Classes. Brahm Capoor
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1 Classes Brahm Capoor
2 Announcements 3 handouts: Assignment #3, Section #3, Bouncing Ball
3 Announcements 3 handouts: Assignment #3, Section #3, Bouncing Ball Assignment #3 YEAH Hours: Tuesday at 6:30 in Bishop Auditorium
4 Announcements 3 handouts: Assignment #3, Section #3, Bouncing Ball Assignment #3 YEAH Hours: Tuesday at 6:30 in Bishop Auditorium Section handouts
5 Announcements 3 handouts: Assignment #3, Section #3, Bouncing Ball Assignment #3 YEAH Hours: Tuesday at 6:30 in Bishop Auditorium Section handouts Assignment 2 Pain Poll
6 Variables Variables allow us to store information about the state of our program Local variables: Temporary state Instance variables: Persistent State
7 Variables Variables allow us to store information about the state of our program Local variables: Temporary state Instance variables: Persistent State Parameters (and return values) allow us to control the flow of this information
8 Variables Variables allow us to store information about the state of our program Local variables: Temporary state Instance variables: Persistent State Parameters (and return values) allow us to control the flow of this information We mostly didn t need parameters in Karel. Why?
9 if (frontisclear())
10 Implementing Classes
11 Imagine a program that manages a single student public void run() { String studentname = Brahm ; int studentid = ; String = brahm@stanford.edu ; int numunits = 180; boolean isinternational = true;
12 Imagine a program that manages a single student public void run() { String studentname = Brahm ; int studentid = ; String = brahm@stanford.edu ; int numunits = 180; boolean isinternational = true; printdetails(studentname, studentid, , numunits, isinternational);
13 What if we needed multiple students? public void run() { String studentname1 = Brahm ; int studentid1 = ; String 1 = brahm@stanford.edu ; int numunits1 = 180; boolean isinternational1 = true; String studentname2 = Trillian ; int studentid2 = ; String 2 = hooloovoo@stanford.edu ; int numunits2 = 143; boolean isinternational2 = false; printdetails(studentname1, studentid1, 1, numunits1, isinternational1); printdetails(studentname2, studentid2, 2, numunits2, isinternational2);
14 What if we needed multiple students? public void run() { String studentname1 = Brahm ; int studentid1 = ; String 1 = brahm@stanford.edu ; int numunits1 = 180; boolean isinternational1 = true; String studentname2 = Trillian ; int studentid2 = ; String 2 = hooloovoo@stanford.edu ; int numunits2 = 143; boolean isinternational2 = false; printdetails(studentname1, studentid1, 1, numunits1, isinternational1); printdetails(studentname2, studentid2, 2, numunits2, isinternational2);
15 Wouldn t this be nice? public void run() { Student s1 = new Student( Brahm, , brahm@stanford.edu, 180, true); Student s2 = new Student( Trillian, , hooloovoo@stanford.edu, 143, false); s1.printdetails(); s2.printdetails();
16 A brief history lesson The first programming languages in the 1960s were called procedural languages Comprised of sequences of statements (kind of like Karel) Emphasized strategic top down design Examples: C, BASIC, COBOL, Fortran
17 A brief history lesson In the late 1960s, a language called Smalltalk was made to facilitate object-oriented programming The design of a language for using computers must deal with internal models Translation: programming involves things which have properties and behaviours
18 A brief history lesson Object Oriented Programming began to gain popularity in the 1970s Bjorn Stroustrup added it to C to make C++, the first commercially-used language with OOP
19 A brief history lesson The first major Object-Oriented Programming language to be designed from the ground up was Java
20 A brief history lesson The first major Object-Oriented Programming language to be designed from the ground up was Java Which brings us to
21 public class <ClassName> { // sick code here
22 I m defining a thing called classname public class <ClassName> { // sick code here
23 public class <ClassName> extends <SuperClass> { // sick code here
24 I m defining a thing called Classname Classname is a kind of SuperClass public class <ClassName> extends <SuperClass> { // sick code here
25 public class Student { If you don t extend anything, you re implicitly extending Object // sick code here
26 Student.java These should match public class Student { // sick code here
27 Student.java Stanford.java Creating objects of type Student public class Student { // sick code here public void run() { Student s1; Student s2; Student s3; // more sick code here
28 public class Student { // sick code here 1) Programming involves things 2)...which have properties 3)...and behaviour
29 Instance variables Defined as part of a class, but not within any particular method
30 Instance variables Defined as part of a class, but not within any particular method public void run() { String studentname = Brahm ; int studentid = ; String = brahm@stanford.edu ; int numunits = 180; boolean isinternational = true;
31 Instance variables Defined as part of a class, but not within any particular method public class Student { private String studentname; private int studentid; private String ; private int numunits; private boolean isinternational;
32 Instance variables Defined as part of a class, but not within any particular method s1, s2 and s3 all have their own independent properties public class Student { private String studentname; private int studentid; private String ; private int numunits; private boolean isinternational; public void run() { Student s1; Student s2; Student s3;
