How to use the WeakHashMap.compute method in Java
Overview
WeakHashMap is a kind of Map that is implemented based on the HashTable. The WeakHashMap only stores the WeakReference type as keys. Once the key is no longer used, the respective mapping is automatically removed once the garbage collection is done.
The compute() method
The compute() method uses the mapping function provided as an argument to compute a new value for the specified key. If the specified key is not present, then no operation is performed and the method returns null.
Syntax
Let’s look at the syntax of the function.
public V compute(K key, BiFunction remappingFunction)
Parameters
-
key: The key for which the new value is to be computed. -
remappingFunction:- A
BiFunctionthat takes two arguments as input and returns a single value. - The return value from the
BiFunctionwill be updated as the value of the passed key of the map. - If we return
nullfromBiFunction, then the mapping for the key will be removed. - If the function itself throws an exception, then the exception is rethrown and the current mapping is left unchanged.
- A
Return value
The compute() method returns the new value associated with the key.
Code
The code below demonstrates how to use the compute() method.
import java.util.WeakHashMap;
class Compute {
public static void main( String args[] ) {
WeakHashMap<String, Integer> map = new WeakHashMap<>();
map.put("Maths", 50);
map.put("Science", 60);
map.put("Programming", 70);
System.out.println( "The map is - " + map);
System.out.println( "\nCalling compute function for key Maths");
Integer newVal = map.compute("Maths", (key, oldVal) -> { return oldVal + 10; });
System.out.println("\nThe return value is " + newVal);
System.out.println( "The map is - " + map);
System.out.println( "\n---------------\n");
System.out.println( "Calling compute function for key Economics\n");
newVal =
map.compute("Economics",
(key, oldVal) -> {
System.out.print("Inside BiFunction: The key is ");
System.out.print(key);
System.out.print(". The value is ");
System.out.println(oldVal + ".");
if(oldVal != null) {
return oldVal + 10;
}
return null;
});
System.out.println("\nThe return value is " + newVal);
System.out.println( "The map is - " + map);
}
}Explanation
-
In the code above, we create a
WeakHashMapnamedmapand add some entries to it. -
First, we call the
compute()method on themapfor theMathskey.
Integer newVal = map.compute("Maths", (key, oldVal) -> { return oldVal + 10; });
-
The code computes a new value inside
BiFunctionand updates the value associated with theMathskey. -
Then, we call the
compute()method on themapfor theEconomicskey. There is no mapping associated with theEconomicskey, so the map remains unchanged and returnsnull.
RELATED TAGS
CONTRIBUTOR
View all Courses
