Generic Programming

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Note: Module Under Development

This module is based on Chapter 10 of Introduction of Introduction to Programming Using Java, by David J. Eck. Used with permission.

Generic programming refers to writing code that will work for many types of data. This came up when dealing with dynamic arrays, which are arrays that can resize as items are added to them. It would be inefficient to create a different dynamic array class for each type of item you want to hold. Instead it would make more sense to create a generic dynamic array for holding any type. Java does this by providing the ArrayList class. An ArrayList is essentially a dynamic array of values of type Object. Since every class is a subclass of Object, objects of any type can be stored in an ArrayList. Java goes even further by providing "parameterized types," which let you limit the values that can be stored in the list to objects of a specified type. Parameterized types extend Java's basic philosophy of type-safe programming to generic programming.

The ArrayList class is just one of several standard classes that are used for generic programming in Java. We will spend the next few sections looking at these classes and how they are used, and we'll see that there are also generic methods and generic interfaces. All the classes and interfaces discussed in these sections are defined in the package java.util, and you will need an import statement at the beginning of your program to get access to them. (Before you start putting "import java.util.*" at the beginning of every program, you should know that some things in java.util have names that are the same as things in other packages. For example, both java.util.List and java.awt.List exist, so it is often better to import the individual classes that you need.)

In the final section of this chapter, we will see that it is possible to define new generic classes, interfaces, and methods. Until then, we will stick to using the generics that are predefined in Java's standard library.

It is no easy task to design a library for generic programming. Java's solution has many nice features but is certainly not the only possible approach. To get some perspective on generic programming in general, it might be useful to look very briefly at generic programming in two other languages.

Generic Programming in Other Languages

Generic Programming in Java

Java's generic programming features have gone through several stages of development. The original version of Java had just a few generic data structure classes, such as Vector, that could hold values of type Object. Java version 1.2 introduced a much larger group of generics that followed the same basic model. These generic classes and interfaces as a group are known as the Java Collection Framework. The ArrayList class is part of the Collection Framework. The original Collection Framework was closer in spirit to Smalltalk than it was to C++, since a data structure designed to hold Objects can be used with objects of any type. Unfortunately, as in Smalltalk, the result is a category of errors that show up only at run time, rather than at compile time. If a programmer assumes that all the items in a data structure are strings and tries to process those items as strings, a run-time error will occur if other types of data have inadvertently been added to the data structure. In Java, the error will most likely occur when the program retrieves an Object from the data structure and tries to type-cast it to type String. If the object is not actually of type String, the illegal type-cast will throw an error of type ClassCastException.

Java 5.0 introduced parameterized types, such as ArrayList<String>. This made it possible to create generic data structures that can be type-checked at compile time rather than at run time. With these data structures, type-casting is not necessary, so ClassCastExceptions are avoided. The compiler will detect any attempt to add an object of the wrong type to the data structure; it will report a syntax error and will refuse to compile the program. In Java 5.0, all of the classes and interfaces in the Collection Framework, and even some classes that are not part of that framework, have been parameterized. Java's parameterized classes are similar to template classes in C++ (although the implementation is very different), and their introduction moves Java's generic programming model closer to C++ and farther from Smalltalk. In this chapter, I will use the parameterized types almost exclusively, but you should remember that their use is not mandatory. It is still legal to use a parameterized class as a non-parameterized type, such as a plain ArrayList.

Note that there is a significant difference between parameterized classes in Java and template classes in C++. A template class in C++ is not really a class at all -- it's a kind of factory for generating classes. Every time the template is used with a new type, a new compiled class is created. With a Java parameterized class, there is only one compiled class file. For example, there is only one compiled class file, ArrayList.class, for the parameterized class ArrayList. The parameterized types ArrayList<String> and ArrayList<Integer> both use the same compiled class file, as does the plain ArrayList type. The type parameter -- String or Integer -- just tells the compiler to limit the type of object that can be stored in the data structure. The type parameter has no effect at run time and is not even known at run time. The type information is said to be "erased" at run time. This type erasure introduces a certain amount of weirdness. For example, you can't test "if (list instanceof ArrayList<String>)" because the instanceof operator is evaluated at run time, and at run time only the plain ArrayList exists. Even worse, you can't create an array that has base type ArrayList<String> by using the new operator, as in "new ArrayList<String>[N]". This is because the new operator is evaluated at run time, and at run time there is no such thing as "ArrayList<String>"; only the non-parameterized type ArrayList exists at run time.
Fortunately, most programmers don't have to deal with such problems, since they turn up only in fairly advanced programming. Most people who use the Java Collection Framework will not encounter them, and they will get the benefits of type-safe generic programming with little difficulty.

