CS 10: Problem solving via Object Oriented Programming. Lists Part 1
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1 CS 10: Problem solving via Object Oriented Programming Lists Part 1
2 Agenda 1. Defining a List ADT 2. Generics 3. Singly linked list implementaeon 4. ExcepEons 5. Visibility: public vs. private vs. protected vs. package 2
3 Abstract Data Types specify operaeons on a data set that defines overall behavior Abstract Data Types (ADTs) ADTs specify a set of opera&ons (e.g., get, set, add, ) that define how the ADT behaves on a colleceon of data elements At the ADT level we don t know (and don t really care) how data elements are stored (e.g., linked list or array or something else, it doesn t mater) Also do not care about what kind of data the ADT holds (e.g., Strings, integers, Objects). This is the Abstract in Abstract Data Type Big idea: hide the way data is represented while allowing others to work with the data in a consistent manner 3
4 Example: List ADT defines a set of operaeons List holds mul8ple elements (items) referenced by posi8on in List Index Bob Elvis Alice Denise Charlie 4
5 Example: List ADT defines a set of operaeons List holds mul8ple elements (items) referenced by posi8on in List Index Bob Elvis Alice Denise Charlie get(3) returns Denise 5
6 Example: List ADT defines a set of operaeons List holds mul8ple elements (items) referenced by posi8on in List set(2,abby) replaces Alice with Abby Index Bob Elvis Abby Alice Denise Charlie get(3) returns Denise 6
7 Example: List ADT defines a set of operaeons List holds mul8ple elements (items) referenced by posi8on in List set(2,abby) replaces Alice with Abby add(5,falcon) adds to end of List Index Bob Elvis Abby Denise Charlie Falcon get(3) returns Denise 7
8 Example: List ADT defines a set of operaeons List holds mul8ple elements (items) referenced by posi8on in List set(2,abby) replaces Alice with Abby add(5,falcon) adds to end of List Index Bob Elvis Abby Denise Charlie Falcon get(3) returns Denise ADT defines these opera8ons (and others) How were these items stored? Array? Linked List? We don t know and don t care at the ADT level, we just care that the operaeons (get, set, add, ) work as expected What type of elements are these? Strings, Student Objects? See answer above The type of element doesn t affect how the ADT works! 8
9 ADT can be implemented in different ways, but must provide common funceonality Java Interface: Defines set of ADT operaeons (e.g., get, set, add, remove, ) Interface does not say how to implement, just what to implement List ADT Implementa8on: Code to implement operaeons defined by interface Can be writen using different data structures MUST implement all funceonality defined by interface Can include other funceonality ArrayList Elements stored in an Array LinkedList Elements stored in an Linked List Java has both ArrayList and LinkedList implementaeons of List Both implementaeons provide the same funceonality as 9 required by interface, but store data differently
10 The List Interface describes several operaeons, but not implementaeons List ADT Opera8on size() isempty() get(i) set(i,e) add(i,e) remove(i) Descrip8on Return number of items in List True if no items in List, otherwise false Return the item at index i Replace the item at index i with item e Insert item e at index i, moving all subsequent items one index larger Remove and return item at index i, move all subsequent items one index smaller These operaeons MUST be implemented to complete the ADT Free to implement other methods, but must have these NoEce the familiar look from Java s ArrayList 10
11 Interfaces go in one file, implementaeons go in another file Interface file Specifies required operaeons SimpleList.java Uses keyword interface Linked list implementa8on SinglyLinked.java OR Array implementa8on Implementa8on file Actually implements required operaeons using a specific data structure Same interface could be implemented in different ways (e.g., linked list or array) Use keyword implements to implement an interface 11
12 SimpleList.java is an interface that specifies what operaeons MUST be implemented Interface keyword tells Java this is an interface Methods defined to include parameters and return types (called a signature ) If you are going to create a List, then you MUST implement these methods How you implement is your business 12
