Lists Last Update: Oct 29, 2014 EECS2011: Lists1

Last Update: Oct 29, 2014 EECS2011: Lists2 Topics: Lists Iterators Java Collections Framework

Part 1: Lists Last Update: Oct 29, 2014 EECS2011: Lists3

The java.util.List ADT The java.util.List interface includes the following methods: Last Update: Oct 29, 2014 EECS2011: Lists4

Example A sequence of List operations: Last Update: Oct 29, 2014 EECS2011: Lists5

Array Lists An obvious choice for implementing the list ADT is to use an array, A, where A[i] stores (a reference to) the element with index i. With a representation based on an array A, the get(i) and set(i, e) methods are easy to implement by accessing A[i] (assuming i is a legitimate index). Last Update: Oct 29, 2014 EECS2011: Lists6 A 012n i

Insertion In an operation add(i, o), we need to make room for the new element by shifting forward the n - i elements A[i], …, A[n - 1] In the worst case (i = 0), this takes O(n) time Last Update: Oct 29, 2014 EECS2011: Lists7 A 012n i A 012n i A 012n o i

Element Removal In an operation remove(i), we need to fill the hole left by the removed element by shifting backward the n - i - 1 elements A[i + 1], …, A[n - 1] In the worst case (i = 0), this takes O(n) time Last Update: Oct 29, 2014 EECS2011: Lists8 A 012n i A 012n o i A 012n i

Performance In an array-based implementation of a dynamic list: o The space used by the data structure is O(n) o Indexing the element at i takes O(1) time o add and remove run in O(n) time In an add operation, when the array is full, instead of throwing an exception, we can replace the array with a larger one … Last Update: Oct 29, 2014 EECS2011: Lists9

Java Implementation Last Update: Oct 29, 2014 EECS2011: Lists10

Java Implementation, 2 Last Update: Oct 29, 2014 EECS2011: Lists11

Growable Array-based Array List push(o) is the operation that adds element o at the end of the list A when the array is full, replace the array with a larger one how large should the new array be?  Incremental strategy: increase the size by a constant c  Doubling strategy: double the size Last Update: Oct 29, 2014 EECS2011: Lists12 Algorithm push(o) n  A.length if t = n  1 then S  new larger array // copy A’s contents into S: for i  0.. n-1 do S[i]  A[i] // rename: A  S // add new element: t  t + 1 A[t]  o Algorithm push(o) n  A.length if t = n  1 then S  new larger array // copy A’s contents into S: for i  0.. n-1 do S[i]  A[i] // rename: A  S // add new element: t  t + 1 A[t]  o

Comparison of the Strategies We compare the incremental strategy and the doubling strategy by analyzing the total time T(n) needed to perform a series of n push operations We assume that we start with an empty list represented by a growable array of size 1 We call amortized time of a push operation the average time taken by a push operation over the series of operations, i.e., T(n)/n Last Update: Oct 29, 2014 EECS2011: Lists13

Incremental Strategy Analysis Over n push operations, we replace the array k = n/c times, where c is the incremental enlargement constant The total time T(n) of a series of n push operations is proportional to n + c + 2c + 3c + 4c + … + kc = n + c( … + k) = n + ck(k + 1)/2 Since c is a constant, T(n) is O(n + k 2 ), i.e., O(n 2 ) Thus, the amortized time of a push operation is O(n) Last Update: Oct 29, 2014 EECS2011: Lists14

Doubling Strategy Analysis We replace the array k = log 2 n times The total time T(n) of a series of n push operations is proportional to n …+ 2 k = n + 2 k = 3n - 1 T(n) is O(n) The amortized time of a push operation is O(1) Last Update: Oct 29, 2014 EECS2011: Lists15 geometric series

Positional Lists To provide for a general abstraction of a sequence of elements with the ability to identify the location of an element, we define a positional list ADT. A position acts as a marker or token within the broader positional list. A position p is unaffected by changes elsewhere in a list; the only way in which a position becomes invalid is if an explicit command is issued to delete it. A position instance is a simple object, supporting only the following method: – p.getElement( ): Returns the element stored at position p. Last Update: Oct 29, 2014 EECS2011: Lists 16

Positional List ADT Accessor methods: Last Update: Oct 29, 2014 EECS2011: Lists17

Positional List ADT (2) Update methods: Last Update: Oct 29, 2014 EECS2011: Lists18

Example A sequence of Positional List operations: Last Update: Oct 29, 2014 EECS2011: Lists19

Positional List Implementation The most natural way to implement a positional list is with a doubly-linked list. Last Update: Oct 29, 2014 EECS2011: Lists20 prevnext element trailer header nodes/positions elements node

Insertion Insert a new node, q, between p and its successor. Last Update: Oct 29, 2014 EECS2011: Lists21 ABXC ABC p ABC p X q pq

