Java 103

Java 103: Intro to Java Data
Structures

Introduction
• Your Name: Manuela Grindei
• Your day job: Software Developer @ Gamesys
• Your last holiday destination: Vienna

Java 103
• Data Structures
– Arrays
– Java Collections Framework
– Collection Algorithms

Structures
Arrays

Arrays
• Indexed sequence of values of the same type
• Store and manipulate huge quantities of data
• Examples:
– 52 playing cards in a deck
– Thousand undergrads at Hult
– 1 million characters in a book
– 10 million audio samples in an MP3 file
– 4 billion nucleotides in a DNA strand
– 73 billion Google queries per year
– 50 trillion cells in the human body
– 6.02 x 10e23 particles in a mole

Many Variables of the Same Type
• Example: Goal is to store 10 variables of the same
type without Arrays

Many Variables of the Same Type
• Example: Goal is to store 10 variables of the same
type Arrays

Arrays in Java
• Java has special language support for arrays
– To make an array: declare, create, and initialize it
– To access element i of array named a, use a[i]
– Array indices start at 0 and end at a.length - 1
• Compact alternative
– Declare, create, and initialize in one statement
– Default initialization: all numbers automatically set to zero

Two-Dimensional Arrays
• Examples
– Table of data for each experiment and outcome
– Table of grades for each student and assignments
– Table of gray scale values for each pixel in a 2D image
• Mathematical abstraction
– Matrix
• Java abstraction
– 2D array

Two-Dimensional Arrays in Java
• Array Access
– Use a[i][j] to access element in row i and column j
– Zero-based indexing
– Row and column indices start at 0

Setting Array Values at Compile Time
• Initialize 2D array by listing values

Arrays Summary
• Organized way to store huge quantities of data
• Almost as easy to use as primitive types
• Can directly access an element given its index

Hands-on Exercise
Array of Days

Exercise: Array of Days
• Create a new Java project in Eclipse named ArrayOfDays
• Create a java class named DayPrinter that prints out
names of the days in a week from an array.

Solution : Arrays of Days
public class DayPrinter {
public static void main(String[] args) {
//initialise the array with the names of days of the
week
String[] daysOfTheWeek =
{"Sunday","Monday","Tuesday","Wednesday",
"Thuesday","Friday”,"Saturday"};
//loop through the array and print their elements to
//stdout
for (int i= 0;i < daysOfTheWeek.length;i++ ){
System.out.println(daysOfTheWeek[i]);
}
}
}
% javac DayPrinter.java
% java DayPrinter
Sunday
Monday
Tuesday
Wednesday
Thuesday
Friday
Saturday

Structures
Java Collections Framework

What is a Collection?
• An object that groups multiple items into a single
unit
• Used to store, retrieve, manipulate, and
communicate aggregate data
• Represent data items that form a natural group,
– a poker hand (a collection of cards)
– a mail folder (a collection of letters)
– a telephone directory (a mapping of names to phone
numbers)

Collections Framework
• A unified architecture for representing and
manipulating collections
• Usually contain…
– Interfaces
– Implementations
– Algorithms
• Examples of collections frameworks in other
languages
– C++ Standard Template Library (STL)
– Smalltalk's collection hierarchy

Java Collections Framework
• Interfaces
Collection
List Set
Iterable
SortedSet Dequeue
Queue
NavigableSet NavigableMap
SortedMap
Map
Note: Map does not
extend Collection;
but it is a “collection”

Collection Interface
• Contains about a dozen methods that describe
common operations on groups of objects
• Commonly Used Methods
– add(Object x) adds x to the collection
– remove(Object x) removes x from the collection
– contains(Object x) returns true if x is in collection
– size () return number of elements
– toArray() returns an array containing elements
– iterator() returns an Iterator object for accessing the
elements

Hands-on Exercise
Dumping Collection Elements

Exercise: Dumping Collection Elements
• What happens when you run the following program?
import java.util.Arrays;
import java.util.Collection;
import java.util.Iterator;
public class DumpCollection {
Collection days= Arrays.asList(”Mon",”Tues",”Wed",”Thurs",”Fri",”Sat", ”Sun");
dumpCollection(days);
}
public static void dumpCollection(Collection c){
System.out.println("collection has " + c.size() + " elements");
Iterator iterator = c.iterator();
while (iterator.hasNext()){
System.out.println("Next element is " + iterator.next());
}
}
}

List Interface
• Keeps its elements in the order in which they are
added
• Each element has an index starting from 0
(similar to an array)
– add (int index, Object x) insert x at index
– get (int index) returns the element at index
– indexOf(Object x) returns the index of the first
occurrance of x
– remove (int index) removes the element at index

List World
List
ArrayList
LinkedList

List World
List
ArrayList
LinkedList
Fast iteration and fast
random access

List World
List
ArrayList
LinkedList
Fast iteration and fast
random access
Good for adding
elements to the ends,
i.e. stacks and queues

Hands-on Exercise
Creating a TODO List

Exercise: Simple TODO List
• Create a Java project in Eclipse named
SimpleTodoList
• Create a Java program named SimpleTodoList
• Create a List to store your todo list items (use
either ArrayList or LinkedList)
• Use a for-loop to display your list contents

Solution: Simple TODO List
import java.util.ArrayList;
public class SimpleTodoList {
ArrayList<String> todoList = new ArrayList<String>();
todoList.add("make breakfast");
todoList.add("read morning paper");
todoList.add("Doctors appointment");
for (String item : todoList ) {
System.out.println("Item:" + item) ;
}
}
}

