WeakHashMap is a class which is available in java.util package. The WeakHashMap is a class of the Java collections framework provides the functionality of hash table data structure. In Java WeakHashMap, elements are stored in key/value pairs. Keys are unique values associated with individual values. WeakHashMap cannot contains duplicate keys.
The object of a weak reference type can be garbage collected in Java if the reference is no longer used in the program.
WeakHashMap Syntax:
WeakHashMap<K, V> numbers = new WeakHashMap<>();
WeakHashMap<String, Integer> numbers = new WeakHashMap<>();
By default,
| Method Name | Description |
|---|---|
| put() | inserts the specified key/value mapping to the map. |
| putAll() | inserts all the entries from the specified map to this map. |
| putIfAbsent() | inserts the specified key/value mapping to the map if the specified key is not present in the map. |
| entrySet() | returns a set of all the key/value mapping of the map. |
| keySet() | returns a set of all the keys of the map. |
| values() | returns a set of all the values of the map. |
| get() | Returns the value associated with the specified key. If the key is not found, it returns null. |
| getOrDefault() | Returns the value associated with the specified key. If the key is not found, it returns the specified default value. |
| remove(key) | returns and removes the entry associated with the specified key from the map. |
| remove(key, value) | removes the entry from the map only if the specified key mapped to be the specified value and return a boolean value. |
| clear() | removes all the entries from the map. |
| containsKey() | checks if the map contains the specified key and returns a boolean value. |
| containsValue() | checks if the map contains the specified value and returns a boolean value. |
| size() | returns the size of the map. |
| isEmpty() | checks if the map is empty and returns a boolean value. |
import java.util.WeakHashMap;
class Main {
public static void main(String[] args) {
// Creating WeakHashMap of even numbers
WeakHashMap<String, Integer> evenNumbers = new WeakHashMap<>();
String two = new String("Two");
Integer twoValue = 2;
// Using put()
evenNumbers.put(two, twoValue);
String four = new String("Four");
Integer fourValue = 4;
// Using putIfAbsent()
evenNumbers.putIfAbsent(four, fourValue);
System.out.println("WeakHashMap of even numbers: " + evenNumbers);
//Creating WeakHashMap of numbers
WeakHashMap<String, Integer> numbers = new WeakHashMap<>();
String one = new String("One");
Integer oneValue = 1;
numbers.put(one, oneValue);
// Using putAll()
numbers.putAll(evenNumbers);
System.out.println("WeakHashMap of numbers: " + numbers);
}
}
import java.util.WeakHashMap;
class Main {
public static void main(String[] args) {
// Creating WeakHashMap of even numbers
WeakHashMap<String, Integer> numbers = new WeakHashMap<>();
String one = new String("One");
Integer oneValue = 1;
numbers.put(one, oneValue);
String two = new String("Two");
Integer twoValue = 2;
numbers.put(two, twoValue);
System.out.println("WeakHashMap: " + numbers);
// Using entrySet()
System.out.println("Key/Value mappings: " + numbers.entrySet());
// Using keySet()
System.out.println("Keys: " + numbers.keySet());
// Using values()
System.out.println("Values: " + numbers.values());
// Using get()
int value1 = numbers.get("Two");
System.out.println("Using get(): " + value1);
// Using getOrDefault()
int value2 = numbers.getOrDefault("Four", 4);
System.out.println("Using getOrDefault(): " + value2);
}
}
import java.util.WeakHashMap;
class Main {
public static void main(String[] args) {
// Creating WeakHashMap of even numbers
WeakHashMap<String, Integer> numbers = new WeakHashMap<>();
String one = new String("One");
Integer oneValue = 1;
numbers.put(one, oneValue);
String two = new String("Two");
Integer twoValue = 2;
numbers.put(two, twoValue);
System.out.println("WeakHashMap: " + numbers);
// Using remove() with single parameter
int value = numbers.remove("Two");
System.out.println("Removed value: " + value);
// Using remove() with 2 parameters
boolean result = numbers.remove("One", 3);
System.out.println("Is the entry {One=3} removed? " + result);
System.out.println("Updated WeakHashMap: " + numbers);
}
}