Deletion Remove a node, p, from a doubly-linked list. Last Update: Oct 29, 2014 EECS2011: Lists22 ABCD p ABC D p ABC

Part 1: Summary The List ADT The java.util.List ADT Fixed size array implementation – Performance & limitations Growable array implementation – Amortized performance Positional List ADT – Doubly linked list implementation Last Update: Oct 29, 2014 EECS2011: Lists23

Part 2: Iterators & The Java Collections Framework Last Update: Oct 29, 2014 EECS2011: Lists24

The Java Collections Framework (JCF) is a good example of how to apply the principles of object-oriented software engineering to the design of classical data structures. A modular design of classes and interfaces that implement commonly reusable collection data structures.classesinterfacesdata structures Designed and developed primarily by Joshua Bloch (currently Chief Java Architect at Google).Joshua BlochGoogle Last Update: Oct 29, 2014 EECS2011: Lists25 The Java Collections Framework

What is a Collection? An object that groups multiple elements into a single unit. Sometimes called a container. Last Update: Oct 29, 2014 EECS2011: Lists26

What is a Collection Framework? A unified architecture for representing and manipulating collections. Includes: – Interfaces: A hierarchy of ADTs. – Implementations – Algorithms: The methods that perform useful computations, such as searching and sorting, on objects that implement collection interfaces.  These algorithms are polymorphic, that is, the same method can be used on many different implementations of the appropriate collection interface. Last Update: Oct 29, 2014 EECS2011: Lists27

History Apart from JCF, the best-known examples of collections frameworks are: – C++ Standard Template Library (STL) – Smalltalk's collection hierarchy. Last Update: Oct 29, 2014 EECS2011: Lists28

Benefits Reduces programming effort: By providing useful data structures and algorithms, the Collections Framework frees you to concentrate on the important parts of your program rather than on the low-level "plumbing" required to make it work. Increases program speed and quality: Provides high-performance, high-quality implementations of useful data structures and algorithms. Allows interoperability among unrelated APIs: APIs can interoperate seamlessly, even though they were written independently. Reduces effort to learn and to use new APIs Reduces effort to design new APIs Fosters software reuse: New data structures that conform to the standard collection interfaces are by nature reusable.  Last Update: Oct 29, 2014 EECS2011: Lists29

Where is JCF? Package java.util. In this lecture we will survey the interfaces, abstract classes and classes for linear data structures provided by JCF. We will not cover all of the details (e.g., thrown exceptions). For additional details, please see – The Java API – Comments and code in the specific java.util.*.java files provided with your java distribution. – The Collections Java tutorial available herehere – Chan et al, “The Java Class Libraries”, Second Edition. Last Update: Oct 29, 2014 EECS2011: Lists30

Core Collection Interfaces Last Update: Oct 29, 2014 EECS2011: Lists31

Traversing Collections in Java There are two ways to traverse collections: – Using Iterators. – Using the (enhanced) for-each construct Last Update: Oct 29, 2014 EECS2011: Lists32

Iterators An iterator is a software design pattern that abstracts the process of scanning through a sequence of elements, one element at a time. Last Update: Oct 29, 2014 EECS2011: Lists33

The for-each Loop Java’s Iterable class also plays a fundamental role in support of the “for-each” loop syntax: is equivalent to: Last Update: Oct 29, 2014 EECS2011: Lists34

The Iterable Interface The package java.lang defines a parameterized interface, named Iterable, that includes the following single method: Iterator iterator( ) Returns an iterator of the elements of type T in the collection. An instance of a typical collection class in Java, such as an ArrayList, is iterable (but not itself an iterator); it produces an iterator for its collection as the return value of the iterator( ) method. Each call to iterator( ) returns a new iterator instance, thereby allowing multiple (even simultaneous) traversals of a collection. Last Update: Oct 29, 2014 EECS2011: Lists35

Iterators An Iterator is an object that enables you to traverse through a collection and to remove elements from the collection selectively, if desired.Iterator You get an Iterator for a collection by calling the collection’s iterator method. Suppose collection is an instance of a Collection. Then to print out each element on a separate line use: Iterator iter = collection.iterator( ); while (iter.hasNext( )) System.out.println(iter.next( )); Note that next( ) does two things: 1.Returns the current element (initially the first element) 2.Steps to the next element and makes it the current element. Last Update: Oct 29, 2014 EECS2011: Lists36

hasNext( ) returns true if the iteration has more elements next( ) returns the next element in the iteration. – throws exception if iterator has already visited all elements. remove( ) removes the last element that was returned by next. – remove may be called only once per call to next – otherwise throws an exception. – Iterator.remove is the only safe way to modify a collection during iteration Last Update: Oct 29, 2014 EECS2011: Lists37 The Iterator interface public interface Iterator { boolean hasNext( ); E next( ); void remove( ); //optional } public interface Iterator { boolean hasNext( ); E next( ); void remove( ); //optional }

