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CS 1110
Prelim II: Review Session
1
Introduc8on
- My name: Bruno Abrahao
- We have three addi8onal TA’s in the room to help
you individually
- Shuang
- Nam
- Yookyung
- You’re welcome to ask them ques8ons at any 8me
2
Exam Info
- Prelim II: 7:30–9:00PM, Thursday, 12 March,
Uris Hall G
- Look at the previous Prelims
- Arrive early! Helps reducing stress
- Grades released the same evening (morning…)
3
Regrade Requests
- Releasing grades quickly is good for you —
exams serve two purposes:
- Give feedback to student and teacher
- Give grades
That’s one reason we
grade ~180 exams so
quickly
4
Review session
- Let’s make this interac8ve
- Presenta8on is at slower pace than a regular
lecture
- Ask ques8ons
- All ques8ons are smart ones
5
What’s in the exam?
- Everything you needed to know for Prelim I
- Vector / String class, func8ons
- Wri8ng func8ons
- Recursive Func8ons
- apparent/real classes, cas8ng, operator
instanceof , func8on equals
- Abstract classes and methods
6
What’s in the exam?
- Everything you needed to know for Prelim I
- Vector / String class, func8ons
- Wri8ng func8ons
- Recursive Func8ons
- apparent/real classes, cas8ng, operator
instanceof , func8on equals
- Abstract classes and methods
I’m gonna assume
you can do this with
your eyes closed by
now
7
What’s in the exam?
- Everything you needed to know for Prelim I
- Vector / String class, func8ons
- Wri8ng func8ons
- Recursive Func8ons
- apparent/real classes, cas8ng, operator
instanceof , func8on equals
- Abstract classes and methods
8
(Fall’07) Question 1 (15 points). Write the body of the
following function recursively.
/** = n, but with its digits reversed.
Precondition: n >= 0.
e.g. n = 135720, value is "027531".
e.g. n = 12345, value is "54321".
e.g. n = 7, value is "7".
e.g. n = 0, value is "0".*/
public static String rev(int n) {
9
returns a String
Recursive Func8on 4 Principles
- Write the precise specifica8on
10
/** = n, but with its digits reversed.
Precondition: n >= 0.
e.g. n = 135720, value is "027531".
e.g. n = 12345, value is "54321".
e.g. n = 7, value is "7".
e.g. n = 0, value is "0".*/
public static String rev(int n) {
// base case:
//{n has only one digit}
// recursive case:
// {n has at least two digits}
11
Recursive Func8on 4 Principles
- Write the precise specifica8on
- Base Case
12
What’s in the exam?
- Everything you needed to know for Prelim I
- Vector / String class, func8ons
- Wri8ng func8ons
- Recursive Func8ons
- apparent/real classes, cas8ng, operator
instanceof , func8on equals
- Abstract classes and methods
19
Administrivia
- Please remember to fill out your TA evals.
- Open un8ll Friday 13
- Good for you , good for us
- You can give feedback to any TA (not only your lab
instructor)
- A TA who taught you something
- A TA that inspired you
- A TA who you think needs to improve some aspect
- …
20
CS1110 Flix
21
public class Movie {
private String title; // title of movie
private int length; // length in minutes
/** Constructor: document with title t
and len minutes long */
public Movie(String t, int len) {
title= t; length= len;
}
/** = title of this Movie */
public String getTitle()
{ return title; }
/** = length of document, in minutes */
public int getLength()
{ return length; }
/** = the popularity:
shorter means more popular */
public int popularity()
{ return 240 – length; }
}
public class Trailer extends Movie {
/** Constructor: a trailer of movie t.
Trailers are 1 minute long*/
public Trailer(String t)
{super(t, 1);}
}
public class Documentary extends Movie {
private String topic; // …
/** Constructor: instance with title t,
length n, and topic p */
public Documentary(String t, int n,
String p) {
super(t, n);
topic= p;
}
/** = "Documentary" */
public String DocumentaryType()
{ return "Documentary"; }
/** = popularity of this instance */
public int popularity()
{ return 200 - getLength(); }
}
public class Short extends Documentary {
/** Constructor: instance with title t,
length n, and topic p */
public Short(String t, int n, String p)
{ super(t, n, p); }
/** displays acknowledgement */
public String showAck()
{return "We thank our director“;}
/** = "Short Doc" */
public String DocumentaryType()
{ return "Short Doc"; }
}
22
(Fall’05) Ques7on 4 (30 points) For each pair of statements below,
write the value of d aier execu8on. If the statements lead to an error,
write “BAD” and briefly explain the error. (The ques8on con8nues on
the next page.)
Documentary e=
new Short("Man on Wire”, 5, "Bio");
boolean d=
"Short Doc” .equals(e.DocumentaryType());
23
(Fall’05) Ques7on 4 (30 points) For each pair of statements below,
write the value of d aier execu8on. If the statements lead to an error,
write “BAD” and briefly explain the error. (The ques8on con8nues on
the next page.)
