Collections 4132010 Categorizing collections Static Size Dynamic Size

Collections 4/13/2010

Categorizing collections Static Size Dynamic Size Homogeneous elements Array. List Vector Set Heterogeneous elements Record/Structure Hash. Map

Arrays • Homogeneous static structures ▫ Can make heterogeneous by making an array of Object, but then you have to cast Objects to more specific types. ▫ Can simulate a dynamic structure by creating a method to add elements to the structure. • Problems ▫ Holes in the array maybe intentional maybe not ▫ Size limits, must know in advance or call a process to make a new array and copy. ▫ Integer indices – may not match data or processing needs

Array. List Features • A dynamic “array” • Stores Objects Limitations • Still has an integer index • Cannot store primitives (directly – stay tuned)

Some important methods • http: //java. sun. com/j 2 se/1. 5. 0/docs/api/java/u til/Array. List. html • add(Object o) // append • add(int index, Object o) // insert • remove(int index) // returns object … takes out • size() // like length • get(int index) // returns the object…leaves in • contains(Object 0) // true if object in list

Example • Array. List. Demo. java • Illustrates add, get, remove, and size • Array. List. Demo. Safe. java • Illustrates the use of the generic type

Hash. Map • http: //java. sun. com/j 2 se/1. 5. 0/docs/api/java/u til/Hash. Map. html • A dynamic “array”, but unlike an array one accesses the elements through a “key” which can be any object.

Important methods • • • put(Object key, Object item) get(Object key) remove(Object key) is. Empty() size()

Limitations • You access the elements by the key, not by its sequence. • You cannot go through the whole collection to print out the values.

Benefits • Ideally suited for structures where you need to do a fast search on some key element • Ideal for sparsely populated numbers (such as a phone number…you could use as a key to an array, but is much too sparse…too much space for too little return.

Benefits • Ideally suited for structures where you need to do a fast search on some key element • Ideal for sparsely populated numbers (such as a phone number…you could do it as a key to an array, but is much too sparse…too much space for too little return.

Example • Hash. Map demo

Hash. Set collection • A set collection; access to an individual through its value or you can iterate through the collection. • Can process the group or process an individual item. • No duplicate items. • http: //java. sun. com/j 2 se/1. 5. 0/docs/api/java/u til/Hash. Set. html

Important methods – see inheritance structure • • • add(Object o) remove(Object o) is. Empty() size() iterator() // returns an iterator over the elements of the set

Iterator “pattern” • We often want to process all of the elements of a collection whatever that collection is. • We should not know how the elements are stored (violates encapsulation). • So, we need some way of accessing the collection in a “sequential” manner.

Alternatives • Produce a “copy” of the collection in a form that we do understand. ▫ Enum class values() returns an array of the objects in the Enum. ▫ But we still need to understand that this is an array. • Iterable interface requires methods that let us “iterate” through a set of values. ▫ http: //java. sun. com/j 2 se/1. 5. 0/docs/api/java/lan g/Iterable. html

• Iterable interface requires methods that let us “iterate” through a set of values. ▫ http: //java. sun. com/j 2 se/1. 5. 0/docs/api/java/lan g/Iterable. html • Iterator interface requires methods that let us “loop” through the elements of the underlying object.