Implementing Iterators Could make a copy of the collection. – Good: could make copy private. No other objects could change it from under you. – Bad: construction takes O(n) time. Could use the collection itself (the typical choice). – Good: construction, hasNext and next are all O(1). – Bad: if another object makes a structural change to the collection, the results are unspecified. Last Update: Oct 29, 2014 EECS2011: Lists38

The Generality of Iterators iterators are general: they apply to any collection. – Could represent a sequence, set or map. – Could be implemented using arrays or linked lists. Last Update: Oct 29, 2014 EECS2011: Lists39

ListIterators ListIterator extends Iterator to treat the collection as a list, allowing – access to the integer position (index) of elements – forward and backward traversal – modification and insertion of elements. The current position is viewed as being either – Before the first element – Between two elements – After the last element Last Update: Oct 29, 2014 EECS2011: Lists40 Iterator ListIterator

ListIterators Last Update: Oct 29, 2014 EECS2011: Lists41 ListIterators support the following methods: add(e): inserts element e at current position (before implicit cursor) hasNext( ) hasPrevious( ) previous( ): returns element before current position and steps backward next( ): returns element after current position and steps forward nextIndex( ) previousIndex( ) set(e): replaces the element returned by the most recent next( ) or previous( ) call remove( ): removes the element returned by the most recent next( ) or previous( ) call Iterator ListIterator

Part 3: Java Collections Framework Last Update: Oct 29, 2014 EECS2011: Lists42

Levels of Abstraction Recall that Java supports three levels of abstraction: – Interface Java expression of an ADT Includes method declarations with arguments of specified types, but with empty bodies – Abstract Class Implements only a subset of an interface. Cannot be used to instantiate an object. – (Concrete) Class May extend one (abstract or concrete) class Must fully implement any interfaces it implements Can be used to instantiate objects. Last Update: Oct 29, 2014 EECS2011: Lists43

JCF  Ordered Data Types Last Update: Oct 29, 2014 EECS2011: Lists44 Abstract Class Interface Class Iterable Collection Queue Abstract Collection List Abstract Queue Abstract List Priority Queue Abstract Sequential List Array List Vector Stack Linked List Deque

Is an interface in java.lang Allows an Iterator to be associated with an object. The iterator allows an existing data structure to be stepped through sequentially, using the following methods: – hasNext( ) returns true if the iteration has more elements – next( ) returns the next element in the iteration.  throws exception if iterator has already visited all elements. – remove( ) removes last element returned by next. remove may be called only once per call to next otherwise throws an exception. Iterator.remove is the only safe way to modify a collection during iteration Last Update: Oct 29, 2014 EECS2011: Lists45 Iterable

Allows data to be modeled as a collection of objects. In addition to the Iterator interface, provides interfaces to: Last Update: Oct 29, 2014 EECS2011: Lists46 Creation: add(e) addAll(c) Creation: add(e) addAll(c) Querying: size( ) isEmpty( ) contains(e) containsAll(c) toArray( ) equals(e) Querying: size( ) isEmpty( ) contains(e) containsAll(c) toArray( ) equals(e) Modification: remove(e) removeAll(c) retainAll(c) clear( ) Modification: remove(e) removeAll(c) retainAll(c) clear( ) Collection

Skeletal implementation of the Collection interface. For unmodifiable collection, we still need to implement: – iterator (including hasNext and next methods) – size For modifiable collection, we need to also implement: – remove method for iterator – add Last Update: Oct 29, 2014 EECS2011: Lists47 Abstract Collection

Extends the Collections interface to model the data as an ordered sequence of elements with 0-based integer position indexing. Provides interface for creation of a ListIterator Also adds interfaces for: Last Update: Oct 29, 2014 EECS2011: Lists48 List Creation: add(e): append element e to the list add(i,e): insert element e at position i (and shift elements at positions > i one to the right). Creation: add(e): append element e to the list add(i,e): insert element e at position i (and shift elements at positions > i one to the right). Querying: get(i): return element currently stored at position i indexOf(e): return index of first occurrence of specified element e lastIndexOf(e): return index of last occurrence of specified element e subList(i,j): return list of elements from index i to j Querying: get(i): return element currently stored at position i indexOf(e): return index of first occurrence of specified element e lastIndexOf(e): return index of last occurrence of specified element e subList(i,j): return list of elements from index i to j

