Lists - Data Structures and Abstractions with Java - page 312

Question 5 Suppose that the return type of getEntry was Object instead of a generic type.
Would this change affect how you use the method? In particular, would the statement in the
previous example that retrieved the second name in nameList be correct? Why?

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Example. Let's talk a bit more about the previous example. The variable secondName is a reference
to the second object in the list. Using this reference, we can modify the object. For example, we
could change its last name by writing
secondName.setLast("Doe");
If the class Name did not have set methods like setLast , we would be unable to modify the
objects in this list. For instance, if we had a list of strings, we would not be able to alter one of the
strings in this way. The class String has no set methods, so once we create a String object, we
cannot alter it. We could, however, replace an entire object in the list—regardless of its type—by
using the ADT list operation replace .
Recall that Chapter 10 mentioned mutable and immutable objects. Since the class Name has set
methods, its objects are mutable. The class String , on the other hand, does not define set methods,
so its objects are immutable.

Java Class Library: The Interface List
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The standard package java.util contains an interface List for an ADT list that is similar to the list that
our interface describes. One difference between a list in the Java Class Library and our ADT list is the num-
bering of a list's entries. A list in the Java Class Library uses the same numbering scheme as a Java array:
The first entry is at position, or index, 0. In contrast, we begin our list at position 1. The interface List also
declares more methods than our interface does. You'll see a few of those additional methods in Chapter 15.
The following method headers from the interface List are for a selection of methods that are
similar to the ones you have seen in this chapter. We have highlighted where they differ from our
methods. Once again, T is the generic type of the entries in the list.

public boolean add(T newEntry)
public void add( int index, T newEntry)

public T remove( int index)
public void clear()

public T set( int index, T anEntry) // like replace
public T get( int index) // like getEntry
public boolean contains(Object anEntry)
public int size() // like getLength

public boolean isEmpty()
The first add method, which adds an entry to the end of a list, returns a boolean value, whereas our analo-
gous method is a void method. The second add method is a void method. It throws an exception if index is
out of range, instead of returning the boolean value false, as our add method does. The methods remove
and get also throw an exception if index is out of range. Our analogous methods return null instead. The
method set is like our replace method, but it returns a reference to the entry that was replaced in the list
instead of returning a boolean value. It also throws an exception if index is out of range. The data type of
contains ' parameter is Object instead of a generic type. In practice, this difference has little consequence.
Lastly, the method get is like our getEntry , and size is like our getLength .
You can learn more about the interface List in the online documentation for the Java Class Library.

Java Class Library: The Class ArrayList
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The Java Class Library contains an implementation of the ADT list that uses a resizable array. This
class, called ArrayList , implements the interface java.util.List , that we just discussed. The
class also is in the package java.util .