The Java Collection Framework

Java's generic data structures can be divided into two categories: collections and maps. A collection is more or less what it sounds like: a collection of objects. A map associates objects in one set with objects in another set in the way that a dictionary associates definitions with words or a phone book associates phone numbers with names. In Java, collections and maps are represented by the parameterized interfaces Collection<T> and Map<T,S>. Here, "T" and "S" stand for any type except for the primitive types. Map<T,S> is the first example we have seen where there are two type parameters, T and S; we will not deal further with this possibility until we look at maps more closely in Part 3. In this section and the next, we look at collections only.

There are two types of collections: lists and sets. A list is a collection in which the objects are arranged in a linear sequence. A list has a first item, a second item, and so on. For any item in the list, except the last, there is an item that directly follows it. The defining property of a set is that no object can occur more than once in a set; the elements of a set are not necessarily thought of as being in any particular order. The ideas of lists and sets are represented as parameterized interfaces List<T> and Set<T>. These are sub-interfaces of Collection<T>. That is, any object that implements the interface List<T> or Set<T> automatically implements Collection<T> as well. The interface Collection<T> specifies general operations that can be applied to any collection at all. List<T> and Set<T> add additional operations that are appropriate for lists and sets respectively.

Of course, any actual object that is a collection, list, or set must belong to a concrete class that implements the corresponding interface. For example, the class ArrayList<T> implements the interface List<T> and therefore also implements Collection<T>. This means that all the methods that are defined in the list and collection interfaces can be used with, for example, an ArrayList<String> object. We will look at various classes that implement the list and set interfaces in the next section. But before we do that, we'll look briefly at some of the general operations that are available for all collections.

The interface Collection<T> specifies methods for performing some basic operations on any collection of objects. Since "collection" is a very general concept, operations that can be applied to all collections are also very general. They are generic operations in the sense that they can be applied to various types of collections containing various types of objects. Suppose that coll is an object that implements the interface Collection<T> (for some specific non-primitive type T). Then the following operations, which are specified in the interface Collection<T>, are defined for coll:

  • coll.size() -- returns an int that gives the number of objects in the collection.
  • coll.isEmpty() -- returns a boolean value which is true if the size of the collection is 0.
  • coll.clear() -- removes all objects from the collection.
  • coll.add(tobject) -- adds tobject to the collection. The parameter must be of type T; if not, a syntax error occurs at compile time. This method returns a boolean value which tells you whether the operation actually modified the collection. For example, adding an object to a Set has no effect if that object was already in the set.
  • coll.contains(object) -- returns a boolean value that is true if object is in the collection. Note that object is not required to be of type T, since it makes sense to check whether object is in the collection, no matter what type object has. (For testing equality, null is considered to be equal to itself. The criterion for testing non-null objects for equality can differ from one kind of collection to another; see generics below.)
  • coll.remove(object) -- removes object from the collection, if it occurs in the collection, and returns a boolean value that tells you whether the object was found. Again, object is not required to be of type T.
  • coll.containsAll(coll2) -- returns a boolean value that is true if every object in coll2 is also in coll. The parameter can be any collection.
  • coll.addAll(coll2) -- adds all the objects in coll2 to coll. The parameter, coll2, can be any collection of type Collection<T>. However, it can also be more general. For example, if T is a class and S is a sub-class of T, then coll2 can be of type Collection<S>. This makes sense because any object of type S is automatically of type T and so can legally be added to coll.
  • coll.removeAll(coll2) -- removes every object from coll that also occurs in the collection coll2. coll2 can be any collection.
  • coll.retainAll(coll2) -- removes every object from coll that does not occur in the collection coll2. It "retains" only the objects that do occur in coll2. coll2 can be any collection.
  • coll.toArray() -- returns an array of type Object[] that contains all the items in the collection. Note that the return type is Object[], not T[]! However, there is another version of this method that takes an array of type T[] as a parameter: the method coll.toArray(tarray) returns an array of type T[] containing all the items in the collection. If the array parameter tarray is large enough to hold the entire collection, then the items are stored in tarray and tarray is also the return value of the collection. If tarray is not large enough, then a new array is created to hold the items; in that case tarray serves only to specify the type of the array. For example, coll.toArray(new String[0]) can be used if coll is a collection of Strings and will return a new array of type String[].

