Download The Java Collections Framework

Chapter 15 – The Java Collections Framework
Copyright © 2014 by John Wiley & Sons. All rights reserved.
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Chapter Goals
 To learn how to use the collection classes supplied in the Java library
 To use iterators to traverse collections
 To choose appropriate collections for solving programming problems
 To study applications of stacks and queues
Copyright © 2014 by John Wiley & Sons. All rights reserved.
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An Overview of the Collections Framework
 Why do we need collections of objects?
•
When we are only creating a few objects, we can afford to declare
individualized reference variables for these objects:
• E.g. Students s1, s2, s3,
• E.g. Professors profA, profB, profC.
•
At other times, individualized reference variables are impractical.
• Too large no. of objects to create:
– E.g. university course catalog might have hundreds of courses
• We do not know until runtime how many objects to create
– So we can’t predefine no. of reference variables at compile time
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 Fortunately, Java solve this problem by collection
• Special category of object
 What are Collections?
• Way to gather up objects as they are created
• We can manage them as a group
• We can operate on them collectively
• We can refer to them individually when necessary
• Collections hold and organize References to Other Objects
• Not objects themselves!
• E.g. ArrayList
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 A collection organizes references to objects that live in memory
outside of the collection:
BankAccount
Object
BankAccount
Object
BankAccount
Object
An ArrayList Object
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An Overview of the Collections Framework
 Java collections framework:
• a hierarchy of interface types and classes for collecting objects.
• Each interface type is implemented by one or more classes
• E.g. List interface is implemented by classes: ArrayList, Stack and linkedList
• Refer JavaDoc to find out implementing classes
Figure 1 Interfaces and Classes in the Java Collections Framework
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An Overview of the Collections Framework
 The Collection interface is at the root
• All Collection classes implement this interface
• Refer javadoc api
• So all have a common set of methods
• Adding objects : Collections automatically expand as new items are added.
• Removing objects: Collections automatically shrink when items are removed.
• Retrieving specific individual objects
• Iterating through the objects in some predetermined order
• Getting a count of the number of objects presently referenced by the collection
• Answering a true/false question as to whether a particular object’s reference is in
the collection or not
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 Three Generic Types of Collection:
1.
Ordered lists
2.
Dictionaries
3.
Sets
Dictionaries/Maps
Ordered Lists
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Sets
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1. Ordered Lists:
• Allows us to insert items in a particular order
• Allow later retrieving them in that same order
• Specific objects can also be retrieved based on their position in
the list
• Most collection types(e.g. ordered lists, dictionaries, sets)
needn’t be assigned an explicit capacity at the time that they are
instantiated
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• By default, items are added at the end of an ordered list
• unless explicit instructions are given to insert an item at a different
position.
• E.g. a student waiting list:
• Order maintenance is important to be fair in selecting students from
waiting list
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An Overview of the Collections Framework
• Ordered lists are realized in java using List interface and/or
Queue interface
• List is a collection that remembers the order of its elements.
• Refer javadoc api
• Several predefined Java classes implement the notion of ordered
list collections:
• ArrayList
• Stack
• Vector
• LinkedList (List+Queue)
• PriorityQueue (Queue)
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 Interface List
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 Interface Queue:
• Add items to one end (the tail) and remove them from the other end (the
head) (First-In First-Out FIFO)
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2. Dictionaries/ Maps
• Provides a means for storing each object reference along with a
unique lookup key that can later be used to quickly retrieve the
object
• The key is often selected based on one or more of the object’s
attribute values.
• E.g. a Student object’s student ID number would make an excellent key,
because its value is inherently unique for each Student.
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 Example map (set of <key, value> pairs )
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 Items in a dictionary can typically also be iterated through one by
one, in ascending key (or some other predetermined) order.
 Maps are realized in java using Map interface
• Set of <key,Value> pairs.
