程序代写代做 data structure More Objects

More Objects
Daniel Archambault
CS-115: Objects II
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Previously in CS-115
Stephen Train Creative Commons
Instantiate those DVDs!
CS-115: Objects II
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Previously in CS-115
What is an abstract data type (ADT)?
CS-115: Objects II
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Previously in CS-115
What is an abstract data type (ADT)?
What is the difference between an ADT and a data structure?
CS-115: Objects II
3

Previously in CS-115
What is an abstract data type (ADT)?
What is the difference between an ADT and a data structure? Why do we need ADTs in the first place?
CS-115: Objects II
3

Previously in CS-115
What is an abstract data type (ADT)?
What is the difference between an ADT and a data structure? Why do we need ADTs in the first place?
What are the two essential parts of a class?
What is the difference between a class and an instance of a class?
CS-115: Objects II
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Previously in CS-115
But given a class, how do we use it properly?
Objects II
CS-115: Objects II
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Reference Diagrams? Why are you doing this to me?!
We are going over reference diagrams today.
This is the most important lecture of CS-115 – especially overconfident students
Reference diagrams help us understand
􏰀 how data is stored by objects
􏰀 computation on objects
􏰀 data structures
Understand things offline before you start programming. Student story… I don’t know why he made us do that?
CS-115: Objects II
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What are attributes?
The data which defines an object
When we create a new instance of a class
􏰀 We get a copy of all non-static instance variables
􏰀 Static variables have one variable back with the class
Use constructors to initialize instance variables of a new object
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public class Book {
private String title; private String author; private int numPages;
public Book (String title, String author, int numPages) {
this.title = title;
this.author = author; this.numPages = numPages;
} }
public class BookTester {
….
Book b1 = new Book (“Swan Lake”, “Tchaikovsky”, 170);
Book b2 = new Book (“Awake and Dreaming”, “Kit Pearson”, 140); ….
}
CS-115: Objects II
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References are Critical
Remember class types and simple types
􏰀 Class types pass parameters by copying references
􏰀 Simple types pass parameters by copying values
last example no difference, but in this one, a big one!
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Reference Example
public class Book {
private String title; private String author; private int numPages;
public Book (String title, String author, int numPages) {
this.title = title;
this.author = author; this.numPages = numPages;
} }
public class BookTester {
….
String name = “Julie E. Czerneda”;
Book b1 = new Book (“Hidden in Sight”, name, 500); Book b2 = new Book (“SI: Survival”, name, 401);
….
}
CS-115: Objects II
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Calling Methods
Static methods called on a class 􏰀 Book.incCheckins ()
Non-static methods called on a object
􏰀 b1.setAuthor (“Me”)
􏰀 this.setAuthor (“Me”)
Although its allowed do not omit what comes before the “.”
CS-115: Objects II
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Inside Methods
Inside a static method you have access to: 􏰀 static instance variables
Inside a non-static method you have access to 􏰀 static and non-static instance variables
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Non-Static Methods
public class Book {
…
public String setAuthor (String author)
{
this.author = author;
}
… }
public class BookTester {
…
Book b1 = new Book (“Awake and Dreaming”, “Kit Pearson”, 140); (1) b1.setAuthor (“Vincent Thomas”);
(2) Book.setAuthor (“Vincent Thomas”);
… }
Which is all right (1) or (2)
CS-115: Objects II
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Static Methods Example
public class Book {
public static int totalNumberOfCheckins; …
public static void incCheckins() {
totalNumberOfCheckins++; }
… }
public class BookTester {
…
Book b1 = new Book (“Awake and Dreaming”, “Kit Pearson”, 140); (1) b1.incCheckins ();
(2) Book.incCheckins ();
… }
Which is all right (1) or (2)
CS-115: Objects II
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References and Methods
….
Book b1 = new Book (“Awake and Dreaming”, “Kit Pearson”, 140); Book b2 = new Book (“Hidden in Sight”, “Julie E. Czerneda”, 500); b1 = b2;
b1.setAuthor (“Jim John”);
….
Don’t forget that variables of a class type contain references
CS-115: Objects II
