Arraylist

Previous Lecture Continued

ArrayList

• An array of any object type that expands for you as needed
• Primitive types are NOT allowed.
• Use angle brackets to indicate the type:

 ArrayList<String> T = new ArrayList<String>();

• A parameter in the constructor indicates initial capacity:

 ArrayList<Double> D = new ArrayList<Double>(25);

What's so great about ArrayLists?

Pro:

• You don't have to know size beforehand.

• Tedium and bugs of resizing an array yourself eliminated. Con:

• Slower than an array

• No primitive support! (must use wrappers)
• Uses more memory

ArrayList<int>     - 4 bytes per element

ArrayList<Integer> - 4 bytes for the value +       --|
                     4 bytes for the data type +     |
                     4 bytes for reference count +   |--- 16 bytes
                     4 bytes for the reference     --|

Newer Syntax

• "Type inference" allows us to eliminate one of the type specifiers:

 ArrayList<String> T = new ArrayList<String>();
    ArrayList<String> T = new ArrayList<>();

Old Syntax

• Prior to generics, ArrayList worked on Objects. This was bad because of much casting.

Recreation and Practice

Practice Website

• Coding Bat
  • Fun, CS1-level problems
• Project Euler
  • Progressive mathematical problems best solved by writing a small program.
  • I need to do more of these.
• Programming Praxis
  • More open-ended programming problems
• ACM Programming Competition
• Google Code Jam
• Codepad
  • An online practice pad for many languages (not java)

Project Euler No. 15

Starting in the top left corner of a 2x2 grid, there are 6 routes (without backtracking) to the bottom right corner. How many routes are there in a 20x20 grid?

"Dynamic Programming"

Exceptions

Why Exceptions?

• Errors happen all the time
• The language should have construct to deal with unusual circumstances

Exceptional Vocabulary

• Exception
  • an uncommon event or error
• "Throw an exception" (or raise an exception)
  • to generate the exceptional condition.
• "Catch an exception"
  • pass control flow to exception handler
• Exception Handler
  • code that deals with an exception

Java Exception Vocabulary

• Try block
  • code that can throw an exception placed in this block

Simple Example ###

(see PPT)

Exception Programming tips

• Exceptions are objects.
• exceptions must be imported.
• catch specific exceptions seperately for better bug control.