cs 3331 1 collection and input/output classes cs 3331 fall 2009

CS 3331 1

Collection and Input/Output Classes

CS 3331

Fall 2009

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Outline

Collection classes Collection interfaces Implementation classes Iterators Ordering

Input/output classes

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Collection Classes in Java Major types of collections

Sets, bags, lists, maps Defined in the java.util package

Interfaces of collections

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Collection InterfaceMethod Description

add(o) Add a new elementaddAll(c) Add all elements of c

remove(o) Remove an element removeAll(c) Remove all elements found in cretainAll(c) Retain only elements found in cclear() Remove all elements

contains(o) Membership testing containsAll(c) Membership testing

isEmpty() Whether it is empty size() The number of elements

iterator() Return an iterator

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Set InterfaceMethod Description

add(o) Add an element if not already presentaddAll(c) Add each elements of c if not present

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Exercise Define a set-intersection method.

public static Set union(Set x, Set y) { Set s = new HashSet(); // HashSet will be discussed later. s.addAll(x); s.addAll(y); return s;}public static Set intersection(Set x, Set y) { // WRITE YOUR CODE HERE

}

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List InterfaceMethod Description

add(i, o) Insert o at the i-th positionadd(o) Append o at the endaddAll(i, c) Insert all elements of c starting at the i-th positionaddAll(c) Append all elements of c at the end

remove(i) Remove i-th elementremove(o) Remove the first occurrence of o set(i, o) Replace i-th element with o

get(i) Return i-th elementindexOf(o) Return the index of the first occurrence of olastIndexOf(o) Return the index of the last occurrence of olistIteratpr() Return a list iteratorlistIterator(i) Return a list iterator for the sublist starting from isubList(i, j) Retrun a sublist between index iand j

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Map InterfaceMethod Description

put(k,v) Associate v with k

remove(k) Remove the mapping for k clear() Remove all mappings

get(k) The value associated with k

containsKey(k) Whether contains a mapping for k containsValue(v) Whether contains a mapping to v

size() The number of pairs isEmpty() Whether it is empty

….

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Map Interface (Cont.)

Method Description

entrySet() Set of key-value pairs keySet() Set of keys values() The collection of values

k1k2k3...kn

v1v2v3...vn

keySet() values()

entrySet()

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Outline



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Implementation of Collections

Why different implementations?Bounded vs. unboundedTime and space complexity of operations

Sets

Class Interface Description

HashSet Set Hash table LinkedHashSet Set Hash table & DLLTreeSet SortedSet Balanced binary tree

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Implementation (Cont.)

Lists


ArrayList List Resizable array LinkedList List Doubly linked listVector List Legacy of JDK 1.0

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Implementation (Cont.)

Maps


HashMap Map Hash table IdentityHashMap Map Hash table with identity comparisonLinkedHashMap Map Hash table and DLLTreeMap SortedMap Balanced binary treeHashtable Map Legacy of JDK 1.0

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Example (JDK 1.5 or above)

Counting word frequency

public static void frequence(String[] words) { Map<String, Integer> map = new LinkedHashMap<String, Integer>(); for (String w: words) { if (!map.containsKey(w)) { map.put(w, 1); } else { map.put(w, 1 + map.get(w)); } } for (String k: map.keySet()) { System.out.println(k + “:\t” + map.get(k)); }}

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Example (JDK 1.4 or below)

Counting word frequency

public static void frequence(String[] words) { Map map = new LinkedHashMap(); for (int i = 0; i < words.length; i++) { if (!map.containsKey(words[i])) { map.put(words[i], new Integer(1)); } else { map.put(words[i], new Integer(1 + ((Integer) map.get(words[i])).intValue()); } } for (Iterator i = map.keySet().iterator(); i.hasNext(); ) { String word = (String) i.next(); System.out.println(word + “:\t” + map.get(word)); }}

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Exercise Write a method that counts the number of different

words

/** Returns the number of different words in the array words. */public static int numOfDifferentWords(String[] words) { // WRITE YOUR CODE HERE

}

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Iterators of Collections

Iterator ListIterator

Method Description

add(o) Insert o in the current positionremove() Remove the last elementset(o) Replace the current element with o

hasNext() More element in the forward? hasPrevious() More element in the reverse?

next() Return the next element nextIndex() Return the next index

previous() Return the previous element previousIndex() Return the previous index

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Group Work: Set Implementation

Work in group of two or three to write class ArraySet that implements the Set interface. You should use an array to store the elements.

