1 Java intro Part 3

2 Arrays in Java Store fixed number of values of a given type Arrays are objects –have attributes –must be constructed Array declaration: data type [] name = new data type[int expression];

3 Arrays in Java Elements in a new array are automatically set to 0, false, or null (depending on the underlying data type) Size of an array is stored in its length attribute; example: int [] numbers = new int [100]; int size = numbers.length; // stores 100 in size

4 Arrays in Java Access to individual members is achieved via index (subscript), just as it is in C++ Can construct and initialize an array in a single statement, such as: char [] vowels = {‘a’, ‘e’, ‘i’, ‘o’, ‘u’};

5 Anonymous arrays Array reference that is not stored in a variable Can be used in construction of other objects Java’s Polygon class is an example; has constructor: Polygon(int[] xvalues, int[] yvalues, int n) Can construct a triangle using anonymous arrays, as shown below: Polygon triangle = new Polygon(new int[] {0, 10, 5}, new int[] {10, 0, 5}, 3);

6 2-dimensional arrays Declaration & construction: Data type[][] name = new data type [rows][cols]; As in C++, array is indexed in row-major order

7 String[] args The default parameter to every main() method is args, an array of String This array is used to hold command-line arguments –args.length stores the number of arguments –Each argument is stored as a String from args[0] to args[args.length-1]

8 ArrayLists ArrayList class describes object that can hold a collection of objects of any type –objects, not primitive data types –can use wrapper classes (Integer, Double, Character, etc.) if ArrayList of one of these types is desired ArrayList class is part of the java.util package

9 ArrayList methods Constructor: ArrayList myList = new ArrayList(); doesn’t specify data type or size data type is Object, size determined by number of elements placed in List add() –simplest form just adds element to end of List; example: myList.add(“Finish notes before class”); myList.add(“Get haircut”); myList.add(“Lather, rinse, repeat”);

10 ArrayList methods size(): returns number of elements in List get(): returns element at specified position (between 0 and size() - 1) String firstItem = (String)myList.get(0); Since get() returns an Object, need explicit cast set(): replaces element at specified location –first argument: position of element –second argument: object to place myList.set(2, “Buy wig”);

11 ArrayList methods Insertion of elements in the middle of a list is accomplished via a call to add() using two arguments: –first: position of insertion –second: object to insert myList.add(1, “Yet another item”);

12 ArrayList methods remove() method removes the element at the specified position: myList.remove(0); Both add() and remove() move remaining elements up or down as needed

13 LinkList ArrayLists are especially useful for applications in which random access to specific elements is desirable However, they are inefficient for applications in which there are frequent insertions/deletions in the middle of a List The LinkList class provides an alternative for this type of application

14 LinkList Part of Java standard library Like ArrayList, container for collection of Objects Methods include: –constructor: LinkedList aList = new LinkedList(); –add(): places element at end of List –listIterator(): constructs an iterator for traversal of List (described on next slide)

15 ListIterator() Object that can access a position anywhere within a LinkList A LinkList object creates an iterator by calling its listIterator() method: ListIterator cursor = aList.listIterator(); Includes methods for moving through a List and adding and removing an element from the List

16 ListIterator methods hasNext(): returns true if iterator hasn’t reached end of List next(): returns current element; moves iterator to next element while (cursor.hasNext()) { String s = (String)cursor.next(); System.out.println(s); }

17 Adding/removing elements add() called on iterator inserts element in position in front of the iterator: cursor.add(“new element”); remove() called on iterator removes the element in front of the iterator: cursor.remove(); If an iterator reaches the end of a list, can start over by constructing another one

18 Packages Groups of Java classes Named using dot-separated sets of identifiers that correspond to the names of directories in a class’s path Base directory of a package is that directory containing all package directories Compilation of a class in a package must occur in the class’s base directory

19 Placing class in a package As the beginning of the source file, use a statement like the following: package base.sub1.sub2.subN.current; –for example, a class in the MYCLASSES subdirectory on drive a: would contain the statement: package myclasses; –if the class name is CLASS1, its full name would be myclasses.class1 A source file that doesn’t have a package statement is considered to be in the default package

20 Internal documentation & javadoc javadoc is a program included with the SDK (along with javac and java) When code has been commented with specially formatted comments, javadoc produces HTML pages describing your classes

21 Format for javadoc comments Start with /** and end with */ First sentence of each comment will be copied by javadoc to the summary table; should place one such comment strategically before each class and each method at the start of a line to describe a parameter at the start of a line to describe a return value

22 Guidelines for comments in Java code Supply an informative summary comment in the first line of documentation for every class and method Supply a description of every parameter and return value Write the comments first, then write the code: –for class, can use description from CRC card –for methods, can borrow information from responsibilities

23 Running javadoc At command line, type javadoc *.java Produces: –one HTML file per class –index.html containing links to other files When code is updated, update javadoc comments at the same time; run javadoc to update your HTML files

24 Java intro Part 3