visual basic barcode printing 7: Objects and Collections in Java

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7: Objects and Collections
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HashMap The HashMap gives you an unsorted, unordered Map. When you need a Map and you don t care about the order (when you iterate through it), then HashMap is the way to go; the other maps add a little more overhead. HashMap allows one null key in a collection and multiple null values in a collection. Hashtable Like Vector, Hashtable has been in from prehistoric Java times. For fun, don t forget to note the naming inconsistency: HashMap vs. Hashtable. Where s the capitalization of t Oh well, you won t be expected to spell it. Anyway, just as Vector is a synchronized counterpart to the sleeker, more modern ArrayList, Hashtable is the synchronized counterpart to HashMap. Remember that you don t synchronize a class, so when we say that Vector and Hashtable are synchronized, we just mean that the key methods of the class are synchronized. Another difference, though, is that while HashMap lets you have null values as well as one null key, a Hashtable doesn t let you have anything that s null. LinkedHashMap Like its Set counterpart, LinkedHashSet, the LinkedHashMap collection maintains insertion order (or, optionally, access order). Although it will be somewhat slower than HashMap for adding and removing elements, you can expect faster iteration with a LinkedHashMap. (LinkedHashMap is a new collection class as of version 1.4.) TreeMap You can probably guess by now that a TreeMap is a sorted Map. And you already know that this means sorted by the natural order of the elements. But like TreeSet, TreeMap lets you pass your own comparison rules in when you construct a TreeMap, to specify how the elements should be compared to one another when they re being ordered.
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Look for incorrect mixtures of interfaces with classes. You can easily eliminate some answers right away if you recognize that, for example, a Map can t be the collection class you choose when you need a name/value pair collection, since Map is an interface and not a concrete implementation class. The wording on the exam is explicit when it matters, so if you re asked to choose an interface, choose an interface rather than a class that implements that interface. The reverse is also true if you re asked to choose an implementation class, don t choose an interface type.
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Whew! That s all the collection stuff you ll need for the exam, and Table 7-2 puts it in a nice little summary.
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Collections (Exam Objective 9.1)
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TABLE 7-2
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Collection Interface Concrete Implementation Classes
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Class HashMap Hashtable TreeMap LinkedHashMap HashSet TreeSet LinkedHashSet ArrayList Vector LinkedList
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Map X X X X
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List
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Ordered No No Sorted By insertion order or last access order
Sorted No No By natural order or custom comparison rules No No By natural order or custom comparison rules No No No No
X X X X X X
No Sorted By insertion order or last access order By index By index By index
Be sure you know how to interpret Table 7-2 in a practical way. For the exam, you might be expected to choose a collection based on a particular requirement, where that need is expressed as a scenario. For example, which collection would you use if you needed to maintain and search on a list of parts, identified by their unique alphanumeric serial where the part would be of type Part Would you change your answer at all if we modified the requirement such that you also need to be able to print out the parts in order, by their serial number For the first question, you can see that since you have a Part class, but need to search for the objects based on a serial number, you need a Map. The key will be the serial number as a String, and the value will be the Part instance. The default choice should be HashMap, the quickest Map for access. But now when we amend the requirement to include getting the parts in order of their serial number, then we need a TreeMap which maintains the natural order of the keys. Since the key is a String, the natural order for a String will be a standard alphabetical sort. If the requirement had been to keep track of which part was last accessed, then we d probably need a LinkedHashMap.
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