• Refer javadoc api
• Key provide easy/faster lookup of objects based on key
• E.g. you can lookup a student object based on student id
• Key must be unique to value
• Map has no duplicate keys
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 Some predefined Java classes that implement the notion
of a dictionary are:
• HashMap
• TreeMap
• The map is sorted according to the natural ordering of its keys, or by
a Comparator provided at map creation time
• guaranteed log(n) time cost for
the containsKey, get, put and remove operations
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3. Sets
•
An unordered collection
•
i.e. you CANNOT ask for a particular item by number/position
once it has been inserted into the set.
•
We can iterate though elements one by one
•
•
Duplicate entries aren’t allowed in a set
•
•
But, order is not predetermined
Unlike lists
E.g. group employees by department
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 Realized using Set interface
• Refer javadoc api
 Two predefined Java classes that implement the
notion of a set are:
• HashSet
• TreeSet
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Question
A gradebook application stores a collection of quizzes.
Should it use a list or a set?
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Answer
A list is a better choice because the application will
want to retain the order in which the quizzes were
given.
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Question
A student information system stores a collection of student
records for a university. Should it use a list or a set?
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Answer
A set is a better choice. There is no intrinsically
useful ordering for the students. For example, the
registrar's office has little use for a list of all students
by their GPA.
By storing them in a set, adding, removing, and
finding students can be efficient.
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Question
Why is a queue of books a better choice than a stack for
organizing your required reading?
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Answer
With a stack, you would always read the latest
required reading, and you might never get to the
oldest readings.
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Question
As you can see from Figure 1, the Java collections
framework does not consider a map a collection. Give a
reason for this decision.
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Answer
A collection stores elements, but a map stores
associations between elements.
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 Let’s now look at some important Collection classes in detail:
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Linked Lists
 A data structure used for collecting a sequence of objects:
 Allows efficient addition and removal of elements in the
middle of the sequence.
 A linked list consists of a number of nodes
• Each node stores element + has a reference to the next node.
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Linked Lists
 Adding and removing elements in the middle of a linked list is
efficient.
 Visiting the elements of a linked list in sequential order is efficient.
 Random access is NOT efficient.
Figure 6 Example of a linked list
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Linked Lists
 When inserting or removing a node:
• Only the neighboring node references need to be updated
• Unlike arrays!
Figure 7 Inserting a Node into a Linked List
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Linked Lists
Figure 8 Removing a Node From A Linked List
 Visiting the elements of a linked list in sequential order is efficient.
 Random access is NOT efficient.
 When to use a linked list:
• You are concerned about the efficiency of inserting or removing elements
• You rarely need element access in random order
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The LinkedList Class of the Java Collections Framework
 Generic class
• Specify type of elements in angle brackets:
LinkedList<Product> = new LinkedList<Product>();
LinkedList<Integer> = new LinkedList<Integer>();
 Package: java.util
• LinkedList has the methods of the Collection interface.
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The LinkedList Class of the Java Collections Framework
 Some additional LinkedList methods:
Refer javadoc api
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List Iterator
 Use a list iterator to access elements inside a linked list.
 Think of an iterator as pointing between two elements:
 To get a list iterator, use the listIterator method of the
LinkedList class.
LinkedList<String> employeeNames = new ListIterator<String>();
ListIterator<String> iterator = employeeNames.listIterator();
 Also a generic type.
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List Iterator
 Initially points before the first element.
 Move the position with next method:
if (iterator.hasNext())
{
iterator.next();
}
 The next method returns the element that the iterator is passing.
 The return type of the next method matches the list iterator's type
parameter.
Copyright © 2014 by John Wiley & Sons. All rights reserved.
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List Iterator
 To traverse all elements in a linked list of strings
 Using while loop:
while (iterator.hasNext())
{
String name = iterator.next();
//Do something with name
}
 To use the “for each” loop:
for (String name : employeeNames)
{
//Do something with name
}
Copyright © 2014 by John Wiley & Sons. All rights reserved.
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List Iterator
 The nodes of the LinkedList class store two links:
• One to the next element
• One to the previous element
• Called a doubly-linked list
 To move the list position forward, use methods:
• hasNext
• next
 To move the list position backwards, use methods:
• hasPrevious
• previous
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List Iterator
 The add method:
•
adds an object after the iterator.
