Notes

Outline

Chapter 12:  Data Structures Presentation slides for Java Software Solutions Foundations of Program Design Third Edition by John Lewis and William Loftus Java Software Solutions is published by Addison-Wesley Presentation slides are copyright 2002 by John Lewis and William Loftus. All rights reserved. Instructors using the textbook may use and modify these slides for pedagogical purposes.

Data Structures Now we can now explore some convenient techniques for organizing and managing information Chapter 12 focuses on: collections Abstract Data Types (ADTs) dynamic structures and linked lists queues and stacks non-linear data structures predefined collection classes

Collections A collection is an object that serves as a repository for other objects A collection usually provides services such as adding, removing, and otherwise managing the elements it contains Sometimes the elements in a collection are ordered, sometimes they are not Sometimes collections are homogeneous, sometimes the are heterogeneous

Abstract Data Types Collections can be implemented in many different ways An abstract data type (ADT) is an organized collection of information and a set of operations used to manage that information The set of operations defines the interface to the ADT As long as the ADT fulfills the promises of the interface, it doesn't really matter how the ADT is implemented Objects are a perfect programming mechanism to create ADTs because their internal details are encapsulated

Abstraction Our data structures should be abstractions That is, they should hide unneeded details We want to separate the interface of the structure from its underlying implementation This helps manage complexity and makes it possible to change the implementation without changing the interface

Static vs. Dynamic Structures A static data structure has a fixed size This meaning is different from the meaning of the static modifier Arrays are static;  once you define the number of elements it can hold, the number doesn’t change A dynamic data structure grows and shrinks at execution time as required by its contents A dynamic data structure is implemented using links

Object References Recall that an object reference is a variable that stores the address of an object A reference also can be called a pointer References often are depicted graphically:

References as Links Object references can be used to create links between objects Suppose a Student class contains a reference to another Student object

References as Links References can be used to create a variety of linked structures, such as a linked list:

Intermediate Nodes The objects being stored should not be concerned with the details of the data structure in which they may be stored For example, the Student class should not have to store a link to the next Student object in the list Instead, we can use a separate node class with two parts: 1) a reference to an independent object and 2) a link to the next node in the list The internal representation becomes a linked list of nodes

Magazine Collection Let’s explore an example of a collection of Magazine objects The collection is managed by the MagazineList class, which has an private inner class called MagazineNode Because the MagazineNode is private to MagazineList, the MagazineList methods can directly access MagazineNode data without violating encapsulation See MagazineRack.java (page xxx) See MagazineList.java (page xxx) See Magazine.java (page xxx)

MagazineRack.java

MagazineList.java

Magazine.java

Magazine Collection A method called insert could be defined to add a node anywhere in the list, to keep it sorted, for example (Figure 12.2 here)

Magazine Collection A method called delete could be defined to remove a node from the list (Figure 12.3 here)

Other Dynamic List Representations It may be convenient to implement as list as a doubly linked list, with next and previous references

Other Dynamic List Implementations It may be convenient to use a separate header node, with a count and references to both the front and rear of the list

Other Dynamic List Implementations A linked list can be circularly linked in which case the last node in the list points to the first node in the list If the linked list is doubly linked, the first node in the list also points to the last node in the list The representation should facilitate the intended operations and should make them easy to implement

Classic Data Structures Classic linear data structures include queues and stacks Classic nonlinear data structures include trees, binary trees, graphs, and digraphs

Queues A queue is similar to a list but adds items only to the rear of the list and removes them only from the front It is called a FIFO data structure:  First-In, First-Out Analogy:  a line of people at a bank teller’s window

Queues We can define the operations for a queue enqueue - add an item to the rear of the queue dequeue (or serve) - remove an item from the front of the queue empty - returns true if the queue is empty As with our linked list example, by storing generic Object references, any object can be stored in the queue Queues often are helpful in simulations or any situation in which items get “backed up” while awaiting processing

Queues A queue can be represented by a singly-linked list; it is most efficient if the references point from the front toward the rear of the queue A queue can be represented by an array, using the mod operator (%) to “wrap around” when the end of the array is reached and space is available at the front of the array

Stacks A stack ADT is also linear, like a list or a queue Items are added and removed from only one end of a stack It is therefore LIFO:  Last-In, First-Out Analogies:  a stack of plates in a cupboard, a stack of bills to be paid, or a stack of hay bales in a barn

Stacks Stacks often are drawn vertically:

Stacks Some stack operations: push - add an item to the top of the stack pop - remove an item from the top of the stack peek (or top) - retrieves the top item without removing it empty - returns true if the stack is empty A stack can be represented by a singly-linked list; it doesn’t matter whether the references point from the top toward the bottom or vice versa A stack can be represented by an array, but the new item should be placed in the next available place in the array rather than at the end of the array

Stacks The java.util package contains a Stack class Like ArrayList operations, the Stack operations operate on Object references See Decode.java (page xxx)

Decode.java

Trees and Binary Trees A tree is a non-linear data structure that consists of a root node and potentially many levels of additional nodes that form a hierarchy Nodes that have no children are called leaf nodes Nodes except for the root and leaf nodes are called internal nodes (Figure 12.8 here)

Trees and Binary Trees A binary tree is defined recursively.  Either it is empty (the base case) or it consists of a root and two subtrees, each of which is a binary tree Binary trees and trees typically are represented using references as dynamic links, though it is possible to use fixed representations like arrays

Graphs and Digraphs A graph is a non-linear structure Unlike a tree or binary tree, a graph does not have a root Any node in a graph can be connected to any other node by an edge Analogy: the highway system connecting cities on a map (Figure 12.9 here)

Graphs and Digraphs In a directed graph or digraph, each edges has a specific direction. Edges with direction sometimes are called arcs Analogy: airline flights between airports (Figure 12.10 here)

Graphs and Digraphs Both graphs and digraphs can be represented using dynamic links or using arrays. As always, the representation should facilitate the intended operations and make them convenient to implement

Collection Classes The Java standard library contains several classes that represent collections, often referred to as the Java Collections API Their underlying implementation is implied in the class names such as ArrayList and LinkedList Several interfaces are used to define operations on the collections, such as List, Set, SortedSet, Map, and SortedMap

Summary Chapter 12 has focused on: collections Abstract Data Types (ADTs) dynamic structures and linked lists queues and stacks non-linear data structures predefined collection classes