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Parameter Passing
A value is copied into a local parameter for the function Once the function terminates, the value is destroyed
􏰀 For simple types, it is the actual value stored in the variable
􏰀 For class types, it is a reference to an object
References outside the method scope do not change For variable declarations
􏰀 simple type (small letter) type (int, char) value stored
􏰀 class type, a reference to the object is stored
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public class ParamTest {
public static void primSwap (int x, int y)
{
int temp = x; x = y;
y = temp;
}
public static void arraySwap (int x[], int y[]) {
int temp = x[0]; x[0] = y[0];
y[0] = temp;
}
public static void main (String args[]) {
int x = 6;
int y = 7;
int a[] = {1, 2, 3};
int b[] = {4, 5, 6}; ParamTest.primSwap (x, y); ParamTest.arraySwap (a, b);
} }
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public class ParamTest {
public static void primSwap (int x, int y)
{
int temp = x; x = y;
y = temp;
}
public static void arraySwap (int x[], int y[]) {
int temp = x[0]; x[0] = y[0];
y[0] = temp;
x = {7, 8, 6};
}
public static void main (String args[]) {
int x = 6;
int y = 7;
int a[] = {1, 2, 3};
int b[] = {4, 5, 6}; ParamTest.primSwap (x, y); ParamTest.arraySwap (a, b);
} }
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public class BookModify {
public static void bookChange (Book b1, Book b2)
{
b1.setAuthor (“Thomas William”); b2.setTitle (“Simple Recipes”);
}
public static void main (String args[]) {
Book b1 = new Book (“Timothy Taylor”, “Stanley Park”, 400); Book b2 = new Book (“Frank McCourt”, “Angela’s Ashes”, 340); BookModify.bookChange (b1, b2);
} }
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public class BookModify {
public static void bookChange (Book b1, Book b2)
{
b1.setAuthor (“Thomas William”); b2.setTitle (“Simple Recipes”);
}
public static void main (String args[]) {
Book b1 = new Book (“Timothy Taylor”, “Stanley Park”, 400);
BookModify.bookChange (b1, b1); }
}
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public class BookModify {
public void bookChange (Book b1, Book b2)
{
b1.setAuthor (“Thomas William”); b2.setTitle (“Simple Recipes”);
}
public static void main (String args[]) {
Book b1 = new Book (“Timothy Taylor”, “Stanley Park”, 400);
BookModify.bookChange (b1, b1); }
}
Does this compile?
CS-115: Objects II
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public class BookModify {
public static void bookChange (Book b1, Book b2)
{
b1.setAuthor (“Thomas William”);
b2 = new Book (“Dr. S.”, “One Fish Two Fish”, 14); b2.setTitle (“Simple Recipes”);
}
public static void main (String args[]) {
Book b1 = new Book (“Timothy Taylor”, “Stanley Park”, 400); Book b2 = new Book (“Frank McCourt”, “Angela’s Ashes”, 340); BookModify.bookChange (b1, b2);
} }
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Arrays of Class Types
􏰀 arrays of references
􏰀 they behave like references in parameter passing
􏰀 you have an array of arrows to objects
Initially, all the arrows point nowhere null
CS-115: Objects II
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public class BookArrayModify {
public static void bookArrayChange (Book[] bookArray)
{
bookArray[0].setAuthor (“Frank John”); bookArray[2].setTitle (“Simple Recipes”);
}
public static void main (String args[]) {
Book b1 = new Book (“Timothy Taylor”, “Stanley Park”, 400); Book b2 = new Book (“Dr. S.”, “One Fish Two Fish”, 14);
Book b3 = new Book (“Frank McCourt”, “Angela’s Ashes”, 340); Book[] bookList = new Book[3]; //understood null, null, null BookArrayModify.bookArrayChange (bookList);
} }
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public class BookArrayModify {
public static void bookArrayChange (Book[] bookArray)
{
bookArray[0].setAuthor (“Frank John”); bookArray[2].setTitle (“Simple Recipes”);
}
public static void main (String args[]) {
Book b1 = new Book (“Timothy Taylor”, “Stanley Park”, 400); Book b2 = new Book (“Dr. S.”, “One Fish Two Fish”, 14);
Book b3 = new Book (“Frank McCourt”, “Angela’s Ashes”, 340); Book[] bookList = {b1, b2, b3}; BookArrayModify.bookArrayChange (bookList);
} }
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public class BookArrayModify {
public static void bookArrayChange (Book[] bookArray)
{
bookArray[0].setAuthor (“Frank John”); bookArray[2] = bookArray[1]; bookArray[2].setTitle (“Simple Recipes”);
}
public static void main (String args[]) {
Book b1 = new Book (“Timothy Taylor”, “Stanley Park”, 400); Book b2 = new Book (“Dr. S.”, “One Fish Two Fish”, 14);
Book b3 = new Book (“Frank McCourt”, “Angela’s Ashes”, 340); Book[] bookList = {b1, b2, b3} BookArrayModify.bookArrayChange (bookList);
} }
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