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Abstract Data Types. many slides taken from Mike Scott, UT Austin. Data Structures. Data structure is a representation of data and the operations allowed on that data. Abstract Data Types.
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Abstract Data Types many slides taken from Mike Scott, UT Austin
Data Structures • Data structure is a representation of data and the operations allowed on that data.
Abstract Data Types • In Object Oriented Programming data and the operations that manipulate that data are grouped together in classes • Abstract Data Types (ADTs) or data structures or collections store data and allow various operations on the data to access and change it
Why Abstract? • Specify the operations of the data structure and leave implementation details to later • in Java use an interface to specify operations • many, many different ADTs • picking the right one for the job is an important step in design • "Get your data structures correct first, and the rest of the program will write itself." -Davids Johnson • High level languages often provide built in ADTs, • the C++ STL, the Java standard library
The Core Operations • Every Collection ADT should provide a way to: • add an item • remove an item • find, retrieve, or access an item • Many, many more possibilities • is the collection empty • make the collection empty • give me a sub set of the collection • and on and on and on… • Many different ways to implement these items each with associated costs and benefits
Implementing ADTs • when implementing an ADT the operations and behaviors are already specified • think Java interface • Implementer’s first choice is what to use as the internal storage container for the concrete data type • the internal storage container is used to hold the items in the collection • often an implementation of an ADT • initially slim pickings for choice of storage containers: arrays anyone?
The Grand Tour • Why study ADTs? • Why reimplement some of them? • How many of you will actually go out and create your own linked list ADT from scratch? • Remember, the primary goal is to learn how to learn how to use and create ADTs • also learn the behavior of some of the more conventional ADTs
Bags and Sets • Simplest ADT is a Bag • items can be added, removed, accessed • no implied order to the items • duplicates allowed • Set • same as a bag, except duplicate elements not allowed • union, intersection, difference, subset
Lists • Items have a position in this Collection • Random access or not? • Array Lists • internal storage container is native array • Linked Lists public class Node { private Object data; private Node next; } last first
Stacks • Collection with access only to the last element inserted • Last in first out • insert/push • remove/pop • top • make empty Data4 Top Data3 Data2 Data1
Queues • Collection with access only to the item that has been present the longest • Last in last out or first in first out • enqueue, dequeue, front • priority queues and deque Front Back Data1 Data2 Data3 Data4
Stacks and Queues in the Java Collection API • No queue in the Java collections ADT • Stack extends Vector (which is almost exactly like ArrayList) • Hmmm? • One reason the Java Collections Library is often said to be broken • no Queue in Collection API
Trees • Similar to a linked list public class TreeNode { private Object data; private TreeNode left; private TreeNode right; } Root
Other Types of Trees • Binary Search Trees • sorted values • Heaps • sorted via a different algorithm • AVL and Red-Black Trees • binary search trees that stay balanced • Splay Trees • B Trees
HashTables • Take a key, apply function • f(key) = hash value • store data or object based on hash value • Sorting O(N), access O(1) if a perfect hash function and enough memory for table • how deal with collisions?
Other ADTs • Maps • a.k.a. Dictionary • Collection of items with a key and associated values • similar to hash tables, and hash tables often used to implement Maps • Graphs • Nodes with unlimited connections between other nodes • Sparse vectors and sparse matrices
The Java Collection Interface boolean isEmpty() int size() boolean add(Object x) boolean contains(Object x) boolean remove(Object x) void clear() Object[] toArray() Iterator iterator()
Generic Containers • ADTs or Collection classes should be generic • only write them once, hold lots or all types of data • Java achieves genericity through inheritance and polymorphism • ADTs have an internal storage container • What is storing the stuff, • implementation vs. abstraction • in Java, usually holds Objects. Why?
