Types of collections

Thursday Mar 1st 2001

The standard Java 1.0 and 1.1 library comes with a bare minimum set of collection classes, but they’re probably enough to get by with for many of your programming projects. (As you’ll see at the end of this chapter, Java 1.2 provides a radically redesigned and filled-out library of collections.)

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The standard Java 1.0 and 1.1 library comes with a bare minimum set of collection classes, but they’re probably enough to get by with for many of your programming projects. (As you’ll see at the end of this chapter, Java 1.2 provides a radically redesigned and filled-out library of collections.)


The Vector is quite simple to use, as you’ve seen so far. Although most of the time you’ll just use addElement( ) to insert objects, elementAt( ) to get them out one at a time, and elements( ) to get an Enumeration to the sequence, there’s also a set of other methods that can be useful. As usual with the Java libraries, we won’t use or talk about them all here, but be sure to look them up in the electronic documentation to get a feel for what they can do.

Crashing Java

The Java standard collections contain a toString( ) method so they can produce a String representation of themselves, including the objects they hold. Inside of Vector, for example, the toString( ) steps through the elements of the Vector and calls toString( ) for each one. Suppose you’d like to print out the address of your class. It seems to make sense to simply refer to this (in particular, C++ programmers are prone to this approach):

//: CrashJava.java
// One way to crash Java
import java.util.*;
public class CrashJava {
  public String toString() {
    return "CrashJava address: " + this + "\n";
  public static void main(String[] args) {
    Vector v = new Vector();
    for(int i = 0; i < 10; i++)
      v.addElement(new CrashJava());
} ///:~ 

It turns out that if you simply create a CrashJava object and print it out, you’ll get an endless sequence of exceptions. However, if you place the CrashJava objects in a Vector and print out that Vector as shown here, it can’t handle it and you don’t even get an exception; Java just crashes. (But at least it didn’t bring down my operating system.) This was tested with Java 1.1.

What’s happening is automatic type conversion for Strings. When you say:

"CrashJava address: " + this

The compiler sees a String followed by a ‘ +’ and something that’s not a String, so it tries to convert this to a String. It does this conversion by calling toString( ), which produces a recursive call. When this occurs inside a Vector, it appears that the stack overflows without the exception-handling mechanism getting a chance to respond.

If you really do want to print the address of the object in this case, the solution is to call the Object toString( ) method, which does just that. So instead of saying this, you’d say super.toString( ). (This only works if you're directly inheriting from Object or if none of your parent classes have overridden the toString( ) method).


A BitSet is really a Vector of bits, and it is used if you want to efficiently store a lot of on-off information. It’s efficient only from the standpoint of size; if you’re looking for efficient access, it is slightly slower than using an array of some native type.

In addition, the minimum size of the BitSet is that of a long: 64 bits. This implies that if you’re storing anything smaller, like 8 bits, a BitSet will be wasteful, so you’re better off creating your own class to hold your flags.

In a normal Vector, the collection will expand as you add more elements. The BitSet does this as well – sort of. That is, sometimes it works and sometimes it doesn’t, which makes it appear that the Java version 1.0 implementation of BitSet is just badly done. (It is fixed in Java 1.1.) The following example shows how the BitSet works and demonstrates the version 1.0 bug:

//: Bits.java
// Demonstration of BitSet
import java.util.*;
public class Bits {
  public static void main(String[] args) {
    Random rand = new Random();
    // Take the LSB of nextInt():
    byte bt = (byte)rand.nextInt();
    BitSet bb = new BitSet();
    for(int i = 7; i >=0; i--)
      if(((1 << i) &  bt) != 0)
    System.out.println("byte value: " + bt);
    short st = (short)rand.nextInt();
    BitSet bs = new BitSet();
    for(int i = 15; i >=0; i--)
      if(((1 << i) &  st) != 0)
    System.out.println("short value: " + st);
    int it = rand.nextInt();
    BitSet bi = new BitSet();
    for(int i = 31; i >=0; i--)
      if(((1 << i) &  it) != 0)
    System.out.println("int value: " + it);
    // Test bitsets >= 64 bits:
    BitSet b127 = new BitSet();
    System.out.println("set bit 127: " + b127);
    BitSet b255 = new BitSet(65);
    System.out.println("set bit 255: " + b255);
    BitSet b1023 = new BitSet(512);
// Without the following, an exception is thrown
// in the Java 1.0 implementation of BitSet:
//    b1023.set(1023);
    System.out.println("set bit 1023: " + b1023);
  static void printBitSet(BitSet b) {
    System.out.println("bits: " + b);
    String bbits = new String();
    for(int j = 0; j < b.size() ; j++)
      bbits += (b.get(j) ? "1" : "0");
    System.out.println("bit pattern: " + bbits);
} ///:~ 