Since these methods are part of the Collection<T> interface, they must be defined for every object that implements that interface. There is a problem with this, however. For example, the size of some collections cannot be changed after they are created. Methods that add or remove objects don't make sense for these collections. While it is still legal to call the methods, an exception will be thrown when the call is evaluated at run time. The type of the exception is UnsupportedOperationException. Furthermore, since Collection<T> is only an interface, not a concrete class, the actual implementation of the method is left to the classes that implement the interface. This means that the semantics of the methods, as described above, are not guaranteed to be valid for all collection objects; they are valid, however, for classes in the Java Collection Framework.

There is also the question of efficiency. Even when an operation is defined for several types of collections, it might not be equally efficient in all cases. Even a method as simple as size() can vary greatly in efficiency. For some collections, computing the size() might involve counting the items in the collection. The number of steps in this process is equal to the number of items. Other collections might have instance variables to keep track of the size, so evaluating size() just means returning the value of a variable. In this case, the computation takes only one step, no matter how many items there are. When working with collections, it's good to have some idea of how efficient operations are and to choose a collection for which the operations that you need can be implemented most efficiently. We'll see specific examples of this in the next two sections.

Iterators and for-each Loops

The interface Collection<T> defines a few basic generic algorithms, but suppose you want to write your own generic algorithms. Suppose, for example, you want to do something as simple as printing out every item in a collection. To do this in a generic way, you need some way of going through an arbitrary collection, accessing each item in turn. We have seen how to do this for specific data structures: For an array, you can use a for loop to iterate through all the array indices. For a linked list, you can use a while loop in which you advance a pointer along the list. For a binary tree, you can use a recursive subroutine to do an inorder traversal. Collections can be represented in any of these forms and many others besides. With such a variety of traversal mechanisms, how can we even hope to come up with a single generic method that will work for collections that are stored in wildly different forms? This problem is solved by iterators. An iterator is an object that can be used to traverse a collection. Different types of collections have iterators that are implemented in different ways, but all iterators are used in the same way. An algorithm that uses an iterator to traverse a collection is generic, because the same technique can be applied to any type of collection. Iterators can seem rather strange to someone who is encountering generic programming for the first time, but you should understand that they solve a difficult problem in an elegant way.

The interface Collection<T> defines a method that can be used to obtain an iterator for any collection. If coll is a collection, then coll.iterator() returns an iterator that can be used to traverse the collection. You should think of the iterator as a kind of generalized pointer that starts at the beginning of the collection and can move along the collection from one item to the next. Iterators are defined by a parameterized interface named Iterator<T>. If coll implements the interface Collection<T> for some specific type T, then coll.iterator() returns an iterator of type Iterator<T>, with the same type T as its type parameter. The interface Iterator<T> defines just three methods. If iter refers to an object that implements Iterator<T>, then we have:

  • -- returns the next item, and advances the iterator. The return value is of type T. This method lets you look at one of the items in the collection. Note that there is no way to look at an item without advancing the iterator past that item. If this method is called when no items remain, it will throw a NoSuchElementException.
  • iter.hasNext() -- returns a boolean value telling you whether there are more items to be processed. In general, you should test this before calling
  • iter.remove() -- if you call this after calling, it will remove the item that you just saw from the collection. Note that this method has no parameter. It removes the item that was most recently returned by This might produce an UnsupportedOperationException, if the collection does not support removal of items.