Method Description

add(o) Add a new elementaddAll(c) Add all elements of c

remove(o) Remove an element removeAll(c) Remove all elements found in cretainAll(c) Retain only elements found in cclear() Remove all elements

contains(o) Membership testing containsAll(c) Membership testing

isEmpty() Whether it is empty size() The number of elements

iterator() Return an iterator

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Outline



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Ordering and Sorting

Partial order (or order)Binary relation that is transitiveTotal order if a < b and b < a implies a = b

How to define order on objects?Natural order by implementing the

Comparable interfaceArbitrary order by comparators (classes

implementing the Comparator interface)

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Comparable Interface

Method compareTo: Result < 0, if the receiver precedes o Result = 0, if neither the receiver precedes o, nor o precedes the receiver Result > 0, if o precedes the receiver

Properties (or constraints) a.compareTo(b) > 0 implies that b.compareTo(a) < 0 a.compareTo(b) < 0 implies that b.compareTo(a) > 0 a.compareTo(b) = 0 implies that b.compareTo(a) = 0; Consistent with the definition of equals, i.e., a.equals(b) is true iff a.compareTo(b) is 0

public interface Comparable { int compareTo(Object o);}

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Exercise Define a natural order for the Person class based on

the person’s name. (Hint: the String class implements the Comparable interface.)

public class Person implements Comparable { private /*@ non_null @*/ String name;

public int compareTo(Object other) { // YOUR CODE HERE …

}}

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Comparator Interface

Method compare: Result < 0, if o1 precedes o2 Result = 0, if neither o1 precedes o2, nor o2 precedes o1 Result > 0, if o2 precedes o1

Properties (or constraints) c.compare(a,b) > 0 implies that c.compare(b,a) < 0 c.compare(a,b) < 0 implies that c.compare(b,a) > 0 c.compare(a,b) = 0 implies that c.compare(a,b) = 0; Consistent with equals, i.e., c.compare(a,b) is 0 iff a.equals(b) and b.equals(a)

public interface Comparator { int compare (Object o1, Object o2);}

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Exercise Define a total ordering for the Person class based on

the person’s SSN.

Public class Person { //@ ensures \result > 0; public int getSSN() { /*… */ } // other declarations …}

public class PersonComparator implements Comparator { public int compare(Object p1, Object p2) { // YOUR CODE HERE …

}}

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Sorted Collections

InterfacesSortedSet and SortedMap

ImplementationsTreeSet and TreeMap

ExampleSortedSet s1 = new TreeSet();

SortedSet s2 = new TreeSet(new PersonComparator());

SortedMap m1 = new TreeMap();

SortedMap m2 = new TreeMap(new PersonComparator());

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SortedSet InterfaceMethod Description

comparator() Return the comparator

first() Return the first (lowest) elementlast() Return the last (highest) element

headSet(o) Return elements less than otailSet(o) Return elements greater than or equal to osubSet(o1,o2) Return elements between o1 and o2

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Exercise Write a method that, given an array of Person objects, returns a

sorted array of the argument array. Assume that the Person class has a natural order defined.

public static Person[] sort(/*@ non_null @*/ Person[] persons) { // YOUR CODE HERE …

}

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SortedMap InterfaceMethod Description

comparator() Return the comparator

firstKey() Return the first (lowest) keylastKey() Return the last (highest) key

headMap(k) Return maplets less than ktailMap(k) Return maplets greater than or equal to ksubMap(k1,k2) Return maplets between k1 and k2

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Outline



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Input/Output Classes

Two types of input/outputStream I/O

Sequential reading and writing Opened for reading or writing, but not both Byte streams vs. character streams

Random access I/O Non-sequential reading and writing Can be opened for both reading and writing

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Byte Streams

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Character Streams

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Example Usage

Reading lines from file

String fileName = …;BufferedReader reader = new BufferedReader( new InputStreamReader(new FileInputStream(fileName)));String line;while ((line = reader.readLine()) != null) { // do something with line …}

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Decorator Pattern To add additional responsibility or capability to an object

dynamically

cs 3331 1 collection and input/output classes cs 3331 fall 2009

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