The random number generator is used to create a random byte, short, and int, and each one is transformed into a corresponding bit pattern in a BitSet. This works fine because a BitSet is 64 bits, so none of these cause it to increase in size. But in Java 1.0, when the BitSet is greater than 64 bits, some strange behavior occurs. If you set a bit that’s just one greater than the BitSet’s currently-allocated storage, it will expand nicely. But if you try to set bits at higher locations than that without first just touching the boundary, you’ll get an exception, since the BitSet won’t expand properly in Java 1.0. The example shows a BitSet of 512 bits being created. The constructor allocates storage for twice that number of bits. Then if you try to set bit 1024 or greater without first setting bit 1023, you’ll throw an exception in Java 1.0. Fortunately, this is fixed in Java 1.1, but avoid using the BitSet if you write code for Java 1.0.


A Stack is sometimes referred to as a “last-in, first-out” (LIFO) collection. That is, whatever you “push” on the Stack last is the first item you can “pop” out. Like all of the other collections in Java, what you push and pop are Objects, so you must cast what you pop.

What’s rather odd is that instead of using a Vector as a building block to create a Stack, Stack is inherited from Vector. So it has all of the characteristics and behaviors of a Vector plus some extra Stack behaviors. It’s difficult to know whether the designers explicitly decided that this was an especially useful way to do things, or whether it was just a naïve design.

Here’s a simple demonstration of Stack that reads each line from an array and pushes it as a String:

//: Stacks.java
// Demonstration of Stack Class
import java.util.*;
public class Stacks {
  static String[] months = { 
    "January", "February", "March", "April",
    "May", "June", "July", "August", "September",
    "October", "November", "December" };
  public static void main(String[] args) {
    Stack stk = new Stack();
    for(int i = 0; i < months.length; i++)
      stk.push(months[i] + " ");
    System.out.println("stk = " + stk);
    // Treating a stack as a Vector:
    stk.addElement("The last line");
      "element 5 = " + stk.elementAt(5));
    System.out.println("popping elements:");
} ///:~ 

Each line in the months array is inserted into the Stack with push( ), and later fetched from the top of the stack with a pop( ). To make a point, Vector operations are also performed on the Stack object. This is possible because, by virtue of inheritance, a Stack is a Vector. Thus, all operations that can be performed on a Vector can also be performed on a Stack, such as elementAt( ).


A Vector allows you to select from a sequence of objects using a number, so in a sense it associates numbers to objects. But what if you’d like to select from a sequence of objects using some other criterion? A Stack is an example: its selection criterion is “the last thing pushed on the stack.” A powerful twist on this idea of “selecting from a sequence” is alternately termed a map, a dictionary, or an associative array . Conceptually, it seems like a vector, but instead of looking up objects using a number, you look them up using another object ! This is often a key process in a program.

The concept shows up in Java as the abstract class Dictionary . The interface for this class is straightforward: size( ) tells you how many elements are within, isEmpty( ) is true if there are no elements, put(Object key, Object value) adds a value (the thing you want), and associates it with a key (the thing you look it up with). get(Object key) produces the value given the corresponding key, and remove(Object key) removes the key-value pair from the list. There are enumerations: keys( ) produces an Enumeration of the keys, and elements( ) produces an Enumeration of all the values. That’s all there is to a Dictionary.

