Using Objects and Dictionaries as Associative Arrays in Java

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Using Objects and Dictionaries as Associative Arrays
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An associative array is a data structure that stores items by index. It functions like a ling cabinet in that if you know the name of what you re looking for, you can nd it by looking it up. The name, or the thing printed on the edge of the folders in the analogy, is called a key, and the item itself, or the folder you nd, is called a value. Associative arrays, like a ling cabinet, let you add a new folder, nd a folder by its name, remove a folder from the cabinet, look to see if a folder by a certain name exists, and ip through all the folders in the cabinet. The essential operation, however, is the lookup: retrieving values by name is what makes an associative array. Associative arrays maintain a many-to-one relationship between keys and values. A key refers to the same value every time, an essential rule. However, you could easily have one value that is led under multiple keys. For example, you might want to create a long, thin Noodle object and store it under Angel Hair, but also under Cappellini. Both names refer to the same kind of pasta, and both keys can refer to the same value: the instance of Noodle. You might also hear associative arrays referred to as hashes, hashtables, maps, or dictionaries. These are all names for the same kind of data type. The terms hashtable, hash, and map all refer to a particular way to implement this data structure, in which a hash function is applied to the keys to evenly distribute the values in memory and to later look them up. All this is handled inside the ActionScript Virtual Machine; all you need to know is that an Object can be used as an associative array.
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This kind of associative data is incredibly useful and is found all around us, in common programming situations, exposed to plain view on the internet, and in metaphors for the real world you can use to structure code. Most search engines, for example, expose associative data to you right in the address. Consider the URL www.google.com/search q=hashtable&safe=active. After the question mark ( ) is an associative array of keys and values. The query is stored as the key q, and its value is hashtable because you re searching for information on hashtables. The parameter for the search engine s content ltering feature is named safe, and its value is active because you don t want inappropriate results. Who knows what kind of lewd examples students in computer science courses might be posting right now This example uses key-value pairs to represent parameters, as with any form on the internet that uses HTTP GET. It s one key organizational technique to refer to complex data by reference, which is just what associative arrays do. In the example from the previous section, I demonstrated most of the necessary properties of an associative array using an Object. You stored and retrieved properties, or values, with dot and bracket notation. In this section, you see how to check for the existence of values, iterate through values, and remove values.
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8 explained how to use an array to store associative arrays. I recommend that you use arrays only for numerically indexed data. Using an Array object creates the impression that the data can be accessed by numeric index and that useful Array methods such as indexOf() and splice() can be used. However, none of these methods apply to properties that are stored by key rather than index. You could create even more confusion by storing values in an Array instance both by index and by associative key. Or consider the capability to store values with keys such as 12. Using an array to access a property like that could confuse your code beyond use. Use Objects to store values by a textual key, and use Arrays to store values by numeric index.
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Objects are ideal for storing key-value pairs. Using bracket notation with an Object instance looks just like looking up an array index by number, so you can think of it like an Array.
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Dictionaries (or, I should say, Dictionarys) can also be used to store key-value pairs. They have an important enhancement, however.
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Objects use strings as keys. This is suf cient for most purposes, but say you need a quick-and-dirty way to annotate class instances with certain additional information. Strings may not always be suf cient for this purpose. For example, you might be meeting a bunch of new people at a party.
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var guy1:Person = new Person("Bob"); var guy2:Person = new Person("Charlie"); var girl1:Person = new Person("Alice");
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You re trying to remember stuff about each person, so you use an Object to store information by the person s name.
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var notes:Object = new Object(); notes["Bob"] = "Likes games"; notes["Charlie"] = "Super organized"; notes["Alice"] = "Enjoys drawing";
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This works, until you meet a second person named Alice! Suddenly, there s no way to keep track of both Alices independently. Or heaven forbid, you might forget some of their names, but without a name you can t nd anything.
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var girl2:Person = new Person("Alice");
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