Java程序辅导

Monday 9/20
Announcements:
 Lab 1 due today at 11 PM
 Lab 2 and Homework 2 posted
 Wednesday Quiz 1 : 15-minute quiz
 Chapter 1 only
Next time
 No new reading assignment
 Study for the quiz
 Do the “before lab” for Lab 2 
Today:
 Using a UML to write a class
 Scope of variables
 checked and unchecked exceptions 
 try-catch
 List interface and ArrayList
 Lab 2  (15 minutes)
UML Diagrams
 Write the class employee
 Scope of a field variable
 Scope of a local variable
Exceptions
Unchecked
Exceptions
Checked
Exceptions
Checked and Unchecked
 What is the difference between checked and 
unchecked exceptions?
 Statements or methods that may throw a checked 
exception must be handled
Throw exception to calling method -- throws
Surround statement with try-catch
 Two examples
FileNotFoundException (checked)
InputMismatchException (unchecked)
 Use try-catch to handle checked exceptions
Starting after Lab 1
try-catch
//code inside main or some method
String filename = "data.dat";
File infile = new File (filename);
Scanner scanFile = null;
try {
scanFile = new Scanner (infile);
} catch (FileNotFoundException e) {
System.out.println("Could not open file :" + filename);
//e.printStackTrace(); 
//System.out.println(e.getMessage());
//open some default file
//System.exit(-1);
}
try contains code that may
throw an Exception
catch has code 
that is executed 
when the Exception 
is thrown
Visibility/Access Modifiers
Visibility Applied to Class Member
- private visibility only inside the class
~ (blank) default visibility to classes in this package
#   protected visibility to classes in this package 
and all subclasses
+   public all classes
Client View: Array vs. ArrayList
 Properties of a primitive array
Once created the length is fixed
User must keep track of “logical size”
Must shift to add/remove elements 
Can create an array of primitives
 Properties of an ArrayList
Automatically grow and shrink as needed
Methods for
• Size
• Search 
• Traversal either direction without subscripts 
Add/Remove without shifting
Can only create ArrayList of Objects
ArrayList
Generic Class
 Generic ArrayList
Type parameter String
 Example:
ArrayList names = new ArrayList();
names.add(“Mary”);
…
String s = names.get(0);  
ArrayList is a raw type
compile time error, why?
(Java 1.5) java.util.List Interface
Required List methods:
add(i, data)
get(i)
size()
isEmpty()
indexOf(data)
…
Part of List Interface
(ArrayList and LinkedList)
Client View of ArrayList
Table 2.1: Some methods of Class ArrayList
public E get(int i)
public E set (int i, E entry)
public int size()
public boolean add(E entry)
public void add(int i, E entry)
public int indexOf(E target)
public E remove(int index)
Similar to Problem 2.2.1
String temp;
List myList = new ArrayList<> ();
myList.add("Pokey");
myList.add("Campy");
int i = myList.indexOf("Happy");
temp = myList.set(i, "Bouncy");
temp = myList.remove(myList.size() - 2);
temp = myList.get(1);
temp = myList.set(1, temp.toUpperCase());
“Bashful” “Awful” “Doc” “Jumpy” “Happy” “Dopey”
ArrayList Practice
Assume we are in a main method
 Declare and instantiate an ArrayList of String
 Create the List: Marge, Homer, Lisa, and Bart 
 Print the size of the ArrayList
 Retrieve and print the 3rd element in the list
 Find and print the index of Bart
 Write a loop that prints all the elements  
Lab 2
 Questions
 The “before lab”