A Dictionary isn’t terribly difficult to implement. Here’s a simple approach, which uses two Vectors, one for keys and one for values:

//: AssocArray.java
// Simple version of a Dictionary
import java.util.*;
public class AssocArray extends Dictionary {
  private Vector keys = new Vector();
  private Vector values = new Vector();
  public int size() { return keys.size(); }
  public boolean isEmpty() {
    return keys.isEmpty();
  public Object put(Object key, Object value) {
    return key;
  public Object get(Object key) {
    int index = keys.indexOf(key);
    // indexOf() Returns -1 if key not found:
    if(index == -1) return null;
    return values.elementAt(index);
  public Object remove(Object key) {
    int index = keys.indexOf(key);
    if(index == -1) return null;
    Object returnval = values.elementAt(index);
    return returnval;
  public Enumeration keys() {
    return keys.elements();
  public Enumeration elements() {
    return values.elements();
  // Test it:
  public static void main(String[] args) {
    AssocArray aa = new AssocArray();
    for(char c = 'a'; c <= 'z'; c++)
    char[] ca = { 'a', 'e', 'i', 'o', 'u' };
    for(int i = 0; i < ca.length; i++)
      System.out.println("Uppercase: " +
} ///:~ 

The first thing you see in the definition of AssocArray is that it extends Dictionary . This means that AssocArray is a type of Dictionary, so you can make the same requests of it that you can a Dictionary. If you make your own Dictionary, as is done here, all you need to do is fill in all the methods that are in Dictionary. (And you must override all the methods because all of them – with the exception of the constructor – are abstract.)

The Vectors keys and values are linked by a common index number. That is, if you call put( ) with a key of “roof” and a value of “blue” (assuming you’re associating the various parts of a house with the colors they are to be painted) and there are already 100 elements in the AssocArray, then “roof” will be the 101 element of keys and “blue” will be the 101 element of values. And if you look at get( ), when you pass “roof” in as the key, it produces the index number with keys.indexOf( ), and then uses that index number to produce the value in the associated values vector.

The test in main( ) is simple; it’s just a map of lowercase characters to uppercase characters, which could obviously be done in a number of more efficient ways. But it shows that AssocArray is functional.

The standard Java library contains only one embodiment of a Dictionary, called Hashtable.[34] Java’s Hashtable has the same basic interface as AssocArray (since they both inherit Dictionary), but it differs in one distinct way: efficiency. If you look at what must be done for a get( ), it seems pretty slow to search through a Vector for the key. This is where Hashtable speeds things up. Instead of the tedious linear search for the key, it uses a special value called a hash code . The hash code is a way to take some information in the object in question and turn it into a “relatively unique” int for that object. All objects have a hash code, and hashCode( ) is a method in the root class Object. A Hashtable takes the hashCode( ) of the object and uses it to quickly hunt for the key. This results in a dramatic performance improvement. [35] The way that a Hashtable works is beyond the scope of this book [36] – all you need to know is that Hashtable is a fast Dictionary, and that a Dictionary is a useful tool.

As an example of the use of a Hashtable, consider a program to check the randomness of Java’s Math.random( ) method. Ideally, it would produce a perfect distribution of random numbers, but to test this you need to generate a bunch of random numbers and count the ones that fall in the various ranges. A Hashtable is perfect for this, since it associates objects with objects (in this case, the values produced by Math.random( ) with the number of times those values appear):

//: Statistics.java
// Simple demonstration of Hashtable
import java.util.*;
class Counter { 
  int i = 1; 
  public String toString() { 
    return Integer.toString(i); 
class Statistics {
  public static void main(String[] args) {
    Hashtable ht = new Hashtable();
    for(int i = 0; i < 10000; i++) {
      // Produce a number between 0 and 20:
      Integer r = 
        new Integer((int)(Math.random() * 20));
        ht.put(r, new Counter());
} ///:~ 

In main( ), each time a random number is generated it is wrapped inside an Integer object so that handle can be used with the Hashtable. (You can’t use a primitive with a collection, only an object handle.) The containsKey( ) method checks to see if this key is already in the collection. (That is, has the number been found already?) If so, the get( ) methods gets the associated value for the key, which in this case is a Counter object. The value i inside the counter is then incremented to indicate that one more of this particular random number has been found.

If the key has not been found yet, the method put( ) will place a new key-value pair into the Hashtable. Since Counter automatically initializes its variable i to one when it’s created, it indicates the first occurrence of this particular random number.