• Then moves the iterator position past the new element.
iterator.add("Juliet");
Figure 8 A Conceptual View of the List Iterator
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List Iterator
 The remove method:
• Removes object that was returned by the last call to next or previous
• To remove all names that fulfill a certain condition:
while (iterator.hasNext())
{
String name = iterator.next();
if (condition is fulfilled for name)
iterator.remove();
}
• Be careful when calling remove:
• It can be called only ONCE after calling next or previous
• You CANNOT call it immediately after a call to add
• If you call it improperly, it throws an IllegalStateException
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List Iterator
 ListIterator interface extends Iterator interface.
 Methods of the Iterator and ListIterator interfaces
Refer javadoc api
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Programming Question
 Write a tester class ListDemo that :
• Create a linked list staff to maintain names of staff of a company.
• Inserts 4 names into the end of the list (Diana, Harry, Romeo, Tom)
• Iterates through the list (use ListIterator)
• After iterating the second element, add two new names (Juliet, Nina)
• Remove the last traversed element
• Prints the list
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Answer
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import java.util.LinkedList;
import java.util.ListIterator;
/**
This program demonstrates the LinkedList class.
*/
public class ListDemo
{
public static void main(String[] args)
{
LinkedList<String> staff = new LinkedList<String>();
staff.addLast("Diana");
staff.addLast("Harry");
staff.addLast("Romeo");
staff.addLast("Tom");
// | in the comments indicates the iterator position
ListIterator<String> iterator = staff.listIterator(); // |DHRT
iterator.next(); // D|HRT
iterator.next(); // DH|RT
// Add more elements after second element
iterator.add("Juliet"); // DHJ|RT
iterator.add("Nina"); // DHJN|RT
iterator.next(); // DHJNR|T
Continued
// Remove last traversed element
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iterator.remove(); // DHJN|T
// Print all elements
System.out.println(staff);
System.out.println("Expected: [Diana, Harry, Juliet, Nina, Tom]");
}
}
Program Run:
[Diana Harry Juliet Nina Tom]
Expected: [Diana Harry Juliet Nina Tom]
Copyright © 2014 by John Wiley & Sons. All rights reserved.
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Question
Do linked lists take more storage space than arrays of the
same size?
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Answer
Yes, for two reasons.
1. A linked list needs to store the neighboring node
references, which are not needed in an array.
2. Moreover, there is some overhead for storing an
object. In a linked list, each node is a separate object
that incurs this overhead, whereas an array is a
single object.
Copyright © 2014 by John Wiley & Sons. All rights reserved.
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Programming Question
Modify ListDemo class by writing a loop that removes all names with
length less than 5 from staff list.
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Answer
ListIterator<String> iter = staff.iterator();
while (iter.hasNext())
{
String str = iter.next();
if (str.length() < 5) { iter.remove(); }
}
Copyright © 2014 by John Wiley & Sons. All rights reserved.
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Question
Write a loop that prints every second element of a linked list of
strings called words.
Answer:
ListIterator<String> iter = words.iterator();
while (iter.hasNext())
{
System.out.println(iter.next());
if (iter.hasNext())
{
iter.next(); // Skip the next element
}
}
Copyright © 2014 by John Wiley & Sons. All rights reserved.
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Sets
 A set organizes its values in an order that is optimized for efficiency.
 May not be the order in which you add elements.
 Inserting and removing elements is more efficient with a set than
with a list.
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Sets
 The Set interface has the same methods as the Collection
interface.
 A set does not admit duplicates.
 Two implementing classes
• HashSet
o based on hash table
• TreeSet
o based on binary search tree
 A Set implementation arranges the elements so that it can locate
them quickly.
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Sets
 HashSet:
• Elements are internally grouped according to a hashcode
• E.g. Try MD5 algorithm hash generation:
echo -n 'text to be encrypted' | md5sum –
• E.g. Try SHA1 algorithm hash generation:
echo -n "yourpassword" | openssl sha1
 E.g.