Using iterators, we can write code for printing all the items in any collection. Suppose, for example, that coll is of type Collection<String>. In that case, the value returned by coll.iterator() is of type Iterator<String>, and we can say:

Iterator<String> iter;          // Declare the iterator variable.
iter = coll.iterator();         // Get an iterator for the collection.
while ( iter.hasNext() ) {
   String item =;   // Get the next item.

The same general form will work for other types of processing. For example, the following code will remove all null values from any collection of type Collection<JButton> (as long as that collection supports removal of values):

Iterator<JButton> iter = coll.iterator():
while ( iter.hasNext() ) {
    JButton item =;
    if (item == null)

(Note, by the way, that when Collection<T>, Iterator<T>, or any other parameterized type is used in actual code, they are always used with actual types such as String or JButton in place of the "formal type parameter" T. An iterator of type Iterator<String> is used to iterate through a collection of Strings; an iterator of type Iterator<JButton> is used to iterate through a collection of JButtons; and so on.)

An iterator is often used to apply the same operation to all the elements in a collection. In many cases, it's possible to avoid the use of iterators for this purpose by using a for-each loop. The for-each loop was discussed earlier for use with enumerated types and with arrays. A for-each loop can also be used to iterate through any collection. For a collection coll of type Collection<T>, a for-each loop takes the form:

for ( T x : coll ) { // "for each object x, of type T, in coll"
   //  process x  

Here, x is the loop control variable. Each object in coll will be assigned to x in turn, and the body of the loop will be executed for each object. Since objects in coll are of type T, x is declared to be of type T. For example, if namelist is of type Collection<String>, we can print out all the names in the collection with:

for ( String name : namelist ) { 
   System.out.println( name );

This for-each loop could, of course, be written as a while loop using an iterator, but the for-each loop is much easier to follow.

Equality and Comparison

There are several methods in the Collection interface that test objects for equality. For example, the methods coll.contains(object) and coll.remove(object) look for an item in the collection that is equal to object. However, equality is not such a simple matter. The obvious technique for testing equality -- using the == operator -- does not usually give a reasonable answer when applied to objects. The == operator tests whether two objects are identical in the sense that they share the same location in memory. Usually, however, we want to consider two objects to be equal if they represent the same value, which is a very different thing. Two values of type String should be considered equal if they contain the same sequence of characters. The question of whether those characters are stored in the same location in memory is irrelevant. Two values of type Date should be considered equal if they represent the same time.

The Object class defines the boolean-valued method equals(Object) for testing whether one object is equal to another. This method is used by many, but not by all, collection classes for deciding whether two objects are to be considered the same. In the Object class, obj1.equals(obj2) is defined to be the same as obj1 == obj2. However, for most sub-classes of Object, this definition is not reasonable, and it should be overridden. The String class, for example, overrides equals() so that for a String str, str.equals(obj) if obj is also a String and obj contains the same sequence of characters as str.

If you write your own class, you might want to define an equals() method in that class to get the correct behavior when objects are tested for equality. For example, a Card class that will work correctly when used in collections could be defined as:

public class Card {  // Class to represent playing cards.
   int suit;  // Number from 0 to 3 that codes for the suit --
              // spades, diamonds, clubs or hearts.
   int value; // Number from 1 to 13 that represents the value.
   public boolean equals(Object obj) {
       try {
          Card other = (Card)obj;  // Type-cast obj to a Card.
          if (suit == other.suit && value == other.value) {
                // The other card has the same suit and value as
                // this card, so they should be considered equal.
             return true;
             return false;
       catch (Exception e) {
              // This will catch the NullPointerException that occurs if obj
              // is null and the ClassCastException that occurs if obj is
              // not of type Card.  In these cases, obj is not equal to
              // this Card, so return false.
           return false;
    . // other methods and constructors

Without the equals() method in this class, methods such as contains() and remove() in the interface Collection<Card> will not work as expected.

