[JAVA API] java.util.HashMap<K,V>

HashMap 사용 예제

import java.util.*;

class Ex107_HashMap 
{
	public static void main(String[] args)
	{
		//Ex107_HashMap.java
		//	- HashTable, HashSet, HashMap, Dictionary
		m3();
	}//end main

	public static void m1() {
		HashMap list = new HashMap();
		//HashMap

		//데이터 삽입
		list.put("apple", "사과");	//put(key,value)
		list.put("dog", "강아지");
		list.put("cat", "고양이");
		list.put("java", "자바");	

		System.out.println(list.size());

		System.out.println(list.get("dog"));
		System.out.println(list.get("cat"));
	}//end m1()

	public static void m2() {
		HashMap list = new HashMap();

		list.put(1,"하나");
		list.put(2,"둘");
		list.put(3,"셋");

		System.out.println(list.get(1));	//index(x) -> key(o)

	}//end m2()

	public static void m3() {
		HashMap list = new HashMap();

		list.put("홍길동",true);
		list.put("아무게",true);
		list.put("하하하",false);
		list.put("호호호",true);
		list.put("이순신",false);

		Set set = list.keySet();
		Iterator iter = set.iterator(); //이터레이터 구현
		
		while (iter.hasNext()) {
			String key = iter.next();
			System.out.println(key);

			System.out.println(list.get(key));
		}

	}//end m3()

}

 

 

java.util

Class HashMap<K,V>

• java.lang.Object
• • java.util.AbstractMap<K,V>
  • • java.util.HashMap<K,V>

• Type Parameters:

  K - the type of keys maintained by this map

  V - the type of mapped values

  All Implemented Interfaces:

  Serializable, Cloneable, Map<K,V>

  Direct Known Subclasses:

  LinkedHashMap, PrinterStateReasons

  ---

  
  

  public class HashMap<K,V>
  extends AbstractMap<K,V>
  implements Map<K,V>, Cloneable, Serializable

  Hash table based implementation of the Map interface. This implementation provides all of the optional map operations, and permits null values and the null key. (The HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls.) This class makes no guarantees as to the order of the map; in particular, it does not guarantee that the order will remain constant over time.

  This implementation provides constant-time performance for the basic operations (get and put), assuming the hash function disperses the elements properly among the buckets. Iteration over collection views requires time proportional to the "capacity" of the HashMap instance (the number of buckets) plus its size (the number of key-value mappings). Thus, it's very important not to set the initial capacity too high (or the load factor too low) if iteration performance is important.

  An instance of HashMap has two parameters that affect its performance: initial capacity and load factor. The capacity is the number of buckets in the hash table, and the initial capacity is simply the capacity at the time the hash table is created. The load factor is a measure of how full the hash table is allowed to get before its capacity is automatically increased. When the number of entries in the hash table exceeds the product of the load factor and the current capacity, the hash table is rehashed (that is, internal data structures are rebuilt) so that the hash table has approximately twice the number of buckets.

  As a general rule, the default load factor (.75) offers a good tradeoff between time and space costs. Higher values decrease the space overhead but increase the lookup cost (reflected in most of the operations of the HashMap class, including get and put). The expected number of entries in the map and its load factor should be taken into account when setting its initial capacity, so as to minimize the number of rehash operations. If the initial capacity is greater than the maximum number of entries divided by the load factor, no rehash operations will ever occur.

  If many mappings are to be stored in a HashMap instance, creating it with a sufficiently large capacity will allow the mappings to be stored more efficiently than letting it perform automatic rehashing as needed to grow the table.

  Note that this implementation is not synchronized. If multiple threads access a hash map concurrently, and at least one of the threads modifies the map structurally, it must be synchronized externally. (A structural modification is any operation that adds or deletes one or more mappings; merely changing the value associated with a key that an instance already contains is not a structural modification.) This is typically accomplished by synchronizing on some object that naturally encapsulates the map. If no such object exists, the map should be "wrapped" using the Collections.synchronizedMap method. This is best done at creation time, to prevent accidental unsynchronized access to the map:

     Map m = Collections.synchronizedMap(new HashMap(...));

  The iterators returned by all of this class's "collection view methods" are fail-fast: if the map is structurally modified at any time after the iterator is created, in any way except through the iterator's own remove method, the iterator will throw a ConcurrentModificationException. Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future.

  Note that the fail-fast behavior of an iterator cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast iterators throw ConcurrentModificationException on a best-effort basis. Therefore, it would be wrong to write a program that depended on this exception for its correctness: the fail-fast behavior of iterators should be used only to detect bugs.

  This class is a member of the Java Collections Framework.

  Since:

  1.2

  See Also:

  Object.hashCode(), Collection, Map, TreeMap, Hashtable, Serialized Form

• • Nested Class Summary

    • Nested classes/interfaces inherited from class java.util.AbstractMap

      AbstractMap.SimpleEntry<K,V>, AbstractMap.SimpleImmutableEntry<K,V>

  • Constructor Summary

    Constructors 

    Constructor and Description
    HashMap() Constructs an empty HashMap with the default initial capacity (16) and the default load factor (0.75).
    HashMap(int initialCapacity) Constructs an empty HashMap with the specified initial capacity and the default load factor (0.75).
    HashMap(int initialCapacity, float loadFactor) Constructs an empty HashMap with the specified initial capacity and load factor.
    HashMap(Map<? extends K,? extends V> m) Constructs a new HashMap with the same mappings as the specified Map.

  • Method Summary

    Methods 

    Modifier and Type	Method and Description
    void	clear() Removes all of the mappings from this map.
    Object	clone() Returns a shallow copy of this HashMap instance: the keys and values themselves are not cloned.
    boolean	containsKey(Object key) Returns true if this map contains a mapping for the specified key.
    boolean	containsValue(Object value) Returns true if this map maps one or more keys to the specified value.
    Set<Map.Entry<K,V>>	entrySet() Returns a Set view of the mappings contained in this map.
    V	get(Object key) Returns the value to which the specified key is mapped, or null if this map contains no mapping for the key.
    boolean	isEmpty() Returns true if this map contains no key-value mappings.
    Set<K>	keySet() Returns a Set view of the keys contained in this map.
    V	put(K key, V value) Associates the specified value with the specified key in this map.
    void	putAll(Map<? extends K,? extends V> m) Copies all of the mappings from the specified map to this map.
    V	remove(Object key) Removes the mapping for the specified key from this map if present.
    int	size() Returns the number of key-value mappings in this map.
    Collection<V>	values() Returns a Collection view of the values contained in this map.

    • Methods inherited from class java.util.AbstractMap

      equals, hashCode, toString

    • Methods inherited from class java.lang.Object

      finalize, getClass, notify, notifyAll, wait, wait, wait

    • Methods inherited from interface java.util.Map

      equals, hashCode