33 What does private mean? Private (and public) are called visibility specifiers If a method or variable is private, it is only accessible in the class in which it is defined If a method or variable is public, it is accessible in any class
34 Initializing your instance variables in the constructor public class Student { public Student(String name, int id, String , int numunits, boolean isinternational) { studentname = name; studentid = id; = ; numunits = numunits; isinternational = isinternational; /* instance variables go down here */
35 Initializing your instance variables in the constructor public class Student { public Student(String name, int id, String , int numunits, boolean isinternational) { studentname = name; studentid = id; = ; // BUGGY: Setting a variable to itself! numunits = numunits; isinternational = isinternational; /* instance variables go down here */
36 Initializing your instance variables in the constructor public class Student { public Student(String name, int id, String , int numunits, boolean isinternational) { studentname = name; studentid = id; this. = ; // to disambiguate between variables this.numunits = numunits; this.isinternational = isinternational; /* instance variables go down here */
37 Now we can make students! public class Student { public Student(String name, int id, String , int numunits, boolean isinternational) {... public void run() { Student s1 = new Student( Brahm, , brahm@stanford.edu, 180, true);
38 Now we can make students! s1 studentname Brahm studentid numunits 180 isinternational true public void run() { Student s1 = new Student( Brahm, , brahm@stanford.edu, 180, true);
39 It s also possible for objects to share the same variable public class Student { public static final double UNITS_TO_GRADUATE = 180; /*etc*/
40 It s also possible for objects to share the same variable public class Student { public static final double UNITS_TO_GRADUATE = 180; /*etc*/ static means that every instance of the class has the same variable
41 How does this look? Student s1 = new Student( Brahm, , brahm@stanford.edu, 180, true); Student s2 = new Student( Sophia, , hooloovoo@stanford.edu, 143, false); s1 studentname Brahm studentid brahm@stanford.edu numunits 180 isinternational true UNITS_TO_GRADUATE UNITS_TO_GRADUATE 180 s2 studentname Trillian studentid hooloovoo@stanford.edu numunits 143 isinternational false UNITS_TO_GRADUATE
42 Under the hood Student s1 = new Student( Brahm, , brahm@stanford.edu, 180, true); Stack frame s1 studentname Brahm studentid brahm@stanford.edu numunits 180 isinternational true
43 Under the hood Student s1 = new Student( Brahm, , brahm@stanford.edu, 180, true); Stack frame s1 A reference studentname Brahm studentid brahm@stanford.edu numunits 180 isinternational true
44 s1 and s2 store references to Student objects public class Student { public Student(String name, int id, String , int numunits, boolean isinternational) {... public void run() { Student s1 = new Student( Brahm, , brahm@stanford.edu, 180, true); Student s2 = new Student( Trillian, , hooloovoo@stanford.edu, 143, false);
45 s1 and s2 store references to Student objects public class Student { public Student(String name, int id, String , int numunits, boolean 1) Programming isinternational) involves things { 2)...which have properties 3)...and behaviour public void run() { Student s1 = new Student( Brahm, , brahm@stanford.edu, 180, true); Student s2 = new Student( Trillian, , hooloovoo@stanford.edu, 143, false);
46 Using an object s properties Variables are only useful because we can get and set their values This is also true for instance variables
47 Using an object s properties Variables are only useful because we can get and set their values Instance variables are private We can get and set their values in the same class It would be nice to be able to do so in other classes as well A Stanford program should be able to change a student s number of units, for example
48 Our first Getters and Setters public class Student { public Student(int unitcount) { numunits = unitcount; public void run() { Student s1 = new Student(42); public int getunits() { return numunits; public void setunits(int newunits) { numunits = newunits; private int numunits;
49 Our first Getters and Setters public class Student { public Student(int unitcount) { numunits = unitcount; public int getunits() { return numunits; public void run() { Student s1 = new Student(42); println( Curr: + s1.getunits()); public void setunits(int newunits) { numunits = newunits; private int numunits;
50 Our first Getters and Setters public class Student { public Student(int unitcount) { numunits = unitcount; public int getunits() { return numunits; public void setunits(int newunits) { numunits = newunits; public void run() { Student s1 = new Student(42); println( Curr: + s1.getunits()); s1.setunits(60); private int numunits;
51 Our first Getters and Setters: some notes public class Student { public Student(int unitcount) { numunits = unitcount; Getter and Setter methods are public (exported) so we can call them in other classes and programs public int getunits() { return numunits; public void setunits(int newunits) { numunits = newunits; private int numunits;