To display the Hashtable, it is simply printed out. The Hashtable toString( ) method moves through all the key-value pairs and calls the toString( ) for each one. The Integer toString( ) is pre-defined, and you can see the toString( ) for Counter. The output from one run (with some line breaks added) is:

{19=526, 18=533, 17=460, 16=513, 15=521, 14=495,
 13=512, 12=483, 11=488, 10=487, 9=514, 8=523,
 7=497, 6=487, 5=480, 4=489, 3=509, 2=503, 1=475,

You might wonder at the necessity of the class Counter, which seems like it doesn’t even have the functionality of the wrapper class Integer. Why not use int or Integer? Well, you can’t use an int because all of the collections can hold only Object handles. After seeing collections the wrapper classes might begin to make a little more sense to you, since you can’t put any of the primitive types in collections. However, the only thing you can do with the Java wrappers is to initialize them to a particular value and read that value. That is, there’s no way to change a value once a wrapper object has been created. This makes the Integer wrapper immediately useless to solve our problem, so we’re forced to create a new class that does satisfy the need.

Creating “key” classes
In the previous example, a standard library class ( Integer) was used as a key for the Hashtable. It worked fine as a key, because it has all the necessary wiring to make it work correctly as a key. But a common pitfall occurs when using Hashtables when you create your own classes to be used as keys. For example, consider a weather predicting system that matches Groundhog objects to Prediction objects. It seems fairly straightforward: you create the two classes and use Groundhog as the key and Prediction as the value:

//: SpringDetector.java
// Looks plausible, but doesn't work right.
import java.util.*;
class Groundhog {
  int ghNumber;
  Groundhog(int n) { ghNumber = n; }
class Prediction {
  boolean shadow = Math.random() > 0.5;
  public String toString() {
      return "Six more weeks of Winter!";
      return "Early Spring!";
public class SpringDetector {
  public static void main(String[] args) {
    Hashtable ht = new Hashtable();
    for(int i = 0; i < 10; i++)
      ht.put(new Groundhog(i), new Prediction());
    System.out.println("ht = " + ht + "\n");
      "Looking up prediction for groundhog #3:");
    Groundhog gh = new Groundhog(3);
} ///:~ 

Each Groundhog is given an identity number, so you can look up a Prediction in the Hashtable by saying “Give me the Prediction associated with Groundhog number 3.” The Prediction class contains a boolean that is initialized using Math.random( ), and a toString( ) that interprets the result for you. In main( ), a Hashtable is filled with Groundhogs and their associated Predictions. The Hashtable is printed so you can see that it has been filled. Then a Groundhog with an identity number of 3 is used to look up the prediction for Groundhog #3.

It seems simple enough, but it doesn’t work. The problem is that Groundhog is inherited from the common root class Object (which is what happens if you don’t specify a base class, thus all classes are ultimately inherited from Object). It is Object’s hashCode( ) method that is used to generate the hash code for each object, and by default it just uses the address of its object. Thus, the first instance of Groundhog(3) does not produce a hash code equal to the hash code for the second instance of Groundhog(3) that we tried to use as a lookup.

You might think that all you need to do is write an appropriate override for hashCode( ). But it still won’t work until you’ve done one more thing: override the equals( ) that is also part of Object. This method is used by the Hashtable when trying to determine if your key is equal to any of the keys in the table. Again, the default Object.equals( ) simply compares object addresses, so one Groundhog(3) is not equal to another Groundhog(3).

Thus, to use your own classes as keys in a Hashtable, you must override both hashCode( ) and equals( ), as shown in the following solution to the problem above:

//: SpringDetector2.java
// If you create a class that's used as a key in
// a Hashtable, you must override hashCode()
// and equals().
import java.util.*;
class Groundhog2 {
  int ghNumber;
  Groundhog2(int n) { ghNumber = n; }
  public int hashCode() { return ghNumber; }
  public boolean equals(Object o) {
    if ((o != null) && (o instanceof Groundhog2))
        ghNumber == ((Groundhog2)o).ghNumber;
    else return false;
public class SpringDetector2 {
  public static void main(String[] args) {
    Hashtable ht = new Hashtable();
    for(int i = 0; i < 10; i++)
      ht.put(new Groundhog2(i),new Prediction());
    System.out.println("ht = " + ht + "\n");
      "Looking up prediction for groundhog #3:");
    Groundhog2 gh = new Groundhog2(3);
} ///:~ 

Note that this uses the Prediction class from the previous example, so SpringDetector.java must be compiled first or you’ll get a compile-time error when you try to compile SpringDetector2.java .