Documentary e=
new Short("Man on Wire”, 5, "Bio");
boolean d=
"Short Doc” .equals(e.DocumentaryType());
24
True. method equals here is from the string object
Movie c=
new Documentary(null, 3, "Carter Peace Center");
int d= c.popularity();
25
public class Movie {
private String title; // title of movie
private int length; // length in minutes
/** Constructor: document with title t
and len minutes long */
public Movie(String t, int len) {
title= t; length= len;
}
/** = title of this Movie */
public String getTitle()
{ return title; }
/** = length of document, in minutes */
public int getLength()
{ return length; }
/** = the popularity:
shorter means more popular */
public int popularity()
{ return 240 – length; }
}
public class Trailer extends Movie {
/** Constructor: a trailer of movie t.
Trailers are 1 minute long*/
public Trailer(String t)
{super(t, 1);}
}
public class Documentary extends Movie {
private String topic; // …
/** Constructor: instance with title t,
length n, and topic p */
public Documentary(String t, int n,
String p) {
super(t, n);
topic= p;
}
/** = "Documentary" */
public String DocumentaryType()
{ return "Documentary"; }
/** = popularity of this instance */
public int popularity()
{ return 200 - getLength(); }
}
public class Short extends Documentary {
/** Constructor: instance with title t,
length n, and topic p */
public Short(String t, int n, String p)
{ super(t, n, p); }
/** displays acknowledgement */
public String showAck()
{return "We thank our director“;}
/** = "Short Doc" */
public String DocumentaryType()
{ return "Short Doc"; }
}
26
27
a
Animal
Cat Cat(String, int)
getNoise()
toString()
getWeight()
age
Animal(String, int)
isOlder(Animal)
QUESTION: Which method is called by
Animal t= new Cat(“A”,5); t.toString()?
A. the one in the hidden partition for
Object of a
B. the one in partition Animal of a
C. the one in partition Cat of a
D. None of these
Object
Animal
Cat
the class hierarchy:
Movie c=
new Documentary(null, 3, "Carter Peace Center");
int d= c.popularity();
- What is the apparent class?
- Answer: 197. method popularity of
class Documentary is called
Movie
Documentary Trailer
Short
28
Short b= (Short)(new Documentary("", 2, "WMD"));
int d= b.DocumentaryType().length();
29
Short b= (Short)(new Documentary("", 2, "WMD"));
int d= b.DocumentaryType().length();
Movie
Documentary Trailer
Short
- From documentary, can go (cast) up and back down
to documentary.
- Think what would happen for the call b.showAck()
30
(Fall’05) Ques7on 4 (24 points). (a) Write an instance method
equals(Object obj) for class Documentary
public class Documentary extends Movie {
/** = "obj is a Documentary with the same values
in its fields as this Documentary" */
public boolean equals(Object obj) {
37
public class Documentary extends Movie {
/** = "obj is a Documentary with the same values
in its fields as this Documentary" */
public boolean equals(Object obj) {
if (!(obj instanceof Documentary) {
38
public class Documentary extends Movie {
/** = "obj is a Documentary with the same values
in its fields as this Documentary" */
public boolean equals(Object obj) {
if (!(obj instanceof Documentary) {
return false;
39
public class Documentary extends Movie {
/** = "obj is a Documentary with the same values
in its fields as this Documentary" */
public boolean equals(Object obj) {
if (!(obj instanceof Documentary) {
return false;
Documentary docObj= (Documentary)obj;
40
Don’t forget to cast.
This is a legal cast. (Why?)
public class Documentary extends Movie {
/** = "obj is a Documentary with the same values
in its fields as this Documentary" */
public boolean equals(Object obj) {
if (!(obj instanceof Documentary) {
return false;
Documentary docObj= (Documentary)obj;
return
getTitle().equals(docObj.getTitle()) &&
getLength() == docObj.getLength() &&
topic.equals(docObj.topic);
41
What’s in the exam?
- Everything you needed to know for Prelim I
- Vector / String class, func8ons
- Wri8ng func8ons
- Recursive Func8ons
- apparent/real classes, cas8ng, operator
instanceof , func8on equals
- Abstract classes and methods
42
Let’s capture the essence of animals
/** representation of an animal */
public class Animal {
private int birthDate; // animal’s birth date
private String predator; // predator of this animal
private String prey; // class of animals this hunts
…
// move the animal to direction…
public void move(…){
…
}
// make the animal eat…
public void eat (…){
…
}
…
}
43
Problems
- Animal is an abstract concept
- Crea8ng an abstract animal doesn’t make sense in the real
world
- Dogs, cats, snakes, birds, lizards, all of which are animals,
must have a way to eat so as to get energy to move
- However…
- Class Animal allows us to create a UFA (uniden8fied flying
animal), i.e. instance of Animal
- If we extend the class to create a real animal, nothing
prevent us from crea8ng a horse that doesn’t move or eat.
44
Solu8ons
- How to prevent one from crea8ng a UFA?
- Make class Animal abstract
- Class cannot be instan7ated
- How? Put in keyword abstract
- How to prevent crea8on paralyzed dogs or starving sharks?
- Make the methods move and eat abstract
- Method must be overridden
- How? Put in keyword abstract and replace the body with ";"
45
Making things abstract
/** representation of an animal */
public abstract class Animal{
private int birthDate; // birth date
private String predator; // animal’s predator
private String prey; // What animal hunts
// Move the animal move in direction …
public abstract void move(…);
// Make the animal eat…
public abstract void eat (…);
46
Good Luck!
47
