Cs2110 sw development methods
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CS2110: SW Development Methods. Textbook readings: MSD, Chapter 8 (Sect. 8.1 and 8.2) But we won ’ t implement our own, so study the section on Java ’ s Map classes… MSD, Chapter 9 (Sect. 9.3.2 and 9.3.4). Sets and Maps in Java. Set and HashSet in Java Collections.

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CS2110: SW Development Methods

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Cs2110 sw development methods

CS2110: SW Development Methods

  • Textbook readings:

    • MSD, Chapter 8 (Sect. 8.1 and 8.2)

      • But we won’t implement our own, sostudy the section on Java’s Map classes…

    • MSD, Chapter 9 (Sect. 9.3.2 and 9.3.4)

Sets and Maps in Java


Set and hashset in java collections

Set and HashSet in Java Collections

  • We’ve focused on List and ArrayList, but now…

  • A Set is a collection of items that

    • are unordered

    • are unique (no duplicates)

  • What kind of operations do we think of?

    • element membership (add, remove, isMember, etc.)

    • manipulation (union, intersection,…)

  • Java has:

    • Set – an interface

    • Several implementation classes, including HashSet


Java s set interface see msd p 625

Java’s Set Interface (see MSD, p. 625)

  • The Set interface looks just like the Collection interface, but with more specific behaviors

    • boolean add(elem) – returns false if already there

    • boolean remove(elem) – returns false if not there

  • What’s sweet here:

    • Try to add something that’s already there?Remove something that’s not there? No problem!

    • It basically ignores that attempt!

  • This allows some powerful methods to be very short

    • See examples on pages 625 and 626


Examples using set

Examples using Set

  • How many unique items in any Collection object?

    int numUniq = new HashSet(aCollection).size();

    • Creates a new set “on the fly” using the constructor that adds all items from an existing collection

  • Remove duplicates from any Collection object?

    Set noDups = new HashSet( aCollection );aCollection.clear();aCollection.addAll( noDups );

    • Note we don’t have to iterate

    • We rely on useful constructors and bulk operations

  • Procedural Abstraction opportunity: Put these lines of code into a method with a good names


Using sets in java treeset class

Using Sets in Java: TreeSet class

  • TreeSet is a concrete class that implements Set

  • How’s it work inside? How is it implemented?

    • We won’t focus on that in CS2110 (not now anyway)

    • Because it’s a well-implemented abstraction, you don’t need to understand what’s in the black-box

  • The “tree” refers to type of data structure used

  • But you do need one thing:

    • All items must be Comparable

    • All items must correctly implement compareTo()

    • Bonus! Iterating over a TreeSet uses compareTo()’s ordering. So prints in “correct” order


Using sets in java hashset class

Using Sets in Java: HashSet class

  • HashSet is a concrete class that implements Set

  • How’s it work inside? How is it implemented?

    • Right now, we’ll show you the one thing you need to know but not explain the details

  • The “hash” refers to this idea in computing

    • Data objects have state values that identify them uniquely, and…

    • Often we want to get a “hash value” or “ID-like” value for an object:

      • Equal items must have the same hash value

      • Un-equal items may have the same value

    • Hash-value used to link objects to “buckets”


Hash values for java classes

Hash Values for Java Classes

  • Hashing to get a hash-value is very useful, so…

  • Java’s superclass Object defines a method hashCode() for every class

    • If you want to use HashSet or any other class that uses hashing, override this method

    • Important: when you override equals(), you should override hashCode() too!

      • Otherwise things break when you don’t expect it.

      • Make sure that things that are equals() as you define that will also hash to the same value

    • Overriding hashCode() is often very very easy


Over riding hashcode in java

Over-riding hashCode() in Java

  • Goal: make sure that two items that areequals() return the same value when hashCode() is called

  • Focus on what fields determine identity of objects

    • Example #1: Class Person

      • Say two objects are equals() if their SSN fields are equal

    • Example #2: Class Song

      • Say two objects are equals() if their title and artist fields are both equal

  • Caveat: later we’ll see that the “simple” rules you’ll see next may not always have the best performance (but they will work)


Over riding hashcode in java 2

Over-riding hashCode() in Java (2)

  • If your object’s “key” is a String

    • Guess what? Most Java library classes already implement hashCode(). So use this fact!

    • If just one String value, your hashCode() method should just return the result of calling hashCode() on that String object

  • If multiple fields, here’s a simple solution:

    • get the hash-value for each individual field and just add them

  • See example in MSD text for Class Dog (p. 578)

  • Example #2: for class Song where title and artist must be equals(), the method just does:

    return artist.hashCode() + title.hashCode();


Over riding hashcode in java 21

Over-riding hashCode() in Java (2)

  • If your “key” field is numeric?

  • If integers, then:

    • Just return that field

    • Example #2: Class InventoryItem

      • Just return the int value itemID for that object

  • If not an integer but a double etc.

    • Use the wrapper class to create an object

      Double dval = new Double(doubleKeyField);

    • Each wrapper class has hashCode() defined, so…

      return dval.hashCode();

  • Multiple fields affect equals()? Again, sum up the hash-values for each field.


