Java WeakHashMap Example
January 05, 2023
java.util.WeakHashMap in our Java application.
1. The
WeakHashMap is the Hash table based implementation of the Map interface with weak keys.
2. The
WeakHashMap has the weak keys that means the keys in the WeakHashMap is eligible for garbage collector even if it is present in this map. Once the key is garbage collected, the entry of this key is removed from this map.
3. The
WeakHashMap supports null values and null keys both. The performance of WeakHashMap is similar to those of HashMap class.
4. The
WeakHashMap is not synchronized but can be synchronized using Collections.synchronizedMap method.
5. The
WeakHashMap is suitable for such object whose equal method is based on == operator check, for example, String objects. Once such a key is discarded, it can never be recreated, hence if such key is removed after some time, we cannot look up them.
6. The
WeakHashMap can automatically remove the entries and this behavior may surprise us. Automatic removal happens because the garbage collector may discard keys at any time and then that key entry is removed from the WeakHashMap.
7. For
WeakHashMap, even if we synchronize it and invoke none of its mutator methods, it is possible that
the
size method can return smaller values over time,
the
isEmpty method can return false and then true,
the
containsKey method can return true and later false for a given key,
the
get method can return a value for a given key but later return null,
the
put method can return null and the remove method can return false for a key that previously appeared to be in the map,
the successive examinations of the key set, the value collection, and the entry set can yield successively smaller numbers of elements.
Contents
1. Constructors
TheWeakHashMap can be created using following constructors.
1.
public WeakHashMap()
WeakHashMap with the default initial capacity (16) and load factor (0.75).
2.
public WeakHashMap(int initialCapacity)
WeakHashMap with the specified initial capacity and default load factor (0.75).
Throws
IllegalArgumentException, if the initial capacity is negative.
3.
public WeakHashMap(int initialCapacity, float loadFactor)
WeakHashMap with the specified initial capacity and load factor.
Throws
IllegalArgumentException, if the initial capacity is negative, or if the load factor is non-positive.
4.
public WeakHashMap(Map<? extends K,? extends V> m)
WeakHashMap with the same mappings as the specified map.
Throws
NullPointerException if the specified map is null.
2. remove, clear and isEmpty Methods
1. remove
public V remove(Object key)
Returns the previous value associated with the key, or null if key is not available.
2. clear
void clear()
WeakHashMap.
3. isEmpty
boolean isEmpty()
Example:
WeakHashMap<String, String> map = new WeakHashMap<>();
map.put("name", "Mohit");
map.put("city", "Varanasi");
map.put("country", "India");
//remove
String removedVal = map.remove("city");
System.out.println(removedVal); // Varanasi
//clear
map.clear();
//isEmpty
boolean mappings = map.isEmpty();
System.out.println(mappings); // true
3. containsKey and containsValue Methods
1. containsKey
boolean containsKey(Object key)
2. containsValue
boolean containsValue(Object value)
Example:
WeakHashMap<String, String> map = new WeakHashMap<>();
map.put("name", "Mohit");
map.put("city", "Varanasi");
//containsKey
boolean mapKey = map.containsKey("name");
System.out.println(mapKey); // true
//containsValue
boolean mapVal = map.containsValue("Varanasi");
System.out.println(mapVal); // true
4. put and putAll Methods
1. put
public V put(K key, V value)
Returns the previous value associated with the key, or null if there was no mapping for the key.
2. putAll
public void putAll(Map<? extends K,? extends V> m)
WeakHashMap. If any key is already available, the old value is replaced by new value.
Example:
WeakHashMap<String, String> wmap = new WeakHashMap<>();
//put
wmap.put("name", "Mohit");
wmap.put("city", "Varanasi");
System.out.println(wmap);
//putAll
Map<String, String> map = new HashMap<>();
wmap.put("name", "Krishn");
wmap.put("country", "India");
wmap.putAll(map);
System.out.println(wmap);
{city=Varanasi, name=Mohit}
{country=India, city=Varanasi, name=Krishn}
5. get and size Method
1. get
public V get(Object key)
2. size
public int size()
WeakHashMap.
Example:
WeakHashMap<String, String> wmap = new WeakHashMap<>();
wmap.put("name", "Mohit");
wmap.put("city", "Varanasi");
//get
String s = wmap.get("name"); // Mohit
System.out.println(s);
//size
int n = wmap.size();
System.out.println(n); // 2
6. keySet and values Methods
1. keySet
public Set<K> keySet()
WeakHashMap.
2. values
public Collection<V> values()
WeakHashMap.
Example:
WeakHashMap<String, String> wmap = new WeakHashMap<>();
wmap.put("name", "Mohit");
wmap.put("city", "Varanasi");
wmap.put("country", "India");
//keySet
Set<String> setOfKeys = wmap.keySet();
System.out.println(setOfKeys);
//values
Collection<String> mapValues = wmap.values();
System.out.println(mapValues);
[country, city, name] [India, Varanasi, Mohit]
7. entrySet Method
public Set<Map.Entry<K,V>> entrySet()
Example:
WeakHashMap<String, String> wmap = new WeakHashMap<>();
wmap.put("name", "Mohit");
wmap.put("city", "Varanasi");
wmap.put("country", "India");
//entrySet
Set<Map.Entry<String, String>> set = wmap.entrySet();
for(Map.Entry<String, String> m: set) {
System.out.println(m.getKey() + " - " + m.getValue());
}
country - India city - Varanasi name - Mohit
