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THE JAVA COLLECTIONS FRAMEWORK
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[CHAP. 4
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element to be removed. These two versions are invoked at lines 12 and 15. At line 12, the element at index 2, which is the object that holds 44, is removed. At line 15, the element that equals() the object that holds 66 is removed. Note that this would not work:
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list.remove(22); // ERROR: the argument refers to index 22
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Autoboxing occurs only when the types would otherwise be mismatched.
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Autoboxing, which might better be called autowrapping, also works in other contexts where an explicit instantiation of a wrapper class would otherwise be required. For example:
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Double pi = 3.14159; // OK
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Note that automatic implicit instantiation (also called promotion) also occurs when a primitive expression is combined with a String object by means of the concatenation operator +. For example:
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System.out.println("circumference = " + 2*pi*r); // OK
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In this case, the value of the double expression 2*pi*r is automatically wrapped in a new Double object, whose toString() method is then invoked to produce the String object that is concatenated with the String literal "circumference = ". This suggests that the alternative,
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System.out.printf("circumference = %f%n", 2*pi*r); // BETTER
should be preferred. Review Questions
4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 What is the Java Collections Framework What are the four main types defined in the Java Collections Framework What is a queue What is a deque What is a legacy class Which collection classes are implemented with an indexed structure (i.e., a simple array) Which collection classes are implemented with a linked structure Which collection classes are implemented with a linked indexed structure What is an ArrayList object
4.10 What is a LinkedList object 4.11 What is the Collection interface 4.12 What set-theoretic methods are supported by the Java Collections Framework 4.13 What is an iterator 4.14 How are iterators similar to array indexes 4.15 How are iterators different from array indexes 4.16 How can the Arrays class be used to initialize a collection 4.17 How can the Collections class be used to initialize a collection 4.18 What is a generic class 4.19 What is a generic method
CHAP. 4]
THE JAVA COLLECTIONS FRAMEWORK
4.20 What is a generic type parameter 4.21 What is a constrained generic type parameter 4.22 What is a generic type argument 4.23 What is autoboxing
Problems
4.1 Write and test this method:
int chars(List<String> strings) // returns the total number of characters in all the strings
Write and test this method, using an iterator:
void print(Collection c) // prints all the elements of the collection c
Note that this has the same effect as in Example 4.4 on page 76. 4.3 Write and test this generic method:
<E> int frequency(Collection<E> c, E e) { // returns the number of occurrences of e in c
a. using an iterator. b. using an enhanced for loop. 4.4 Write and test this generic method:
<E> E getLast(List<E> list) { // returns the last element of the list
a. using an iterator. b. using an enhanced for loop. 4.5 Write and test this generic method:
<E> E getElementAt(List<E> list, int index) { // returns the list element at the specified index
a. using an iterator. b. using an enhanced for loop. 4.6 Write and test this client method:
<E> Collection<E> toCollection(E[] a)
// returns a generic Collection that contains the elements // of the specified array
Answers to Review Questions
4.1 4.2 4.3 4.4 4.5 The Java Collections Framework is a group of interrelated interfaces and classes that support the creation and use of lists, sets, and iterators. The four main types are List, Queue, Set, and Map. A queue is a first-in-first-out collection that allows elements to be added at only one end (the back) and removed only from the other end (the front). A deque is a double-ended queue that allows elements to be added and removed only from its two ends (the front and back). A legacy class is a class that has been superseded by another class.
THE JAVA COLLECTIONS FRAMEWORK
[CHAP. 4
4.6 4.7 4.8 4.9
The ArrayList, ArrayDeque, ArrayDeque, and HashMap classes are implemented with an array. The LinkedList, PriorityQueue, TreeSet, and TreeMap classes are implemented with a linked structure (either a linked list or a tree). The LinkedHashSet class and the LinkedHashMap class are implemented with a linked indexed structure. An ArrayList object is an instance of the java.util.ArrayList class. As a member of the Java Collections Framework, it supports all the methods of the Collection interface. As an indexed structure, it provides direct access to its elements by their index numbers. A LinkedList object is an instance of the java.util.LinkedList class. As a member of the Java Collections Framework, it supports all the methods of the Collection interface. As a linked structure, it allows elements to be added and removed in constant time, independent of the size of the list. The Collection interface is a Java interface that specifies the 15 methods listed in Figure 4.2 on page 71. The four set-theoretic methods specified by the Collection interface are listed in Figure 4.2 on page 71: addAll() for unions, removeAll() for complements, retainAll() for intersections, and containsAll() for testing the subset relation. An iterator is an object that moves about on a collection, providing access to its elements. An iterator can be used to traverse a collection with a for loop, in the same way that an index can be used to traverse an array. An array index provides direct access (i.e., random access) into an array. An iterator provides only sequential access. The java.util.Arrays class includes an asList() method that returns a List object whose elements are the same as those in the array passed to it. (See Example 4.15 on page 96.) The java.util.Collections class includes an addAll() method that adds all elements that are passed to it. (See Example 4.15 on page 96.) The method takes a variable number of arguments (i.e., it is a varargs method). A generic class is a class that uses generic type parameters, that is, parametrized types. (See page 96.) A generic method is a method that uses generic type parameters, that is, parametrized types. A generic type parameter is a symbol used in a class or method definition that represents a type. When applied, an actual class or interface name must be substituted for the parameter. A constrained generic type parameter is a generic type parameter that is constrained by an extends clause. A generic type argument is a class or interface name that is being substituted for a generic type parameter in a generic class or generic method. The term autoboxing refers to the automatic implicit instantiation of a wrapper class thereby allowing the value of a fundamental type expression to be used where an object is expected, for example, in the Collections.addAll() method. (See Example 4.16 on page 97.)
4.11 4.12
4.13 4.14 4.15 4.16 4.17
4.18 4.19 4.20 4.21 4.22 4.23
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