Summary on set hashset

Summary on Set, HashSet

  • HashSet is an implementation of Set you can use

    • Don’t forget to implement hashCode() for items you want to store!

    • You MUST over-ride hashCode() and equals() together! If you do one, always do the other!

  • Items in Sets are not ordered

    • But you can construct a List from a Set and sort it (in two lines of code)

  • add() handles duplicates, and remove() ignores missing items


Adt table

ADT Table

  • What’s a table? What’s “lookup”?


Adt table1

ADT Table

  • Lookup: find data given some key value

  • ADT Table’s model of information

    • Some set of values associated with a key

  • Operations

    • Lookup: given a key, retrieve the values

    • Is this key stored in the table?

    • Insert, remove, updates, etc.

  • How could we implement this?

    • You’ve been using lists (arrays, ArrayLists) and the contains() and indexOf() methods

    • Disadvantages?


Java type maps

Java Type: Maps

  • Maps

    • Like sets, but have <key, value> instead of <value>

    • Dictionary example

    • Other examples

      • Student’s email to Facebook Profile

      • Host Names to IP addresses

  • How to declare: Map<KeyType,ValueType>…Examples:

    Map<String,Integer> // maps string to a number

    Map<Student,String> // maps Student to a StringMap<String,List<Course>> // e.g. a schedule


Important map methods

Important Map Methods

  • Keys and values

    • put (key, value), get(key), remove(key)

    • containsKey(key), containsValue(value)

  • Can think of a Map as a set of keys and a set of values

    • keySet() // returns a Set of key values

    • values() // returns a Collection of values

    • (Note: Map itself does not extend Collection, but it uses Sets and is part of the Java Collection Framework)

  • Others too! See doc or a Java reference.


Concrete classes

Concrete Classes

  • HashMap

    • Most commonly used. Allows nulls as values.

    • Need a hashCode() method

  • TreeMap

    • Uses a search tree data structure

    • Values stored in a sorted order

    • Keys must have a compareTo() method

      • Or can create with a Comparator object

  • HashTable

    • Old class -- deprecated! Don’t use!


Why are maps useful and important

Why Are Maps Useful and Important?

  • “Lookup values for a key” -- an important task

    • We often want to do this!

    • Need flexibility for what values are stored for a given key

    • Need to use objects other than Strings or ints as key values

  • Efficiency matters for large sets of data

    • HashMap has O(1) lookup cost

    • TreeMap is O(lg n) which is still very good


Summary

Summary

  • Set and Table are important ADTs

    • Java supports these directly

  • Set interface

    • Concrete classes: HashSet, TreeSet

    • Note:

      • How add() and remove() fail “nicely” (I.e. return false)

      • How used to achieve solve more complex tasks with Collections

    • HashSet relies on proper implementation ofhashCode()

      • hashCode() and equals() must be “consistent”

    • TreeSet relies on compareTo()


Summary 2

Summary (2)

  • Map Interface

    • Concrete classes: HashMap and TreeMap

    • Declared using both key-type and value-type, e.g. Map<Student,List<Course>>

    • Important because “lookup” is a frequent problem

    • Lookup for maps is very efficient for large amounts of data

    • Code often needs to take different actions for:

      • The first time a key/value pair is inserted into a map

      • We want to update information for an existing key


Note map not is a collection

Note: Map not IS-A Collection

  • It’s wrong to say a Map is a Collection

    • It does not implement the Collection interface

    • Why not? For example, add(E) doesn’t make sense for a map

    • We need to add a (key,value) pair

  • But Map and its concrete classes are part of the JCF

    • They use Sets as part of their definition and API


Big picture on java collections unit

Big Picture on Java Collections Unit

  • Java Collections Framework

    • A large collections of interfaces and classes

    • Includes things we can use “as is” (concrete classes) and building blocks for new things (abstract classes)

    • Classes that exist to contain static methods, e.g. Collections class (see also Arrays class)

  • Power of inheritance, interfaces, polymorphism

    • See how “type” Collection and List are used in parameter lists and in Collections class’ methods (e.g. sort)

  • Be aware of and start to use Java 5.0 generics with your collections


Big picture on java collections unit 2

Big Picture on Java Collections Unit (2)

  • Iterators: both concepts and the practice

    • Example of procedural abstraction (use, methods) anddata abstraction (what’s really in a iterator object?)

    • Using these in Java for Collections, Lists (methods, cursor, etc.)

  • Procedural abstraction: the concepts, examples

  • Function-objects and procedural abstraction

    • Encapsulating “functionality” or “control” in an object

    • Comparator objects (contrast to implementing Comparable interface)

    • Java trick: anonymous classes


Big picture on java collections unit 3

Big Picture on Java Collections Unit (3)

  • Java skills: Concrete collection classes and methods for you to use

    • ArrayList and ListIterator

    • HashSet

      • Must over-ride hashCode()

  • Collections static methods

    • See JavaDoc or p 587 in MSD textbook

      • E.g. sort, reverse, binarySearch, fill, shuffle, min, max,…

    • Implement Comparable for your collection items

    • Define Comparator classes for more powerful, flexible use


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