•
HashSet<String> set1 = new HashSet<String>();
•
Set<Rectangle> set2 = new HashSet<Rectangle>();
•
HashSet<HashSet<Integer>> = new HashSet<HashSet<Integer>>();
Refer javadoc api
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Sets
 TreeSet
• Elements are kept in sorted order
• The nodes are arranged in a tree shape, not in a linear sequence
• You can form tree sets for any class that implements the Comparable
interface:
• Example: String or Integer.
• Use a TreeSet if you want to visit the set's elements in sorted order.
• Otherwise choose a HashSet
o It is a bit more efficient — if the hash function is well chosen
Refer javadoc api
Copyright © 2014 by John Wiley & Sons. All rights reserved.
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Sets
 Store the reference to a TreeSet or HashSet in a Set<String>
variable:
Set<String> names = new HashSet<String>();
Or
Set<String> names = new TreeSet<String>();
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Working with Sets
 Adding and removing elements:
names.add("Romeo");
names.remove("Juliet");
 Sets don't have duplicates.
• Adding a duplicate is ignored.
 Attempting to remove an element that isn't in the set is ignored.
 The contains method tests whether an element is contained in
the set:
if (names.contains("Juliet")) . . .
• The contains method uses the equals method of the element type
Copyright © 2014 by John Wiley & Sons. All rights reserved.
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Working with Sets
 To process all elements in the set, get an iterator.
 A set iterator visits the elements in the order in which the set
implementation keeps them.
Iterator<String> iter = names.iterator();
while (iter.hasNext())
{
String name = iter.next();
//Do something with name
}
 You can also use the “for each” loop
for (String name : names)
{
//Do something with name
}
Copyright © 2014 by John Wiley & Sons. All rights reserved.
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Working with Sets
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Programming Question
 Write a class called SpellCheck. In main method implement following:
• Read all the correctly spelled words from a dictionary file
(ch15\section_3\words)
• Put them in a set dictinaryWords
• Reads all words from a document (ch15\section_3\alice30.txt)
• Put them in a second set documentWords
• To read file into a set, implement readWords method:
Set<String> dictionaryWords = readWords("words");
...
public static Set<String> readWords(String filename) throws FileNotFoundException
• Print all the words in the second set that are not in the dictionary set.
• This will leave out potential misspellings in documentWords
Copyright © 2014 by John Wiley & Sons. All rights reserved.
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Answer
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import
import
import
import
import
SpellCheck.java
java.util.HashSet;
java.util.Scanner;
java.util.Set;
java.io.File;
java.io.FileNotFoundException;
/**
This program checks which words in a file are not present in a dictionary.
*/
public class SpellCheck
{
public static void main(String[] args)
throws FileNotFoundException
{
// Read the dictionary and the document
Set<String> dictionaryWords = readWords("words");
Set<String> documentWords = readWords("alice30.txt");
// Print all words that are in the document but not the dictionary
for (String word : documentWords)
{
if (!dictionaryWords.contains(word))
{
System.out.println(word);
}
}
}
Copyright © 2014 by John Wiley & Sons. All rights reserved.
Continued
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/**
Reads all words from a file.
@param filename the name of the file
@return a set with all lowercased words in the file. Here, a
word is a sequence of upper- and lowercase letters.
*/
public static Set<String> readWords(String filename)
throws FileNotFoundException
{
Set<String> words = new HashSet<String>();
Scanner in = new Scanner(new File(filename));
// Use any characters other than a-z or A-Z as delimiters
in.useDelimiter("[^a-zA-Z]+");
while (in.hasNext())
{
words.add(in.next().toLowerCase());
}
return words;
}
}
Program Run:
neighbouring
croqueted
pennyworth
dutchess
comfits
xii
dinn
clamour
Copyright © 2014 by John Wiley & Sons. All rights reserved.
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Question
Arrays and lists remember the order in which you added
elements; sets do not. Why would you want to use a set
instead of an array or list?
