The associated package for HashMap and TreeMap classes:
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java.util.HashMap
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java.util.TreeMap
Common constructors of the HashMap and TreeMap classes:
| Constructors | Description |
| HashMap<K,V>() | Creates an empty HashMap using the specified types for the keys and values. |
| TreeMap<K,V>() | Creates an empty TreeMap using the specified types for the keys and values. |
Common methods of the HashMap and TreeMap classes:
| Method | Description |
| clear() | Removes all entries from the map. |
| containsKey(key) | Returns true if the specified key is in the map. |
| containsValue(value) | Returns true if the specified value is in the map. |
| entrySet() | Returns a set of all entries in the map as Map.Entry objects. |
| get(key) | Returns the value for the entry with the secified key. Returns ull if the key is not found. |
| put(key, value) | Adds an entry with the specified key and value, or replaces the value if an entry with the key already exists. |
| remove(key, value) | Removes the entry with the specified key. |
| size() | Returns the number of entries in the map. |
Common mehtods of the Map.Entry interface:
| Method | Description |
| getKey() | Returns the key for the map entry. |
| getValue() | Returns the value for the map entry. |
Descriptions:
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A map is a collection that contains values that are associated with keys. The two most commonly used classes that implement maps are HashMap and TreeMap.
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The main difference between HashMap and TreeMap automaticlly maintains entryies in order based on the key values. If an application doesn't require that the entries be kept in order, a HashMap is often more efficient than a TreeMap.
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Each entry of a map implements the Map.Entry interface in the java.util.Map package. You can use two of the methods provided by this interface to get the key and value for an entry.
Note:
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You can use a custom class for the key objects of a HashMap. To do that, the class must override the hashCode and equals methods inherited from Object.
Example 1: Code that uses a hash map
// create an empty hash map HashMap<String, String> books = new HashMap<String, String>(); // Add the entries books.put( "wooz", "Wizard of Oz"); books.put( "mbdk", "Mobby Dick"); books.put( "wuth", "War and Peace"); // print the entries. for (Map.Entry book : books.entrySet()) System.out.println(book.getKey() + ": " + book.getValue()); // print the entry whose key is "mbdk" System.out.println("\nCode mbk is " + books.get("mbk"));Example 2: Code that uses a tree map
// create an empty tree map TreeMap<String, String> books = new TreeMap<String, String>(); // Add the entries books.put( "wooz", "Wizard of Oz"); books.put( "mbdk", "Mobby Dick"); books.put( "wuth", "War and Peace"); // print the entries. for (Map.Entry book : books.entrySet()) System.out.println(book.getKey() + ": " + book.getValue()); // print the entry whose key is "mbdk" System.out.println("\nCode mbk is " + books.get("mbk"));