JETT Workshop - Java Basics

Computer Science (CSCI) Department
College of Engineering, Computer Science, & Technology (ECST)

JETT Workshops
Java Basics

Fall 2004

Arrays
(Example code and figures from Computing Concepts with Java Essentials, 3rd edition, by Cay Horstmann.
Visit www.wiley.com/college/horstmann for information on the current edition of this excellent text.)

Arrays
- Construct array:
  new double[10]
- Store reference in variable of type double[]
  double[] data = new double[10];
- Arrays have fixed length
- Arrays have elements of a specific type
- Use [] to access element:
  data[4] = 29.95;
- Get array length as data.length (Not a method!)

Copying Arrays

Copying an array reference yields a second reference to the same array
double[] data = new double[10];
// fill array . . .
double[] prices = data;

Use clone to make true copy

double[] prices = (double[])data.clone();

Copying Array Elements

System.arraycopy(from, fromStart, to, toStart, count);

Partially Filled Arrays

Array length = maximum number of elements in array
Usually, array is partially filled
Need companion variable to keep track of current size

DataSet Example
- DataSet.java
- DataSetTest.java

Array Lists
- rather than managing size of array ourselves, we can use ArrayList to store a variable number of objects
  ArrayList coins = new ArrayList();
  coins.add(new Coin(0.1, "dime"));
  . . .
- Purse.java

Storing Numbers in an Array List

Array lists store object references

Use wrapper classes to store numbers

Double wrapper = new Double(29.95);
double unwrapped = wrapper.doubleValue()

ArrayList data = new ArrayList();
data.add(wrapper);
unwrapped = ((Double)data.get(i)).doubleValue();

Also have Integer and Boolean wrappers