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Map (interface) |
Maintains key-value associations (pairs), so you can look up a value using a key. |
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HashMap* |
Implementation based on a hash table. (Use this instead of Hashtable.) Provides constant-time performance for inserting and locating pairs. Performance can be adjusted via constructors that allow you to set the capacity and load factor of the hash table. |
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ArrayMap |
Map backed by an ArrayList. Gives precise control over the order of iteration. Designed for very small Maps, especially those that are frequently created and destroyed. For very small Maps, creation and iteration is substantially cheaper than for HashMap. Performance gets very bad when the Map is large. |
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TreeMap |
Implementation based on a red-black tree. When you view the keys or the pairs, they will be in sorted order (determined by Comparable or Comparator, discussed later). The point of a TreeMap is that you get the results in sorted order. TreeMap is the only Map with the subMap( ) method, which allows you to return a portion of the tree. |
The following example contains two sets of test data and a fill( ) method that allows you to fill any map with any two-dimensional array of Objects. These tools will be used in other Map examples, as well.
//: Map1.java
// Things you can do with Maps
package c08.newcollections;
import java.util.*;
public class Map1 ,
,
,
,
,
,
,
};
public final static String[][] testData2 = ,
,
};
public static Map fill(Map m, Object[][] o)
// Producing a Set of the keys:
public static void printKeys(Map m)
// Producing a Collection of the values:
public static void printValues(Map m)
// Iterating through Map.Entry objects (pairs):
public static void print(Map m)
}
public static void test(Map m)
public static void main(String args[])
} ///:~
The printKeys( ), printValues( ), and print( ) methods are not only useful utilities, they also demonstrate the production of Collection views of a Map. The keySet( ) method produces a Set backed by the keys in the Map; here, it is treated as only a Collection. Similar treatment is given to values( ), which produces a List containing all the values in the Map. (Note that keys must be unique, while values can contain duplicates.) Since these Collections are backed by the Map, any changes in a Collection will be reflected in the associated Map.
The print( ) method grabs the Iterator produced by entries and uses it to print both the key and value for each pair. The rest of the program provides simple examples of each Map operation, and tests each type of Map.
When creating your own class to use as a key in a Map, you must deal with the same issues discussed previously for Sets.
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