13 The List ADT could be implemented with a singly linked list OR an array; either works Examples of List implementa8on Singly linked list We will implement a List both ways Each implementa8on has pros and cons head Alice Bob Charlie Array 0 1 Alice Bob Charlie 2 n- 1 13
14 Agenda 1. Defining a List ADT 2. Generics 3. Singly linked list implementaeon 4. ExcepEons 5. Visibility: public vs. private vs. protected vs. package 14
15 Generics allow a variable to stand in for a Java type Interface declara8on public interface SimpleList<T> {... public T get(int index) throws Exception; public void add(int index, T item) throws Exception; } T stands for whatever object type we instaneate SimpleList<Blob> then T always stands for Blob SimpleList<Point> then T always stands for Point Allows us to write one implementaeon that works regardless of what kind of Object we store in our data set Must use class version of primieves (Integer, Double, etc) By conveneon we name type of variables with a single uppercase leter, oken T for type, later we ll use K for key and V for value 15
16 Agenda 1. Defining a List ADT 2. Generics 3. Singly linked list implementaeon 4. ExcepEons 5. Visibility: public vs. private vs. protected vs. package 16
17 Singly linked list review: elements have data and a next pointer Singly linked list Box- and- pointer diagram Data in Box Pointer to next item in List head head points to first item (index 0) null if list is empty Alice Bob Charlie Slash indicates end of List next pointer is null 17
18 To get an item at index i, start at head and march down get(i) return item at specified index head Alice Bob Charlie Get item at index 2 18
19 To get an item at index i, start at head and march down get(i) return item at specified index Index 0 head Alice Bob Charlie Get item at index 2 1. Start at head (index 0) 19
20 To get an item at index i, start at head and march down get(i) return item at specified index Index 1 head Alice Bob Charlie Get item at index 2 1. Start at head (index 0) 2. Follow next pointer to index 1 20
21 To get an item at index i, start at head and march down get(i) return item at specified index Index 2 head Alice Bob Charlie Get item at index 2 1. Start at head (index 0) 2. Follow next pointer to index 1 3. Follow next pointer to index 2 21
22 To get an item at index i, start at head and march down get(i) return item at specified index Item 2 head Alice Bob Charlie Get item at index 2 1. Start at head (index 0) 2. Follow next pointer to index 1 3. Follow next pointer to index 2 4. Return Charlie (index 2) 22
23 add() splices in a new object anywhere in the list by updaeng next pointers add(1, Bill ) head Alice Bob Charlie Add Bill at index idx=1 Advance from head to idx- 1 (Alice) Bill 23
24 add() splices in a new object anywhere in the list by updaeng next pointers add(1, Bill ) Index 0 (=idx- 1) head Alice Bob Charlie Add Bill at index idx=1 Advance from head to idx- 1 (Alice) Bill 24
25 add() splices in a new object anywhere in the list by updaeng next pointers add(1, Bill ) head Alice Bob Charlie Add Bill at index idx=1 Advance from head to idx- 1 (Alice) Update Bill s next pointer to same address as Alice s next pointer Bill 25
26 add() splices in a new object anywhere in the list by updaeng next pointers add(1, Bill ) head Alice Bob Charlie Bill Add Bill at index idx=1 Advance from head to idx- 1 (Alice) Update Bill s next pointer to same address as Alice s next pointer Update Alice s next pointer to Bill 26
27 add() splices in a new object anywhere in the list by updaeng next pointers add(1, Bill ) head Alice Bob Charlie Bill Add Bill at index idx=1 Advance from head to idx- 1 (Alice) Update Bill s next pointer to same address as Alice s next pointer Update Alice s next pointer to Bill 27
28 remove() takes an item out of the list by updaeng next pointer remove(1) head Alice Bob Charlie Remove Bill at index idx=1 Advance from head to idx- 1 (Alice) Bill 28
29 remove() takes an item out of the list by updaeng next pointer remove(1) head Alice Bob Charlie Bill Remove Bill at index idx=1 Advance from head to idx- 1 (Alice) Set Alice s next pointer to Bill s next pointer (the next element s next) 29
30 remove() takes an item out of the list by updaeng next pointer remove(1) head Alice Bob Charlie Bill Remove Bill at index idx=1 Advance from head to idx- 1 (Alice) Set Alice s next pointer to Bill s next pointer (the next element s next) Bill will be garbage collected (in C we have to call free()) 30