A similar concern arises when items in a collection are sorted. Sorting refers to arranging a sequence of items in ascending order, according to some criterion. The problem is that there is no natural notion of ascending order for arbitrary objects. Before objects can be sorted, some method must be defined for comparing them. Objects that are meant to be compared should implement the interface java.lang.Comparable. In fact, Comparable is defined as a parameterized interface, Comparable<T>, which represents the ability to be compared to an object of type T. The interface Comparable<T> defines one method:

public int compareTo( T obj )

The value returned by obj1.compareTo(obj2) should be negative if and only if obj1 comes before obj2, when the objects are arranged in ascending order. It should be positive if and only if obj1 comes after obj2. A return value of zero means that the objects are considered to be the same for the purposes of this comparison. This does not necessarily mean that the objects are equal in the sense that obj1.equals(obj2) is true. For example, if the objects are of type Address, representing mailing addresses, it might be useful to sort the objects by zip code. Two Addresses are considered the same for the purposes of the sort if they have the same zip code -- but clearly that would not mean that they are the same address.

The String class implements the interface Comparable<String> and defines compareTo in a reasonable way. In this case, the return value of compareTo is zero if and only if the two strings that are being compared are equal. (It is generally a good idea for the compareTo method in classes that implement Comparable to have the analogous property.) If you define your own class and want to be able to sort objects belonging to that class, you should do the same. For example:

 * Represents a full name consisting of a first name and a last name.
public class FullName implements Comparable<FullName> {

   private String firstName, lastName;  // Non-null first and last names.
   public FullName(String first, String last) {  // Constructor.
      if (first == null || last == null)
         throw new IllegalArgumentException("Names must be non-null.");
      firstName = first;
      lastName = last;
   public boolean equals(Object obj) {
      try {
         FullName other = (FullName)obj;  // Type-cast obj to type FullName
         return firstName.equals(other.firstName) 
                                && lastName.equals(other.lastName);
      catch (Exception e) {
         return false;  // if obj is null or is not of type FullName
   public int compareTo( FullName other ) {
      if ( lastName.compareTo(other.lastName) < 0 ) {
             // If lastName comes before the last name of
             // the other object, then this FullName comes
             // before the other FullName.  Return a negative
             // value to indicate this.
         return -1;
      else if ( lastName.compareTo(other.lastName) > 0 ) {
             // If lastName comes after the last name of
             // the other object, then this FullName comes
             // after the other FullName.  Return a positive
             // value to indicate this.
         return 1;
      else {
             // Last names are the same, so base the comparison on
             // the first names, using compareTo from class String.
         return firstName.compareTo(other.firstName);
   . // other methods 

There is another way to allow for comparison of objects in Java, and that is to provide a separate object that is capable of making the comparison. The object must implement the interface Comparator<T>, where T is the type of the objects that are to be compared. The interface Comparator<T> defines the method:

public int compare( T obj1, T obj2 )

This method compares two objects of type T and returns a value that is negative, or positive, or zero, depending on whether obj1 comes before obj2, or comes after obj2, or is considered to be the same as obj2 for the purposes of this comparison. Comparators are useful for comparing objects that do not implement the Comparable interface and for defining several different orderings on the same collection of objects.

In the next two sections, we'll see how Comparable and Comparator are used in the context of collections and maps.


What is meant by generic programming and what is the alternative?

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LinkedList<int> would cause a compile error. What code should you use instead?

(And why?)

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Which of the following is true about an iterator?

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The method iterSum takes in a variable integers, which is of type Collection<Integer>. Fill in the method so it uses an iterator to compute an print the sum of all the integer values in the collection.

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  • This is really to much text and information to understand ( for me it is at least)

  • This module was marked as "under development" for that reason. It covers generic programming in detail, which isn't essential early on. It may make sense to get more programming practice first.

  • I am not sure if this was the way to do it, but it worked.

    my code

  • @ Learneroo Perhaps you are right and that makes sense. But i am doing everything is order that is given.

  • Should i skip this for now and go on to the module games? What is your advice?

  • Unless you're specifically interested in generic programming, you can skip it for now. There are four main Java modules in the beginner Java course. After you've completed them, you should get

  • Hi this is a tough read for me but totally worth it. Helps me think forward!

    Might you know if it will be completed some time?

  • Sorry there isn't any plan to update this right now. I'm leaving the content up since it's still something people can benefit from, but it's not part of the "primary" Java track.

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