Java 103 intro to java data structures

Java 103: Intro to Java Data Structures

Java 103

• Data Structures

– Arrays

– Java Collections Framework

– Collection Algorithms


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.

Array Processing Examples

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 DayPrinterSundayMondayTuesdayWednesdayThuesdayFridaySaturday


Equals and HashCode Methods

Overriding Object methods

• public boolean equals(Object o)

• public int hashCode()

Use @Override!

When not to override Equals?

• Each instance of the class is inherently unique (e.g. Thread)

• You don’t care whether the class provides a “logical equality” test (e.g. Random)

• A superclass has already overridden equals, and the superclass behaviour is appropriate for this class (e.g. Set/List/Map and Abstract*)

• The class is private or package-private, and you are certain that its equals method will never be invoked (arguably, override and throw AssertionError)

Equals Contract

Reflexive x.equals(x) must return trueSymmetric x.equals(y) must return true iif

y.equals(x) returns trueTransitive if x.equals(y) is true and y.equals(z) is

true, then x.equals(z) must return trueConsistent multiple invocations of x.equals(y)

consistently return true or consistently return false, provided no information used in equals comparisons on the objects is modified

Null For any non-null reference value x, x.equals(null) must return false

Equals

• Use == to determine if two reference variables refer to the same object

• Use equals() to determine if two objects are meaningfully equivalent

• If you don't override equals(), your objects won't be useful hashing keys

• When overriding equals(), compare the objects' significant attributes

HashCode

• In real-life hashing, it’s not uncommon to have more than one entry in a bucket

• Hashing retrieval is a two-step process:1. Find the right bucket (using hashCode())2. Search the bucket for the right element (using equals() )

• An efficient hashCode() override distributes keys evenly across its buckets

• It's legal for a hashCode() method to return the same value for all instances (but very inefficient!)

• If you override equals(), override hashCode()• transient variables aren't appropriate for equals() and hashCode()• redundant fields can be excluded from the hash code computation• HashMap, HashSet, Hashtable, LinkedHashMap, & LinkedHashSet

use hashing

HashCode Contract

Consistent: multiple calls to x.hashCode() return the same integerCondition Required Not Required (But Allowed)

x.equals(y) == true x.hashCode() == y.hashCode()

x.hashCode() == y.hashCode()

x.equals(y) == true

x.equals(y) == false No hashCode() requirements

x.hashCode() != y.hashCode()

x.equals(y) == false


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.*;

public class DumpCollection {

public static void main(String[] args) {Collection days= Arrays.asList(”Mon",”Tues",”Wed",”Thurs",”Fri",”Sat", ”Sun");dumpCollection(months);

}

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)• Commonly Used Methods

– 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 occurance of x– remove (int index) removes the element at index

• Implementations– ArrayList– LinkedList

List World

List

ArrayList

LinkedList

Vector

List World

List

ArrayList

LinkedList

Vector

Fast iteration and fast random access

List World

List

ArrayList

LinkedList

Vector


Good for adding elements to the ends, i.e. stacks and queues

List World

List

ArrayList

LinkedList

Vector


Good for adding elements to the ends, i.e. stacks and queues

It's like a slower ArrayList, but it has synchronized methods

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 {

public static void main(String[] args) {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

• Commonly Used Methods

– add (Object x) returns true if x doesn’t already exist and false if it exists

• Implementations

– HashSet

– TreeSet

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


No duplicates; iterates by insertion order

Set World

Set

HashSet

LinkedHashSet

TreeSet


No duplicates; iterates 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 {

public static void main(String[] args) {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• Commonly Used Methods

– 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

• Implementations– HashMap– TreeMap

Map World

Map

Hashtable

LinkedHashMap

Treemap

HashMap

Map World

Map

Hashtable

LinkedHashMap

TreemapLike a slower HashMap (as with Vector, due to its synchronized methods). No null values or null keys allowed

HashMap

Map World

Map

Hashtable

LinkedHashMap


HashMap

Fastest updates (key/values); allows one null key, manynull values

Map World

Map

Hashtable

LinkedHashMap


Faster iterations; iterates by insertion order or last accessed; allows one null key, many null values

HashMap


Map World

Map

Hashtable

LinkedHashMap


Faster iterations; iterates by insertion order or last accessed; allows one null key, many null values

A sorted map

HashMap


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 {

public static void main(String[] args) {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

Collection Interface Concrete Implementation Classes

Class Map Set List Ordered Sorted

HashMap x No No

Hashtable x No No

TreeMap x Sorted By natural order or custom comparison rules

LinkedHashMap x By insertion order or last access order

No

HashSet x No No

TreeSet x Sorted By natural order or custom comparison rules

LinkedHashSet x By insertion order No

ArrayList x By index No

Vector x By index No

LinkedList x By index No

Key Methods in List, Set, and Map

Key Interface Methods List Set Map Descriptions

boolean add(element)boolean add(index, element)

XX

X Add an element. For Lists, optionallyadd 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, orvia the element’s value, or via a key

int size() X X X Return the number of elements in acollection

Object[] toArray()T[] toArray(T[])

X X Return an array containing theelements of the collection

Java 103: Intro to Java Collections

Collection Algorithms

What are 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)

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)

Key Methods in ArraysKey Methods in java.util.Arrays Descriptions

static List asList(T[]) Convert an array to a List (and bind them)

static int binarySearch(Object[], key)static int binarySearch(primitive[], key)

Search a sorted array for a given value, return an index or insertion point

static int binarySearch(T[], key, Comparator) Search a Comparator-sorted array for a value

static boolean equals(Object[], Object[])static boolean equals(primitive[], primitive[])

Compare two arrays to determine if their contents are equal

public static void sort(Object[ ] )public static void sort(primitive[ ] )

Sort the elements of an array by naturalorder

public static void sort(T[], Comparator) Sort the elements of an array using aComparator

public static String toString(Object[])public static String toString(primitive[])

Create a String containing the contents of an array

Key Methods in Collections

Key Methods in java.util.Collections Descriptions

static int binarySearch(List, key)static int binarySearch(List, key, Comparator)

Search a "sorted" List for a given value,return an index or insertion point

static void reverse(List) Reverse the order of elements in a List

static Comparator reverseOrder()static Comparator reverseOrder(Comparator)

Return a Comparator that sorts the reverse of the collection’s current sort sequence

static void sort(List)static void sort(List, Comparator)

Sort a List either by natural order or by aComparator

Hands-on Exercise

Sorting Arrays and Collections

Example: Sorting Arrays

import java.util.Arrays;import java.util.Collections;import java.util.List;

public class ArraySort {

public static void main(String[] args) {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 ArraySortAugDecFebJanJulyJuneMarMayNovOctSept

Example: Sorting Collections

import java.util.Arrays;import java.util.Collections;import java.util.List;

public class CollectionSort {

public static void main(String[] args) {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 CollectionySortAugDecFebJanJulyJuneMarMayNovOctSept

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/7/docs/technotes/guides/collections/

http://docs.oracle.com/javase/tutorial/collections/

http://docs.oracle.com/javase/7/docs/technotes/guides/collections/

Software

Java 103 intro to java data structures