31 SinglyLinked.java: ImplementaEon of List interface implements is a promise to implement all required methods specified by Interface SimpleList isempty() size() add() remove() get() set() 31
32 SinglyLinked.java: ImplementaEon of List interface Type of data is generic T Don t care what kind of data the List holds, could be Strings, Integers, Blob Objects, This way we don t have to write a separate implementa8on if use Strings as elements, and other implementa8on if use Integers, and third implementa8on if use Just implement the List once and hold whatever data type needed for the applica8on 32
33 SinglyLinked.java: ImplementaEon of List interface Define a private class called Element to implement data and next pointers Element constructor takes data as type T and pointer to next Element (could be null) Element is private to SinglyLinked (internal to this file, no need for others to change it) 33
34 SinglyLinked.java: ImplementaEon of List interface Creates head Element and size counter Constructor ini8alizes head to null and size to 0 No8ce head is of type Element but is never newed head will be a pointer to first Element in the List 34
35 SinglyLinked.java: ImplementaEon of List interface size() method just returns instance variable size size will be incremented on add(), decremented on remove() 35
36 SinglyLinked.java: ImplementaEon of List interface advance() helper method Start at head and marches down n items Loop un8l hit n th item or run out of items in List Return n th item (or throw excep8on) Note: return type from advance() is Element advance() not specified by interface, but implementa8ons can have more methods than required 36
37 SinglyLinked.java: ImplementaEon of List interface add()/remove() use advance() to march down list to item before index idx, then adjust pointers 37
38 SinglyLinked.java: ImplementaEon of List interface Safety check for nega8ve index head 38
39 SinglyLinked.java: ImplementaEon of List interface If adding at head (index 0) add(0,15) head 39
40 SinglyLinked.java: ImplementaEon of List interface If adding at head (index 0) Create new element with data set to parameter item add(0,15) head 15 40
41 SinglyLinked.java: ImplementaEon of List interface If adding at head (index 0) Create new element with data set to parameter item Set new element next pointer to where ever head points add(0,15) head 15 41
42 SinglyLinked.java: ImplementaEon of List interface If adding at head (index 0) Create new element with data set to parameter item Set new element next pointer to where ever head points head will ini8ally point to null add(0,15) head 15 42
43 SinglyLinked.java: ImplementaEon of List interface If adding at head (index 0) Create new element with data set to parameter item Set new element next pointer to where ever head points head will ini8ally point to null Set head to new element add(0,15) head 15 43
44 SinglyLinked.java: ImplementaEon of List interface add(0,15) If adding at head (index 0) Create new element with data set to parameter item Set new element next pointer to where ever head points head will ini8ally point to null Set head to new element Finally increment size head 15 44
45 SinglyLinked.java: ImplementaEon of List interface add(0,6) head 15 45
46 SinglyLinked.java: ImplementaEon of List interface If adding at head (index 0) Create new element with data set to parameter item Set new element next pointer to where ever head points add(0,6) head
47 SinglyLinked.java: ImplementaEon of List interface If adding at head (index 0) Create new element with data set to parameter item Set new element next pointer to where ever head points Set head to new element Finally increment size add(0,6) head
48 SinglyLinked.java: ImplementaEon of List interface If adding not at head advance() to e=(idx- 1) th item add(1,3) head
49 SinglyLinked.java: ImplementaEon of List interface If adding not at head advance() to e=(idx- 1) th item add(1,3) head 6 15 Advance to item idx- 1 Here (item 1)- 1 = item 0 So e points to item 0 with data = 6 49
50 SinglyLinked.java: ImplementaEon of List interface If adding not at head advance() to e=(idx- 1) th item Create new Element with data set to item and next to e.next add(1,3) head
51 SinglyLinked.java: ImplementaEon of List interface add(1,3) If adding not at head advance() to e=(idx- 1) th item Create new Element with data set to item and next to e.next Set e.next = new item head
52 SinglyLinked.java: ImplementaEon of List interface add(1,3) If adding not at head advance() to e=(idx- 1) th item Create new Element with data set to item and next to e.next Set e.next = new item Finally, increment size head