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Answer
Adding and removing elements as well as testing for
membership is more efficient with sets.
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Question
Why are set iterators different from list iterators?
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Answer
Sets do not have an ordering, so it doesn't make
sense to add an element at a particular iterator
position, or to traverse a set backward.
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Programming Question
Modify the SpellCheck program to use a TreeSet
instead of a HashSet. How would the output change?
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Answer
The words would be listed in sorted order.
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Maps
 A map allows you to associate elements from a key set with elements from
a value collection.
 Use a map when you want to look up objects by using a key.
Figure 10 A Map
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Maps
 Two implementations of the Map interface:
• HashMap
• TreeMap
 Store the reference to the map object in a Map reference:
Map<String, Color> favoriteColors = new HashMap<String, Color>();
Refer javadoc api
Copyright © 2014 by John Wiley & Sons. All rights reserved.
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Maps
 Use the put method to add an association:
favoriteColors.put("Juliet", Color.RED);
 You can change the value of an existing association by calling put
again:
favoriteColors.put("Juliet", Color.BLUE);
 The get method returns the value associated with a key:
Color favorite = favorite.get("Juliet");
• If you ask for a key that isn't associated with any values, the get method
returns null.
 To remove an association, call the remove method with the key:
favoriteColors.remove("Juliet");
Copyright © 2014 by John Wiley & Sons. All rights reserved.
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Working with Maps
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Maps
 Sometimes you want to enumerate all keys in a map.
 The keySet method yields the set of keys.
 Ask the key set for an iterator and get all keys.
 For each key, you can find the associated value with the get method.
 To print all key/value pairs in a map m:
Set<String> keySet = m.keySet();
for (String key : keySet)
{
Color value = m.get(key);
System.out.println(key + "->" + value);
}
Copyright © 2014 by John Wiley & Sons. All rights reserved.
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Programming Question
 Implement the tester class MapDemo. In the main method, create a
map called favoriteColors with person name as the key and favorite
color of the person as value. Then add <key,value> pairs based on
following diagram:
 Finally print all <key,value> pairs in the map.
Program Run:
Juliet : java.awt.Color[r=0,g=0,b=255]
Adam : java.awt.Color[r=255,g=0,b=0]
Eve : java.awt.Color[r=0,g=0,b=255]
Romeo : java.awt.Color[r=0,g=255,b=0]
Copyright © 2014 by John Wiley & Sons. All rights reserved.
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Answer
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import
import
import
import
MapDemo.java
java.awt.Color;
java.util.HashMap;
java.util.Map;
java.util.Set;
/**
This program demonstrates a map that maps names to colors.
*/
public class MapDemo
{
public static void main(String[] args)
{
Map<String, Color> favoriteColors = new HashMap<String, Color>();
favoriteColors.put("Juliet", Color.BLUE);
favoriteColors.put("Romeo", Color.GREEN);
favoriteColors.put("Adam", Color.RED);
favoriteColors.put("Eve", Color.BLUE);
// Print all keys and values in the map
Set<String> keySet = favoriteColors.keySet();
for (String key : keySet)
{
Color value = favoriteColors.get(key);
System.out.println(key + " : " + value);
}
}
}
Copyright © 2014 by John Wiley & Sons. All rights reserved.
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Question
What is the difference between a set and a map?
Answer: A set stores elements. A map stores
associations between keys and values.
Copyright © 2014 by John Wiley & Sons. All rights reserved.
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Answer
A set stores elements. A map stores associations
between keys and values.
Copyright © 2014 by John Wiley & Sons. All rights reserved.
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Question
Why is the collection of the keys of a map a set and not a
list?
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Answer
The ordering does not matter, and you cannot have
duplicates
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Question
Why is the collection of the values of a map not a set?
Copyright © 2014 by John Wiley & Sons. All rights reserved.
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Answer
Because it might have duplicates.
Copyright © 2014 by John Wiley & Sons. All rights reserved.
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Question
Suppose you want to track how many times each word
occurs in a document. Declare a suitable map variable.