52 Our first Getters and Setters: some notes public class Student { public Student(int unitcount) { numunits = unitcount; Getter and Setter methods are public (exported) so we can call them in other classes and programs Define Getters and Setters whenever you want to grant a client access to or control over an instance variable public int getunits() { return numunits; public void setunits(int newunits) { numunits = newunits; private int numunits;
53 Our first Getters and Setters: some notes public class Student { public Student(int unitcount) { numunits = unitcount; public int getunits() { return numunits; Getter and Setter methods are public (exported) so we can call them in other classes and programs Define Getters and Setters whenever you want to grant a client access to or control over an instance variable These methods are typically very short public void setunits(int newunits) { numunits = newunits; private int numunits;
54 Our first Getters and Setters: some notes public class Student { public Student(int unitcount) { numunits = unitcount; public int getunits() { return numunits; Getter and Setter methods are public (exported) so we can call them in other classes and programs Define Getters and Setters whenever you want to grant a client access to or control over an instance variable These methods are typically very short Why not just make these variables public? public void setunits(int newunits) { numunits = newunits; private int numunits;
55 Our first Getters and Setters: some notes public class Student { public Student(int unitcount) { numunits = unitcount; public int getunits() { return numunits; public void setunits(int newunits) { numunits = newunits; Getter and Setter methods are public (exported) so we can call them in other classes and programs Define Getters and Setters whenever you want to grant a client access to or control over an instance variable These methods are typically very short Why not just make these variables public? Student s1 = new Student(42); s1.numunits = 0; // why is this problematic? private int numunits;
56 Our first Getters and Setters: some notes public class Student { public Student(int unitcount) { numunits = unitcount; Getter and Setter methods are public (exported) so we can call them in other classes and programs Define Getters and Setters whenever you want to grant a client access to or control over an instance variable public int getunits() { return numunits; public void setunits(int newunits) { numunits = newunits; private int numunits; These methods are typically very short They allow more precise control over the value of a variable: public void setunits(int newunits) { if (newunits >= numunits) { numunits = newunits;
57 Why stop there? Now that we know how to use instance variables, we can do even cooler things public boolean cangraduate() { return numunits >= 180; public void dropclass (int classunits) { if (classunits <= 5) { numunits -= classunits;
58 Why stop there? Now that we know how to use instance variables, we can do even cooler things public boolean cangraduate() { return numunits >= 180; public void dropclass (int classunits) { if (classunits <= 5) { numunits -= classunits; Methods allow us to define behaviours for our classes
59 One special method How could we do this? Student s1 = new Student(...); println(s1); //s1 isn t a string!
60 One special method How could we do this? Student s1 = new Student(...); println(s1); //s1 isn t a string! We define a tostring() method public String tostring() { return studentname + " (#" + studentid + ")";
61 One special method How could we do this? Student s1 = new Student(...); println(s1); //s1 isn t a string! We define a tostring() method public String tostring() { return studentname + " (#" + studentid + ")";...and now this magically works! Student s1 = new Student(...); println(s1); //Prints Brahm (# )
62 One special method How could we do this? We define a tostring() method Student s1 = new Student(...); println(s1); //s1 isn t a string! 1) Programming involves things 2)...which have properties 3)...and behaviour public String tostring() { return studentname + " (#" + studentid + ")";...and now this magically works! Student s1 = new Student(...); println(s1); //Prints Brahm (# )
63 Demo
64 Passing an object as a parameter public void run() { int x = 7; dosomething(x); println(x); // prints 7 private void dosomething(int n) { n *= 2;
65 Passing an object as a parameter public void run() { int x = 7; dosomething(x); println(x); // prints 7 private void dosomething(int n) { n *= 2; public void run() { Student s1 = new Student(42); dosomething(s1); println(s1.getunits()); private void dosomething(student s) { s.setunits(s.getunits() * 2);
66 Passing an object as a parameter public void run() { int x = 7; dosomething(x); println(x); // prints 7 private void dosomething(int n) { n *= 2; public void run() { Student s1 = new Student(42); dosomething(s1); println(s1.getunits()); // prints 84 private void dosomething(student s) { s.setunits(s.getunits() * 2);
67 How does this work? Object variables store references It helps to think of a reference as the location of the object elsewhere in your computer When we pass in an object to a method, we pass in a reference to that object
68 How does this work? public void run() { Student s1 = new Student(42); dosomething(s1); println(s1.getunits()); private void dosomething(student s) { s.setunits(s.getunits() * 2);
69 How does this work? public void run() { Student s1 = new Student(..., 42); dosomething(s1); println(s1.getunits()); private void dosomething(student s) { s.setunits(s.getunits() * 2); run() s1 studentname Brahm studentid brahm@stanford.edu numunits 42 isinternational true