53 SinglyLinked.java: ImplementaEon of List interface If removing at head Just set head to next remove(0) head
54 SinglyLinked.java: ImplementaEon of List interface If removing at head Just set head to next remove(0) head
55 SinglyLinked.java: ImplementaEon of List interface If removing at head Just set head to next remove(0) head
56 SinglyLinked.java: ImplementaEon of List interface If removing not at head advance() to idx- 1 (data 3) Set e.next to e.next.next remove(1) head
57 SinglyLinked.java: ImplementaEon of List interface If removing not at head advance() to idx- 1 (data 3) Set e.next to e.next.next remove(1) head
58 SinglyLinked.java: ImplementaEon of List interface If removing not at head advance() to idx- 1 (data 3) Set e.next to e.next.next remove(1) head 3 58
59 SinglyLinked.java: ImplementaEon of List interface get()/set() also use advance() to march down list, this 8me to index idx 59
60 SinglyLinked.java: ImplementaEon of List interface get()/set() also use advance() to march down list, this 8me to index idx tostring() overrides a Java Object method and allows us to print an object as desired If tostring() not overriden, defaults to prin8ng the memory address of object Return value type is String rather than actually prin8ng 60
61 ListTest.java: Test of List implementaeon Declare SinglyLinked List to hold Strings, so T = String in the implementa8on Implementa8on is SinglyLinked which implemented SimpleList interface Next class we ll look at an array implementa8on which will also be a SimpleList 61
62 ListTest.java: Test of List implementaeon tostring() method called in print statements 62
63 ListTest.java: Test of List implementaeon tostring() method called in print statements Remember, tostring() returns a String (doesn t do the prin8ng) 63
64 Agenda 1. Defining a List ADT 2. Generics 3. Singly linked list implementaeon 4. ExcepEons 5. Visibility: public vs. private vs. protected vs. package 64
65 An excepeon indicates that something unexpected happened at run- Eme Cannot check for all errors at compile Eme What if we ask for element at an index of - 1 in an array? There is no clear, always- do- this, answer Maybe we should return null or maybe we should stop execueon ExcepEons provide a way to show something is amiss, and let calling funceons deal with error (or not) ExcepEons not handled by a method are passed to calling method. If excepeon not handled in main() or before, program stops Throw error with throw new Exception( error description ) Java provides structured error- handling via try/catch/finally blocks catch executes only if there is an excepeon in try body catch block can specify the type of error it handles Can have muleple catch blocks for each try 65 Finally block executes regardless whether try succeeds or fails
66 ListExcepEons.java: ExcepEons can be handled with try/catch/finally blocks Create new SinglyLinked List 66
67 ListExcepEons.java: ExcepEons can be handled with try/catch/finally blocks Try block Catch block Only executes if excep8on in try block 67
68 ListExcepEons.java: ExcepEons can be handled with try/catch/finally blocks Trying to add at index - 1 is an error, the catch block will execute because add() throws an excep8on for nega8ve indices 68
69 ListExcepEons.java: ExcepEons can be handled with try/catch/finally blocks Trying to add at index - 1 is an error, the catch block will execute because add() throws an excep8on for nega8ve indices SinglyLinked.java 69
70 ListExcepEons.java: ExcepEons can be handled with try/catch/finally blocks Trying to add at index - 1 is an error, the catch block will execute because add() throws an excep8on for nega8ve indices Catch block on line 15 executes because excep8on thrown on line 10 Lines 11 and 12 never execute because excep8on on line 10 stops execu8on in try block and starts running in catch block I never run and Neither do I are not printed If we didn t catch excep8on, the program would end because main() wouldn t have caught the excep8on (but we did catch 70 it, so main() doesn t end execu8on)
71 ListExcepEons.java: ExcepEons can be handled with try/catch/finally blocks Trying to add at index - 1 is s&ll an error, the catch block will execute 71
72 ListExcepEons.java: ExcepEons can be handled with try/catch/finally blocks Trying to add at index - 1 is s&ll an error, the catch block will execute We can see what the excep8on was by prin8ng e (it is just an object) 72