Set Interface
• Sets contain an unordered list of elements
• They do not allow duplicate elements
– add (Object x) returns true if x doesn’t already
exist and false if it exists

Set World
Set
HashSet
LinkedHashSet
TreeSet

Set World
Set
HashSet
LinkedHashSet
TreeSet
Fast access, assures no
duplicates, provides no
ordering

Set World
Set
HashSet
LinkedHashSet
TreeSet
ordering
No duplicates; iterates
by insertion order
by insertion order

Set World
Set
HashSet
LinkedHashSet
TreeSet
ordering
by insertion order
No duplicates;
iterates in
sorted order

Hands-on Exercise
Set of Points

Example: Set of Points
• What happens when you run the following program?
import java.awt.Point;
import java.util.HashSet;
import java.util.Set;
public class AddTwice {
Set points = new HashSet();
Point p1 = new Point(10,20);
Point p2 = new Point(10,20);
points.add(p1);
points.add(p2);
System.out.println("number of points = " + points.size());
}
}

Map Interface
• Combines two collections called keys and values
• Associates exactly one value with each key
• Keys are used to get values from Maps
– put (Object key, Object value) associates a key and a value
– get(Object key) returns the value associated with key
– containsKey (Object key) return true if the Map associates
some value with key
– keySet() returns a Set containing the Map’s keys
– values() returns a Collection containing the Map’s values

Map World
Map
LinkedHashMap
TreeMap
HashMap

Map World
Map
LinkedHashMap
TreeMap
HashMap
Fastest updates (key/values);
allows one null key, many
null values

Map World
Map
LinkedHashMap
TreeMap
Faster iterations; iterates
by insertion order or last
accessed; allows one null
key, many null values
HashMap
null values

Map World
Map
LinkedHashMap
TreeMap
Faster iterations; iterates
by insertion order or last
accessed; allows one null
key, many null values
A sorted map
HashMap
null values

Hands-on Exercise
Word Frequency Table

Example: Word Frequency Table
import java.util.*;
/**
* Creates a word frequency table from command line arguments
*/
public class WordFrequency {
Map<String, Integer> m = new HashMap<String, Integer>();
for (String a : args) {
Integer freq = m.get(a);
m.put(a, (freq == null) ? 1 : freq + 1);
}
System.out.println(m.size() + " distinct words:");
System.out.println(m);
}
} % java WordFrequency if it is to be it is up to me to delegate
8 distinct words:
{to=3, is=2, it=2, if=1, me=1, delegate=1, up=1, be=1}

Collection Framework Summary
• Reduces programming effort
• Increases program speed and quality
• Allows interoperability among unrelated APIs
• Reduces effort to learn and to use new APIs
• Reduces effort to design new APIs
• Fosters software reuse

Key Methods in List, Set, and Map
Key Interface Methods List Set Map Descriptions
boolean add(element)
boolean add(index, element)
X
X
X Add an element. For Lists, optionally
add the element at an index point
boolean contains(object)
boolean containsKey(object key)
boolean containsValue(object value)
X X
X
X
Search a collection for an object (or,
optionally for Maps a key), return the
result as a boolean
object get(index)
object get(key)
X
X
Get an object from a collection, via an
index or a key
int indexOf(object) X Get the location of an object in a List
Iterator iterator() X X Get an Iterator for a List or a Set
Set keySet() X Return a Set containing a Map’s keys
put(key, value) X Add a key/value pair to a Map
remove(index)
remove(object)
remove(key)
X
X X
X
Remove an element via an index, or
via the element’s value, or via a key
int size() X X X Return the number of elements in a
collection
Object[] toArray() X X Return an array containing the

Java 103: Intro to Java
Collections
Collection Algorithms

Collection Algorithms
• Arrays Class
– Contains static methods for operations on Arrays
– Commonly Used Methods
• asList(Array array)
• sort (Array array)
• Collections Class
– Contains static methods for operations on Collections
– Commonly Used Methods
• sort (List list)
• binarySearch(List list, Object key)

Hands-on Exercise
Sorting Arrays and Collections

Example: Sorting Arrays
import java.util.Collections;
import java.util.List;
public class ArraySort {
String[] months = {"Jan","Feb","Mar","Apr","May","June", "July", "Aug",
"Sept","Oct","Nov","Dec"};
Arrays.sort(months);
for (Object month :months){
System.out.println(month);
}
}
}
% java ArraySort
Aug
Dec
Feb
Jan
July
June
Mar
May
Nov
Oct
Sept

Example: Sorting Collections
import java.util.Collections;
import java.util.List;
public class CollectionSort {
List months = Arrays.asList("Jan","Feb","Mar","Apr","May","June", "July", "Aug",
"Sept","Oct","Nov","Dec");
Collections.sort(months);
for (Object month :months){
System.out.println(month);
}
}
}
% java CollectionSort
Aug
Dec
Feb
Jan
July
June
Mar
May
Nov
Oct
Sept

Collection Algorithms Summary
• Collection algorithms enable sorting and
searching of collections
• Most algorithms are in …
– java.util.Collections
– java.utils.Arrays

Resources
• Collections Framework Tutorial: http://docs.oracle.com/javase/tutorial/collections/
• Official Java Collections Framework Documentation:
http://docs.oracle.com/javase/8/docs/technotes/guides/collections/

Java 103

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