70 How does this work? public void run() { Student s1 = new Student(..., 42); dosomething(s1); println(s1.getunits()); run() s1 dosomething() s private void dosomething(student s) { s.setunits(s.getunits() * 2); studentname Brahm studentid brahm@stanford.edu numunits 42 isinternational true
71 How does this work? public void run() { Student s1 = new Student(..., 42); dosomething(s1); println(s1.getunits()); run() s1 dosomething() s private void dosomething(student s) { s.setunits(s.getunits() * 2); studentname Brahm studentid brahm@stanford.edu numunits 84 isinternational true
72 How does this work? public void run() { Student s1 = new Student(..., 42); dosomething(s1); println(s1.getunits()); private void dosomething(student s) { s.setunits(s.getunits() * 2); run() s1 studentname Brahm studentid brahm@stanford.edu numunits 84 isinternational true
73 A summary public void run() { int x = 7; dosomething(x); println(x); // prints 7 private void dosomething(int n) { n *= 2; public void run() { Student s1 = new Student(42); dosomething(s1); println(s1.getunits()); // prints 84 private void dosomething(student s) { s.setunits(s.getunits() * 2); Passing by copy/value Passing by reference
74 A nuance: we re passing copies of references
75 A nuance: we re passing copies of references
76 What does that mean? public void run() { Student s1 = new Student(...); dosomething(s1); println(s1.getunits()); private void dosomething(student s) { s = new Student(...); run() s1 Brahm brahm@ true dosomething() s
77 What does that mean? public void run() { Student s1 = new Student(...); dosomething(s1); println(s1.getunits()); private void dosomething(student s) { s = new Student(...); run() s1 Brahm brahm@ true dosomething() s Trillian hoolovoo@ false
78 What does that mean? public void run() { Student s1 = new Student(...); dosomething(s1); println(s1.getunits()); private void dosomething(student s) { s = new Student(...); run() s1 Brahm brahm@ true
79 What have we done? We identified a single kind of entity our program interacted with
80 What have we done? We identified a single kind of entity our program interacted with We modelled that entity in Java by specifying its properties and behaviour
81 What have we done? We identified a single kind of entity our program interacted with We modelled that entity in Java by specifying its properties and behaviour We used that model as a new custom-defined type
82 How can we go further? We ve defined a Student class, but there are different kinds of students!
83 How can we go further? We ve defined a Student class, but there are different kinds of students! These type of students are mostly similar but with some unique properties and behaviour
84 How can we go further? We ve defined a Student class, but there are different kinds of students! These type of students are mostly similar but with some unique properties and behaviour How can we leverage our existing Student class to implement these new student types?
85 public class Frosh { // sick code here
86 public class Frosh extends Student { // sick code here
87 I m defining a thing called Classname Classname is a kind of SuperClass public class <ClassName> extends <SuperClass> { // sick code here
88 Frosh.java public class Frosh extends Student { // sick code here
89 Frosh.java public class Frosh extends Student { // sick code here We re subclassing, or making a subclass of, the Student class
90 public Student (String name, int id){... public void setid(int id) { this.id = id; public void setnunits(int units) { nunits = units; private int id, nunits;
91 public Student (String name, int id){... public void setid(int id) { this.id = id; public void setnunits(int units) { nunits = units; private int id, nunits; public class Frosh extends Student { public Frosh (String name, int id) {
92 public Student (String name, int id){... public void setid(int id) { this.id = id; public void setnunits(int units) { nunits = units; private int id, nunits; public class Frosh extends Student { public Frosh (String name, int id) { super(name, id); // call the Student constructor
93 public Student (String name, int id){... public void setid(int id) { this.id = id; public void setnunits(int units) { nunits = units; private int id, nunits; public class Frosh extends Student { public Frosh (String name, int id) { super(name, id); // call the Student constructor setnunits(0);
94 public Student (String name, int id){... public void setid(int id) { this.id = id; public void setnunits(int units) { nunits = units; private int id, nunits; public class Frosh extends Student { public Frosh (String name, int id) { super(name, id); // call the Student constructor setnunits(0); // call an inherited method
95 public Student (String name, int id){... public void setid(int id) { this.id = id; public void setnunits(int units) { nunits = units; private int id, nunits; public class Frosh extends Student { public Frosh (String name, int id) {... public String tostring() { // overriding superclass method return getname() + is a Frosh! ;
96 public Student (String name, int id){... public void setid(int id) { this.id = id; public void setnunits(int units) { nunits = units; private int id, nunits; public class Frosh extends Student { public Frosh (String name, int id) {... public String tostring() { // overriding superclass method return getname() + is a Frosh! ; Subclasses don t have access to private variables of superclass
97 Questions?
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