Using MapsA simple map: HashtableExample use of a HashtableHashtable constructorsWhich constructor should you use?Hashtable constructors (again)The Collections frameworkThe Map interface IThe Map interface IIOptional operationsMap viewsConstructorsHazards IHazards IIFrom Hashtables to HashMapssynchronizedCopying objectsThe Cloneable interfaceThe SortedMap interfaceRequirements for SortedMapSortedMap Methods ISortedMap Methods IIThe TreeMap classTreeMap constructorsQuick summarySetsThe EndUsing MapsA simple map: Hashtable•To create a Hashtable, use:import java.util.*;Hashtable table = new Hashtable();•To put things into a Hashtable, use:table.put(key, value);•To retrieve a value from a Hashtable, use:value = table.get(key);Example use of a Hashtable import java.util.*;public class HashtableUser { public static void main(String[] args) { Hashtable table = new Hashtable(); table.put("one", "un"); table.put("two", "deux"); table.put("three", "trois"); System.out.println("two -> " + table.get("two")); System.out.println("deux -> " + table.get("deux")); }} two -> deuxdeux -> nullHashtable constructors•Hashtable() –Constructs a new, empty Hashtable with a default capacity (11) and default load factor (0.75). •Hashtable(int initialCapacity) –Constructs a new, empty Hashtable with the specified initial capacity and the default load factor (0.75). •Hashtable(int initialCapacity, float loadFactor) –Constructs a new, empty Hashtable with the specified initial capacity and the specified load factor. •Hashtable(Map t) –Constructs a new Hashtable with the same mappings as the given Map.Which constructor should you use?•This is basically a question of efficiency–A hash table that is mostly empty wastes space–If a hash table is nearly full, some searches may take a very long time•The initial capacity of a hash table is the number of entries that it can hold initially•The load factor is a measure of how full it is–A load factor of 75% is usually a good compromise–If the table gets fuller than the load factor, Java creates a new, larger hash table and rehashes everything–Rehashing is an expensive operationHashtable constructors (again)•Hashtable() –Use if the default values are good enough•Hashtable(int initialCapacity) –Use if you have some idea how many entries to expect–Try to ensure it won’t be more than 75% full–If space is not an issue, double or triple the size•Hashtable(int initialCapacity, float loadFactor) –Use if you are trying to be super efficient–Requires careful experimentation and tuning•Hashtable(Map t) –Use to make a Hashtable from some other map–Initial capacity = 2*(size of t), load factor = 0.75The Collections framework•Hashtable is an old (pre-Collections) class•Hashtable has been retrofitted to implement the Map interfaceCollectionSortedSetListSetSortedMapMapHashtableThe Map interface I•Basic operations:–Object put(Object key, Object value)•Returns the previous value associated with key, or null if there was no previous value–Object get(Object key)•Returns null if the key was not found•A return value of null may not mean the key was not found (some implementations of Map allow null keys and values)•Tests:–boolean containsKey(Object key)–boolean containsValue(Object value)•Warning: probably requires linear time!–boolean isEmpty()–boolean equals(Object o)•Returns true if o is also a map and has the same mappingsThe Map interface II•Optional operations:–Object put(Object key, Object value)•(So you could implement an immutable map)–void putAll(Map t)•Adds the mappings from t to this map–void clear()–Object remove(Object key)•Returns the value that was associated with the key, or null•Other:–int size()•Returns the number of key-value mappings–int hashCode()•Returns a hash code value for this mapOptional operations•Question: How can a method declared in an interface be optional?•Answer: you have to implement it, but the implementation may be something like this: public void remove(Object key) throws UnsupportedOperationException { throw new UnsupportedOperationException();}•In fact, HashMap extends AbstractMap, which provides many of the map operations, and implements the optional operations like thisMap views• Set keySet() –Returns a set view of the keys contained in this map.• Collection values() –Returns a collection view of the values contained in this map–Can’t be a set—keys must be unique, but values may be repeated•A view is dynamic access into the Map–If you change the Map, the view changes–If you change the view, the Map changes•The Map interface does not provide any Iterators–However, there are iterators for the above Sets and CollectionsConstructors•Map is an interface, so it cannot require any constructors•However, Java always supplies:–A no-argument constructor for each Map type–A constructor that takes a Map argument, and copies its key-value pairs into the new Map •If you ever implement your own Map class, you should define these constructors–Defining your own Map class is easy:class MyMap implements Map { ... }–There are, however, a lot of methods to implementHazards I•In order for a Hashtable to work correctly,–equals must be defined properly on the keys–hashCode must be defined properly on the keys•This is not a problem if you use Strings for the keys (this is extremely common)•If you use objects of some other class as your keys, you must make sure equals and hashCode are properly defined•Note: equals and hashCode are properly defined for all of Java’s Maps; it’s the keys that you need to be careful withHazards II•You should use immutable objects (like Strings) as keys•If you put a value into a hash table with a mutable key, and you change the key, what happens?–Answer: Nothing good!•Special case #1: A map may not contain itself as a key•Special case #2: A map may contain itself as a value, but equals and hashCode are no longer well-defined•These special cases are really weird and you will probably never get anywhere near themFrom Hashtables to HashMaps•Hashtable has been around a long time, but HashMap is new with Java 1.2•So why am I teaching you the old stuff?–Actually, except for the constructors, I’ve been talking about the Map interface, which both Hashtable and HashMap implement–Both are