Copyright © 2014 by John Wiley & Sons. All rights reserved.
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Answer
Answer:
Map<String, Integer> wordFrequency;
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Question
What is a Map<String, HashSet<String>>? Give a
possible use for such a structure.
Copyright © 2014 by John Wiley & Sons. All rights reserved.
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Answer
Answer: It associates strings with sets of strings. One
application would be a thesaurus that lists synonyms
for a given word. For example, the key "improve"
might have as its value the set ["ameliorate",
"better", "enhance", "enrich", "perfect",
"refine"].
Copyright © 2014 by John Wiley & Sons. All rights reserved.
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Choosing a Collection
 Determine how you access the values.
 Determine the element types or key/value types.
 Determine whether element or key order matters.
 For a collection, determine which operations must be efficient.
 For hash sets and maps, decide whether you need to implement the
hashCode and equals methods.
 If you use a tree, decide whether to supply a comparator.
Copyright © 2014 by John Wiley & Sons. All rights reserved.
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Stacks
 A stack lets you insert and remove elements only at one end:
• Called the top of the stack.
• Removes items in the opposite order than they were added
• Last-in, first-out or LIFO order
 Add and remove methods are called push and pop.
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Stacks
 Example
Stack<String> s = new Stack<String>();
s.push("A");
s.push("B");
s.push("C");
while (s.size() > 0)
{
System.out.print(s.pop() + " "); // Prints C B A
}
 The last pancake that has been added to this stack will be the first
one that is consumed.
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 Stack Animator:
• Array implementation:
• http://www.cs.usfca.edu/~galles/visualization/StackArray.html
• List Implementation:
• http://www.cs.usfca.edu/~galles/visualization/StackLL.html
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92
Stacks
 Many applications for stacks in computer science.
 Consider: Undo function of a word processor
• The issued commands are kept in a stack.
• When you select “Undo”, the last command is popped off the
stack and undone
 Run-time stack that a processor or virtual machine:
• Stores the values of variables in nested methods.
• When a new method is called, its parameter variables and local
variables are pushed onto a stack.
• When the method exits, they are popped off again.
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Stack in the Java Library
 Stack class provides push, pop and peek methods.
Refer javadoc api
Copyright © 2014 by John Wiley & Sons. All rights reserved.
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Programming Question
 Implement a tester ckass StackDemo. The main method should do
following:
• create a stack to hold integers.
• Add values 1,2,3 to the stack.
• Print stack content
• Remove top element
• Print stack after removal
 A sample program run is shown:
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Answer
StackDemo.java
import java.util.*;
public class StackDemo {
public static void main(String args[]) {
// creating stack
Stack<Integer> st = new Stack<Integer>();
// populating stack
st.push(Integer.valueOf(1));
st.push(Integer.valueOf(2));
st.push(Integer.valueOf(3));
//elements before remove
System.out.println("Elelments before removal: "+st);
// removing top object
System.out.println("Removed object is: "+st.pop());
// elements after remove
System.out.println("Elements after remove: "+st);
}
}
Copyright © 2014 by John Wiley & Sons. All rights reserved.
96
Queue
 A queue
• Lets you add items to one end of the queue (the tail)
• Remove items from the other end of the queue (the head)
• Items are removed in the same order in which they were added
• First-in, first-out or FIFO order
 To visualize a queue, think of people lining up.
 Typical application: a print queue.
Copyright © 2014 by John Wiley & Sons. All rights reserved.
97
Queue
 The Queue interface in the standard Java library has:
• an add method to add an element to the tail of the queue,
• A remove method to remove the head of the queue, and
• A peek method to get the head element of the queue without removing it.
 The LinkedList class implements the Queue interface.
 When you need a queue, initialize a Queue variable with a
LinkedList object:
Queue<String> q = new LinkedList<String>();
q.add("A");
q.add("B");
q.add("C");
while (q.size() > 0) { System.out.print(q.remove() + " "); }
// Prints A B C
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98
Queue
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99
 Queue Animator:
• Queues: Array Implementation
• Queues: Linked List Implementation
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100
Question
Why would you want to declare a variable as
Queue<String> q = new LinkedList<String>();
instead of simply declaring it as a linked list?
