- Question: This lesson mentions three ways to traverse a
List
. Describe them, and note the limitations of each. Answer:
- Use the enhanced
for
statement:Limitations: cannot be used to add, remove, or modify elements.List<Thing> list; ... for (Thing thing : list) { ... }- Use the traditional
for
statement together with anIterator
:Limitations: cannot be used to modify elements.List<Thing> list; ... for (Iterator<Thing> it = list.iterator(); it.hasNext(); ) { Thing thing = it.next(); ... }- Use the traditional
for
statement together with anListIterator
:Limitations: none.List<Thing> list; ... for (ListIterator<Thing> it = list.iterator(); it.hasNext(); ) { Thing thing = it.next(); ... }- Question: Consider the four core interfaces,
Set
,List
,Queue
, andMap
. For each of the following four assignments, specify which of the four core interfaces is best-suited, and explain how to use it to implement the assignment. Answer:
- Whimsical Toys Inc (WTI) needs to record the names of all its employees. Every month, an employee will be chosen at random from these records to receive a free toy.
Use aList
. Choose a random employee by picking a number between0
andsize()-1
.- WTI has decided that each new product will be named after an employee — but only first names will be used, and each name will be used only once. Prepare a list of unique first names.
Use aSet
. Collections that implement this interface don't allow the same element to be entered more than once.- WTI decides that it only wants to use the most popular names for its toys. Count up the number of employees who have each first name.
Use aMap
, where the keys are first names, and each value is a count of the number of employees with that first name.- WTI acquires season tickets for the local lacrosse team, to be shared by employees. Create a waiting list for this popular sport.
Use aQueue
. Invokeadd()
to add employees to the waiting list, andremove()
to remove them.- Question: The following program is supposed to print the string "Blue". Instead, it throws an error. Why?
Answer:import java.util.*; public class SortMe { public static void main(String args[]) { SortedSet<StringBuffer> s = new TreeSet<StringBuffer>(); s.add(new StringBuffer("Red")); s.add(new StringBuffer("White")); s.add(new StringBuffer("Blue")); System.out.println(s.first()); } }TreeSort
elements must be instances of a class that implementsComparable
.StringBuffer
does not.
- Exercise: Write a program that prints its arguments in random order. Do not make a copy of the argument array. Answer:
import java.util.*; public class Ran { public static void main(String[] args) { List<String> argList = Arrays.asList(args); Collections.shuffle(argList); for (String arg: argList) { System.out.format("%s ", arg); } System.out.println(); } }- Exercise: Take the
FindDups example
and modify it to use aSortedSet
instead of aSet
. Specify aComparator
so that case is ignored when sorting and identifying set elements. Answer:import java.util.*; public class FindDups { public static void main(String[] args) { Comparator<String> comparator = new Comparator<String>() { public int compare (String s1, String s2) { return s1.compareToIgnoreCase(s2); } }; SortedSet<String> s = new TreeSet<String>(comparator); for (String a : args) if (!s.add(a)) System.out.println("Duplicate detected: " + a); System.out.println(s.size() + " distinct words: " + s); } }- Exercise: Write a method that takes a
List<String>
and appliesString.trim
to each element. To do this, you'll need to pick one of the three iteration idioms that you described in Question 1. Two of these will not give the result you want, so be sure to write a program that demonstrates that the method actually works! Answer: The enhancedfor
statement does not allow you to modify theList
. Using anIterator
allows you to add and delete elements, but not replace existing element. That leavesListIterator
:import java.util.*; public class ListTrim { static void listTrim(List<String> strings) { for (ListIterator<String> lit = strings.listIterator(); lit.hasNext(); ) { lit.set(lit.next().trim()); } } public static void main(String[] args) { List<String> l = Arrays.asList(" red ", " white ", " blue "); listTrim(l); for (String s : l) { System.out.format("\"%s\"%n", s); } } }