Extends the Collections interface to model the data as an ordered sequence of elements with 0-based integer position indexing. Provides interface for creation of a ListIterator Also adds interfaces for: Last Update: Oct 29, 2014 EECS2011: Lists49 List Modification: set(i,e): replace element currently stored at index i with specified element e remove (e): remove the first occurrence of the specified element from the list remove(i): remove the element at position i Modification: set(i,e): replace element currently stored at index i with specified element e remove (e): remove the first occurrence of the specified element from the list remove(i): remove the element at position i

Skeletal implementation of the List interface. For unmodifiable list, we need to implement methods: – get – size For modifiable list, need to implement – set For variable-size modifiable list, need to implement – add – remove Last Update: Oct 29, 2014 EECS2011: Lists50 Abstract List

Random access implementation of the List interface Uses an array for storage. Supports automatic array-resizing Adds methods – trimToSize( ): Trims capacity to current size – ensureCapacity(n): Increases capacity to at least n – clone( ): Create copy of list – removeRange(i, j): Remove elements at positions i to j – RangeCheck(i): throws exception if i not in range – writeObject(s): writes out list to output stream s – readObject(s): reads in list from input stream s Last Update: Oct 29, 2014 EECS2011: Lists51 Array List

Similar to ArrayList. But all methods of Vector are synchronized. – Uses an internal lock to prevent multiple threads from concurrently executing methods for the same vector object. – Other threads trying to execute methods of the object are suspended until the current thread completes. – Helps to prevent conflicts and inconsistencies in multi-threaded code Vector is a so-called legacy class: no longer necessary for new applications, but still in widespread use in existing code. Synchronization can be achieved using synchronization wrappers (we will not cover this). Last Update: Oct 29, 2014 EECS2011: Lists52 Vector

Represents a last-in, first-out (LIFO) stack of objects. Adds 5 methods: – push( ) – pop( ) – peek( ) – empty( ) – search(e): return the 1-based position of e on stack. Last Update: Oct 29, 2014 EECS2011: Lists53 Stack

Skeletal implementation of the List interface. Assumes a sequential access data store (e.g., linked list) We need to implement methods – listIterator( ), size( ) For unmodifiable list, we need to implement list iterator’s methods: – hasNext( ), next( ), hasPrevious( ), previous( ), nextIndex( ), previousIndex( ) For modifiab le list, need to also implement list iterator’s – set(e) For variable-size modifiable list, need to implement list iterator’s – add(e), remove( ) Last Update: Oct 29, 2014 EECS2011: Lists54 Abstract Sequential List

Designed for holding elements prior to processing Typically first-in first-out (FIFO) Defines a head position, which is the next element to be removed. Provides additional insertion, extraction and inspection operations. Extends the Collection interface to provide interfaces for: – offer(e): add e to queue if there is room (return false if not) – poll( ): return and remove head of queue (return null if empty) – remove( ): return and remove head of queue (throw exception if empty) – peek( ): return head of queue (return null if empty) – element( ): return head of queue (throw exception if empty) Last Update: Oct 29, 2014 EECS2011: Lists55 Queue

Implements the List and Queue interfaces. Uses a doubly-linked list data structure. Extends the List interface with additional methods: – getFirst( ) – getLast( ) – removeFirst( ) – removeLast( ) – addFirst(e) – addLast(e) These make it easier to use the LinkedList class to create stacks, queues and deques (double-ended queues). Last Update: Oct 29, 2014 EECS2011: Lists56 Linked List

LinkedList objects are not synchronized by default. However, the LinkedList iterator is fail-fast: if the list is structurally modified at any time after the iterator is created, in any way except through the Iterator's own remove or add methods, the iterator will throw a ConcurrentModificationException. This is detected at the first execution of one of the iterator’s methods after the modification. In this way the iterator will hopefully fail quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future. Last Update: Oct 29, 2014 EECS2011: Lists57 Linked List

Skeletal implementation of the Queue interface. Provides implementations for – add(e) – remove( ) – element( ) – clear( ) – addAll(c) Last Update: Oct 29, 2014 EECS2011: Lists58 Abstract Queue

Based on priority heap Elements are prioritized based either on – natural order, or – a comparator, passed to the constructor. Provides an iterator We will study this in detail when we get to heaps! Last Update: Oct 29, 2014 EECS2011: Lists59 Priority Queue

Part 3: Summary From this part you should understand: – The purpose and advantages of the JCF – How interfaces, abstract classes and classes are used hierarchically to achieve some of the key goals of object- oriented software engineering. – iterators: purpose; how to create & use them. – How JCF can be used to develop code using general collections, lists, array lists, stacks and queues. Last Update: Oct 29, 2014 EECS2011: Lists60

For More Details see The Java API Comments and code in the specific java.util.*.java files provided with your java distribution. The Collections Java tutorial available herehere Chan et al, “The Java Class Libraries”, Second Edition. Last Update: Oct 29, 2014 EECS2011: Lists61

Last Update: Oct 29, 2014 EECS2011: Lists 62