Groundhog2.hashCode( ) returns the ground hog number as an identifier. (In this example, the programmer is responsible for ensuring that no two ground hogs exist with the same ID number.) The hashCode( ) is not required to return a unique identifier, but the equals( ) method must be able to strictly determine whether two objects are equivalent.

The equals( ) method does two sanity checks: to see if the object is null, and if not, whether it is an instance of Groundhog2 (using the instanceof keyword, which is fully explained in Chapter 11). It should be a Groundhog2 to even continue executing equals( ). The comparison, as you can see, is based on the actual ghNumbers. This time, when you run the program, you’ll see it produces the correct output. (Many of the Java library classes override the hashcode( ) and equals( ) methods to be based upon their contents.)

Properties: a type of Hashtable
In the first example in this book, a type of Hashtable was used called Properties. In that example, the lines:

Properties p = System.getProperties();

called the static method getProperties( ) to get a special Properties object that described the system characteristics. The method list( ) is a method of Properties that sends the contents to any stream output that you choose. There’s also a save( ) method to allow you to write your property list to a file in a way that it can be retrieved later with the load( ) method.

Although the Properties class is inherited from Hashtable, it also contains a second Hashtable that acts to hold the list of “default” properties. So if a property isn’t found in the primary list, the defaults will be searched.

The Properties class is also available for use in your programs (an example is ClassScanner.java in Chapter 17). You can find more complete details in the Java library documentation.

Enumerators revisited

We can now demonstrate the true power of the Enumeration: the ability to separate the operation of traversing a sequence from the underlying structure of that sequence. In the following example, the class PrintData uses an Enumeration to move through a sequence and call the toString( ) method for every object. Two different types of collections are created, a Vector and a Hashtable, and they are each filled with, respectively, Mouse and Hamster objects. (These classes are defined earlier in the chapter; notice you must have compiled HamsterMaze.java and WorksAnyway.java for the following program to compile.) Because an Enumeration hides the structure of the underlying collection, PrintData doesn’t know or care what kind of collection the Enumeration comes from:

//: Enumerators2.java
// Revisiting Enumerations
import java.util.*;
class PrintData {
  static void print(Enumeration e) {
class Enumerators2 {
  public static void main(String[] args) {
    Vector v = new Vector();
    for(int i = 0; i < 5; i++)
      v.addElement(new Mouse(i));
    Hashtable h = new Hashtable();
    for(int i = 0; i < 5; i++)
      h.put(new Integer(i), new Hamster(i));
} ///:~ 

Note that PrintData.print( ) takes advantage of the fact that the objects in these collections are of class Object so it can call toString( ). It’s more likely that in your problem, you must make the assumption that your Enumeration is walking through a collection of some specific type. For example, you might assume that everything in the collection is a Shape with a draw( ) method. Then you must downcast from the Object that Enumeration.nextElement() returns to produce a Shape.

[34] If you plan to use RMI (described in Chapter 15), you should be aware that there’s a problem when putting remote objects into a Hashtable. (See Core Java , by Cornell & Horstmann, Prentice-Hall 1997).

[35] If these speedups still don’t meet your performance needs, you can further accelerate table lookup by writing your own hash table routine. This avoids delays due to casting to and from Objects and synchronization built into the Java Class Library hash table routine. To reach even higher levels of performance, speed enthusiasts can use Donald Knuth’s The Art of Computer Programming, Volume 3: Sorting and Searching, Second Edition to replace overflow bucket lists with arrays that have two additional benefits: they can be optimized for disk storage characteristics and they can save most of the time of creating and garbage collecting individual records .

[36] The best reference I know of is Practical Algorithms for Programmers , by Andrew Binstock and John Rex, Addison-Wesley 1995.

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