defaultProps=new Properties();
        Properties userProps=new Properties(defaultProps);

        for (int i=0;i<numElements;i++){
            defaultProps.put(defaultNames[i],
                Integer.toString(defaultValues[i]));
            userProps.put(userNames[i],
                Integer.toString(userValues[i]));
        }
        System.out.println("Default Properties");
        defaultProps.list(System.out);
        System.out.println("\nUser Defined Properties");
        userProps.list(System.out);

        String keyValue;
        System.out.println("\nWhich property to find? ");
        keyValue=dis.readLine();
        System.out.println("Property '"+keyValue+"' is '"+
            userProps.getProperty(keyValue)+"'");
    }
}

Notice that the getProperties() method is used instead of the inherited get() method. The get() method searches only the current Properties object. The getProperties() method must be used to search the default Properties list. An alternative form of the getProperties() method has a second argument: a Properties list that is to be searched instead of the default specified when the Properties object was created.

The propertyNames() method can be used to return an Enumeration, which can be used to index through all the property names. This Enumeration includes the property names from the default Properties list. Likewise, the list() method, which prints the Properties list to the standard output, lists all the properties of the current Properties object and those in the default Properties object.

Properties objects can be written to and read from a stream using the save() and load() methods. In addition to the output or input stream, the save() method has an additional string argument that is written at the beginning of the stream as a header comment.

Table 13.11 summarizes the methods of the Properties class.

The Observable Class

The Observable class acts as a base class for objects you want to have observed by other objects that implement the Observer interface. An Observable object can notify its Observers whenever the Observable object is modified using the notifyObservers() method. This method accomplishes the notification by invoking the update() method of all its Observers, optionally passing a data object that is passed to notifyObservers(). Observable objects can have any number of Observers.

Table 13.12 summarizes the complete interface of the Observable class.

Summary

This chapter described the classes that make up the Java utilities package. This package provides complete implementations of the basic data structures and some of the most useful data types (other than the fundamental numeric types) needed by programmers. Many of the data types and data structures that you will develop using Java will be based on the classes found in the utilities package. For smaller applets, many of these classes are not necessary. However, as your applets increase in complexity, you will find these classes to be very useful. In any case, this chapter has been a good starting point for understanding the utility of these important Java classes and for understanding how to use them effectively.