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101
Answer
This way, we can ensure that only queue operations
can be invoked on the q object.
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102
Stack and Queue Applications
 A stack can be used to check whether parentheses in an expression
are balanced.
When you see an opening parenthesis, push it on the stack.
When you see a closing parenthesis, pop the stack.
If the opening and closing parentheses don't match
The parentheses are unbalanced. Exit.
If at the end the stack is empty
The parentheses are balanced.
Else
The parentheses are not balanced.
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103
Stack and Queue Applications
 Walkthrough of the sample expression:
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104
Stack and Queue Applications
 Use a stack to evaluate expressions in reverse Polish notation.
If you read a number
Push it on the stack.
Else if you read an operand
Pop two values off the stack.
Combine the values with the operand.
Push the result back onto the stack.
Else if there is no more input
Pop and display the result.
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105
Stack and Queue Applications
 Walkthrough of evaluating the expression 3 4 5 + ×:
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section_6_2/Calculator.java
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import java.util.Scanner;
import java.util.Stack;
/**
This calculator uses the reverse Polish notation.
*/
public class Calculator
{
public static void main(String[] args)
{
Scanner in = new Scanner(System.in);
Stack<Integer> results = new Stack<Integer>();
System.out.println("Enter one number or operator per line, Q to quit. ");
boolean done = false;
while (!done)
{
String input = in.nextLine();
// If the command is an operator, pop the arguments and push the result
if (input.equals("+"))
{
results.push(results.pop() + results.pop());
}
else if (input.equals("-"))
{
Integer arg2 = results.pop();
results.push(results.pop() - arg2);
}
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Continued
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section_6_2/Calculator.java
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else if (input.equals("*") || input.equals("x"))
{
results.push(results.pop() * results.pop());
}
else if (input.equals("/"))
{
Integer arg2 = results.pop();
results.push(results.pop() / arg2);
}
else if (input.equals("Q") || input.equals("q"))
{
done = true;
}
else
{
// Not an operator--push the input value
results.push(Integer.parseInt(input));
}
System.out.println(results);
}
}
}
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108
Evaluating Algebraic Expressions with Two Stacks
 Using two stacks, you can evaluate expressions in standard algebraic
notation.
• One stack for numbers, one for operators
 Evaluating the top: 3 + 4
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109
Evaluating Algebraic Expressions with Two Stacks
 Evaluate 3 x 4 + 5
• Push until you get to the +
• x (top of operator stack) has higher precedence than + , so evaluate the top
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110
Evaluating Algebraic Expressions with Two Stacks
 Evaluate 3 + 4 × 5
• Add x to the operator stack so we can get the next number
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111
Evaluating Algebraic Expressions with Two Stacks
• Keep operators on the stack until they are ready to be evaluated
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112
Evaluating Algebraic Expressions with Two Stacks
 Evaluating parentheses: 3 × (4 + 5)
• Push ( on the stack
• Keep pushing until we reach the )
• Evaluate until we find the matching (
Copyright © 2014 by John Wiley & Sons. All rights reserved.
113
Evaluating Algebraic Expressions with Two Stacks
 The algorithm
If you read a number
Push it on the number stack.
Else if you read a (
Push it on the operator stack.
Else if you read an operator op
While the top of the stack has a higher precedence than op
Evaluate the top.
Push op on the operator stack.
Else if you read a )
While the top of the stack is not a (
Evaluate the top.
Pop the (.
Else if there is no more input
While the operator stack is not empty
Evaluate the top.
At the end, the value on the number stack the the value of the
expression
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114
Evaluating Algebraic Expressions with Two Stacks
 Helper method to evaluate the top:
Pop two numbers off the number stack.
Pop an operator off the operator stack.
Combine the numbers with that operator.
Push the result on the number stack.
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115
References
 From : Beginning Java Objects, JACQUIE BARKER
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