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CS2136:Paradigms of Computation

Class 17:Java:

ContainersEnumerations & Iterators

Copyright 2001, 2002, 2003, Michael J. Ciaraldi and David Finkel

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But First…Remember that objects are active.Generally…

You don’t do things to them. You ask them to do things to

themselves.

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But Second…A little more on polymorphism,

overloading, and overriding.Remember:

Overloading means same operation name.Same or different classes.

Overriding means same operation signature.Different classes.Polymorphism only applies to overriding.

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PolyOver2.javaArt

method1(aa : Art) : voidmethod1(dd : Drawing) : voidmethod2(aa : Art) : voidmethod2(dd : Drawing) : voidmethod3(aa : Art) : voidmethod4(dd : Drawing) : void

Drawing

method1(aa : Art) : voidmethod1(dd : Drawing) : voidmethod3(dd : Drawing) : voidmethod4(aa : Art) : void

Cartoon

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Containers

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ContainersObjects which hold objects

What they hold are of class Object. Not any specific subclass.

Categories Collection

SetList

Map

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ContainersIn Java 1.3, Collections and Maps are

Interfaces. Need to actually use concrete classes

which implement these interfaces. Or create your own.

Containers were completely revamped in Java 2 (as implemented in JDK 1.2). Old ways, e.g. Vector, are de-

emphasized.

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Container Taxonomy (Eckel, pg. 502)

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Collections vs. MapsCollections hold individual elements.Maps hold pairs.

Key and value.

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CollectionsHold individual elementsTypes (subinterfaces):

A Set cannot have duplicate elements A List holds elements in a particular

sequence.Prints in square brackets, separated

by commas.

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MapsA Map holds a group of pairs of

objects.Each pair is a key and a value.a.k.a. associative array.Acts like a simple database.No duplicate keys.Prints in curly braces, with pairs

shown as key=value.

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CollectionsMethods common to all:

boolean add(Object o)Returns true if collection has changed.

boolean remove(Object o)Returns true if collection has changed.

void clear()

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ListHolds elements in a particular

sequence.Automatically expandsYou should use one of the concrete

classes which implement this interface: ArrayList LinkedList

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ArrayListCreating:

new ArrayList() new ArrayList(int initialCapacity)

Measuring: int size()

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ArrayListStoring:

boolean add(Object o) boolean add(int index, Object element) Object set(int index, Object element)

Retrieving: Object get(int index) Object remove(int index)

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ArrayListTesting:

boolean isEmpty() boolean contains(Object elem)

Finding (failure = -1): int indexOf(Object elem) int lastIndexOf(Object elem)

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SetHolds elements in no particular

sequence.No duplicates allowed.Automatically expands.You should use one of the concrete

classes which implement this interface: HashSet TreeSet

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MapStores values and keys.Duplicate keys not allowed.Can be tricky to set up.Concrete implementations:

Attributes HashMap Hashtable

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MapStoring:

Object put(Object key, Object value)Note: Both must be objects, not primitives.

Retrieving: Object get(Object key) Object remove(Object key)

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MapTesting:

boolean containsKey(Object key) boolean containsValue(Object value)

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Problems With MapMatching keysOverriding equals() and hashCode().

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HashMapImplements a hash table.Efficient, constant-time access.Two functions must be properly

implemented for the class used as the key: boolean equals() int hashCode()

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Overriding equals()The default method compares object

addresses: no good because you want to compare contents.

Return true if contents are equal, by whatever criteria are important in your case.

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Overriding hashCode()The hashCode() function must

generate an int derived from the key.The default method uses object

address: no good.Two objects with the same contents

must return the same hash. Vice versa not required.

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Enumerations & Iterators:A Brief Look

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Enumerations & IteratorsObjects used to step through a container.

Available for some standard container classes.

Work properly even if the container changes. Order might or might not be significant.

Java has two variations: Enumeration (old: since JDK 1.0) Iterator (new: since JDK 1.2)

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EnumerationsTo get an Enumeration e for container v:

Enumeration e = v.elements(); This is initialized at the start of the list.

To get the first and subsequent elements: someObject = e.nextElement()

To test if all have been accessed: e.hasMoreElements()

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Enumeration Examplefor (Enumeration e = v.elements(); e.hasMoreElements(); ) { System.out.println( e.nextElement());}

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IteratorsTo get an Iterator i for container v:

Iterator i = v.iterator(); This is initialized at the start of the list.

To get the first and subsequent elements: someObject = i.next()

To test if all have been accessed: i.hasNext()

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Iterator Examplefor (Iterator i = v.iterator(); i.hasNext(); ) { System.out.println(i.next());}

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Next TimeExceptionsNumbers