notes5

Lecture Summary 5a

An array is an ordered list of values, which are of the same type. That type can be primitive types or reference types (class or interface).
Arrays are objects. Array types are reference types.
Arrays are of fixed size, once declared of certain size it cannot be changed.
Arrays are always bound checked, performed by the index operator. If the index is out of range, an appropriate exception is thrown.
The length of array: ia.length
It is held in a public constant that can be referenced directly.
Index starts from 0. An array of size N is indexed from 0 to N-1
Examples:
int[] ia = new int[3]; //declaration and instantiation which allocates
//memory space to store 3 integers. int [] is the type
ia[0] = 1; ia[1] = 2; ia[2] = 3; //assign values to these integers
int ia[] = new int[3]; //alternative array syntax. The previous one is recommended
ia[0] = 1; ia[1] = 2; ia[2] = 3;
int[] ia = { 1, 2, 3}; //initialization using an initializier list. Number of items provides the size. // Only use at declaration.
float[][] mat = new float[4][4]; //2-dimentional array (matrix).

for (int y = 0; y < mat.length; y++)
{
for (int x = 0; x < mat[y].length; x++)
mat[y][x] = 0.0;
}
Example: LetterCount.java

Array as parameters
- An entire array can be passed to a method as a parameter
- Like any other object, the reference to the array is passed, making the formal and actual parameters aliases of each other
- Changing an array element in the method changes the original
Arrays of objects
- The elements of an array can be object references
- The following declaration reserves space to store 25 references to String objects
  String[] words = new String[25]; // words is an array of references to String objects
- It does NOT create the String objects themselves
- Each object stored in an array must be instantiated separately
- Example: GradeRange.java
Command line arguments
- public static void main(String[] args)
- Example CommandLineArguments.java
- Output:
Example: CD.java, CDCollection.java, Tunes.java