1. Arrays in Java
What, why and how
Copyright Curt Hill

2. What is an array? Multiple occurrences of values of one type
One name many values
Item is selected at run time by an integer index
A true data type in Java, unlike C++
Always an object
Always dynamic
The mark of arrays is a pair of square brackets: [ ]
Copyright Curt Hill

3. Why do we need arrays?
We often need to select a value by an index rather than just name
If a picture has 240,000 pixels, do we really want 240,000 different variable names?
Instead we find one name and use an integer subscript to choose which of the many we want
Copyright Curt Hill

4. Counting For Example static void process(Picture p){ // Enhance red
Pixel [] pixels = p.getPixels();
for(int i=0;i<pixels.length;i++){
Pixel ap = pixels[i];
int r = 3 *ap.getRed()/2;
ap.setRed(r); }
Copyright Curt Hill

5. Explanation A reference to pixels either refers to the whole object:
Such as pixels.length which describes how many
Otherwise a bracket selects which one:
pixels[i].setRed(red);
Set the ith pixel’s red value
Copyright Curt Hill

6. For All Example static void process(Picture p){ // Enhance red
Pixel [] pixels = p.getPixels(); for(Pixel ap: pixels){
int r = 3 * ap.getRed()/2;
ap.setRed(r); }
Copyright Curt Hill

7. Explanation This version of the for does two things for us
It infers the length of the array
It hides the subscripting of the pixel
Copyright Curt Hill

8. Arrays One name, many values
Arrays elements demonstrate identical form and function
The form is that each element has exactly the same type
Polymorphism can come into play here, but not considered further until much later
The function is that they are there for the same purpose
Copyright Curt Hill

9. What is needed? Declaration Allocation Access and reference
Marked by the []
Allocation
Determines the size
Access and reference
Indexing or subscripting
Answers to these questions:
What can we do to an array element?
What can we do to the whole array?
Copyright Curt Hill

10. Array declaration Two similar declarations:
int [ ] a, b, c[ ];
int d[];
Brackets may precede or follow the variable name
No size is allowed in declaration
Brackets are attached to variable name
b is not an array, but a simple integer
Even an array of primitives is an object
Copyright Curt Hill

11. Array Allocation Two step for objects or primitives
Allocate the array handle
This determines the size of the array
Allocate (or initialize) the individual elements
Primitives need to be initialized
Objects are usualy initialized by the allocation
In either case, usually done with a for
Copyright Curt Hill

12. Preface to Allocation
At least for a time we will get an array from somewhere else
A picture
We will not use the new operator on it, nor will we initialize it by accessing individual elements
We can create and allocate arrays with new
The technique is seen in next slides
Copyright Curt Hill

13. Allocation examples Two statements One statement
int x[];
x = new int [20];
One statement
int a[] = new int[20];
The initialization of both of these still needs to be done
Copyright Curt Hill

14. Using whole arrays Suppose the following declaration
int [] a = new int [5], b[];
Without subscript an array name is just an object handle
b = a;
Have both handles refer to same array a b
Copyright Curt Hill

15. Subscripting Using a subscript an element is obtained
a[k] = a[j+2]-2;
Anywhere an int can be used a[k] can be used
First element has subscript zero
Last element has subscript size minus one
Copyright Curt Hill

16. A Subscript
The item in brackets chooses which element of array to be used
The item may be a
integral constant
integral variable
integral expression
No doubles or float subscripts
There is no element between [2] and [3]
Must be in the correct range
0 to length of array-1
Copyright Curt Hill

17. Subscript Errors Suppose: int [] a = new int [10]; . . . a[k] = 2;
If k is less than zero or greater than 9 a subscript error occurs
ArrayIndexOutOfBoundsException is thrown
It is the programmers responsibility to guarantee that an index is in the correct bounds
Copyright Curt Hill

18. Subscript Traceback
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: -2
at pd1.PictureDemo1.process(PictureDemo1.java:23)
at pd1.PictureDemo1.main(PictureDemo1.java:16)
ArrayIndexOut OfBounds is the error
The -2 is the actual subscript
Find the first (from the top) line that is in your code.
In this case it is line 23
Copyright Curt Hill

19. Discussion
Since an array is one name with many values, an array element has a two piece name
The array name and the subscript
We may assign to this name or use in an expression
Processing an array sequentially is most often done with a for statement
Copyright Curt Hill

20. Array extras Arrays can be parameters or results of methods
Lowest subscript is always zero
The number of elements of an array can be accessed by a property:
a.length
One larger than largest valid subscript
Copyright Curt Hill

21. The questions What can we do to an array element?
Anything reasonable to do to a variable of the same type
What can we do to the whole array?
Pass as a parameter
Return as a result
Assign the handle
Get the length
Copyright Curt Hill

22. Array Handles Like any object what you have is the handle
This handle can be reused int [] ar = new int[20]; … ar = new int[40];
Thus the length of the array is dependent on the current version
A different length depending on where it is checked
Copyright Curt Hill

23. Tracing Somewhat problematic
Since one name has many values we have to be careful to keep track of them properly
Moreover, we do not know how many elements there are until the allocation
Copyright Curt Hill

24. Discussion A primitive passed to a method just passes a copy
The original cannot be changed
An object just passes a handle
The object may changed but not the handle
Arrays may thus have their values changed
Strings are the exception
The string is immutable
Copyright Curt Hill

25. Conclusions Arrays are a valuable means to structure data
They are an object without an object name
They require a subscript (or index) to access individual elements
Methods may change array parameters
Multiple dimension arrays are also possible but will be discussed later
Copyright Curt Hill