73 ListExcepEons.java: ExcepEons can be handled with try/catch/finally blocks finally always executes, regardless of whether excep8on in try block catch only executes if excep8on occurs in try block, otherwise catch code does not execute If excep8on in try block, execu8on in the try block stops at the point of the excep8on and picks up in first line of catch block Code in the try block afer the line that caused the excep8on is not executed 73
74 ListExcepEons.java: ExcepEons can be handled with try/catch/finally blocks This is valid, so catch block does not execute 74
75 ListExcepEons.java: ExcepEons can be handled with try/catch/finally blocks This is valid, so catch block does not execute finally always executes, even if no excep8on in try block 75
76 Agenda 1. Defining a List ADT 2. Generics 3. Singly linked list implementaeon 4. ExcepEons 5. Visibility: public vs. private vs. protected vs. package 76
77 Java allows us to break up major poreons of code into Projects, Packages and Classes Example of master project for a company Main Project Company Project 77
78 Java allows us to break up major poreons of code into Projects, Packages and Classes Example of master project for a company Main Project Company Project Packages within Project AccounEng Package MarkeEng Package Manufacturing Package 78
79 Java allows us to break up major poreons of code into Projects, Packages and Classes Example of master project for a company Main Project Company Project Packages within Project AccounEng Package MarkeEng Package Manufacturing Package Classes within Package AccounEng Class 1 Manufacturing Class 1 MarkeEng Class 1 AccounEng Class n Manufacturing Class n MarkeEng Class n 79
80 Adapted from Java documentaeon Visibility depends on modifier applied Example: Visibility of Alpha class Company Project Alpha is a class in Accoun&ng package, which is in Company project Packages (Pkg) Classes AccounEng Package Alpha Beta Subclass MarkeEng Package AlphaSub Gamma Y = can access N = cannot access Alpha.x Accoun8ng Pkg Marke8ng Pkg If Alpha.x is: accessed by: Alpha Beta AlphaSub Gamma public Any class Y Y Y Y protected Pkg + Subclass Y Y Y N No modifier Pkg - Subclass Y Y N N private This class only Y N N N 80
81 Adapted from Java documentaeon Visibility depends on modifier applied Example: Visibility of Alpha class Company Project Alpha is a class in Accoun&ng package, which is in Company project Packages (Pkg) Classes AccounEng Package Alpha Beta Subclass MarkeEng Package AlphaSub Gamma Y = can access N = cannot access Alpha.x Accoun8ng Pkg Marke8ng Pkg If Alpha.x is: accessed by: Alpha Beta AlphaSub Gamma public Any class Y Y Y Y protected Pkg + Subclass Y Y Y N No modifier Pkg - Subclass Y Y N N private This class only Y N N N 81
82 Adapted from Java documentaeon Visibility depends on modifier applied Example: Visibility of Alpha class Company Project Alpha is a class in Accoun&ng package, which is in Company project Packages (Pkg) Classes AccounEng Package Alpha Beta Subclass MarkeEng Package AlphaSub Gamma Y = can access N = cannot access Alpha.x Accoun8ng Pkg Marke8ng Pkg If Alpha.x is: accessed by: Alpha Beta AlphaSub Gamma public Any class Y Y Y Y protected Pkg + Subclass Y Y Y N No modifier Pkg - Subclass Y Y N N private This class only Y N N N 82
83 Adapted from Java documentaeon Visibility depends on modifier applied Example: Visibility of Alpha class Company Project Alpha is a class in Accoun&ng package, which is in Company project Packages (Pkg) Classes AccounEng Package Alpha Beta Subclass MarkeEng Package AlphaSub Gamma Y = can access N = cannot access Alpha.x Accoun8ng Pkg Marke8ng Pkg If Alpha.x is: accessed by: Alpha Beta AlphaSub Gamma public Any class Y Y Y Y protected Pkg + Subclass Y Y Y N No modifier Pkg - Subclass Y Y N N private This class only Y N N N 83
84 Adapted from Java documentaeon Visibility depends on modifier applied Example: Visibility of Alpha class Company Project Alpha is a class in Accoun&ng package, which is in Company project Packages (Pkg) Classes AccounEng Package Alpha Beta Subclass MarkeEng Package AlphaSub Gamma Y = can access N = cannot access Alpha.x Accoun8ng Pkg Marke8ng Pkg If Alpha.x is: accessed by: Alpha Beta AlphaSub Gamma public Any class Y Y Y Y protected Pkg + Subclass Y Y Y N No modifier Pkg - Subclass Y Y N N private This class only Y N N N 84
85 85
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