Full Answer
What is the use of computeIfAbsent in Java?
HashMap computeIfAbsent() method in Java with Examples. The computeIfAbsent(Key, Function) method of HashMap class is used to compute value for a given key using the given mapping function, if key is not already associated with a value (or is mapped to null) and enter that computed value in Hashmap else null.
How does the computeIfAbsent method work?
The computeIfAbsent method takes two parameters. The first parameter is the key, and the second parameter is the mappingFunction. It's important to know that the mapping function is only called if the mapping isn't present. 2.1. Key Related to a Non-Null Value First, it checks if the key is present in the map.
What is the use of computeIfAbsent in HashMap?
The computeIfAbsent(Key, Function) method of HashMap class is used to compute value for a given key using the given mapping function, if key is not already associated with a value (or is mapped to null) and enter that computed value in Hashmap else null. If mapping function of this method returns null, then no mapping is recorded for that key.
How many parameters does computeIfAbsent () method take?
The computeIfAbsent () method takes 2 parameters: Note: The remapping function cannot take two arguments. Note: If remappingFunction results null, then the mapping for the specified key is removed. In the above example, we have created a hashmap named prices.
How does computeIfAbsent work?
The computeIfAbsent(Key, Function) method of HashMap class is used to compute value for a given key using the given mapping function, if key is not already associated with a value (or is mapped to null) and enter that computed value in Hashmap else null.
What does computeIfAbsent return?
The computeIfAbsent() method checks the value associated with the provided key parameter and returns one of the following: If key has an associated value, the value is returned and no new value is computed. If key does not have an associated value, a new value is computed and returned.
Is computeIfAbsent Atomic?
It's important to notice that with computeIfAbsent(), a compound operation is not necessarily atomic because updates are performed outside of the method.
What is putIfAbsent in Java?
The putIfAbsent(Key, value) method of Hashtable class which allows to map a value to a given key if given key is not associated with a value or mapped to null. A null value is returned if such key-value set is already present in the HashMap.
What is Java mapping?
A Map is an object that maps keys to values. A map cannot contain duplicate keys: Each key can map to at most one value. It models the mathematical function abstraction.
How do I retrieve a value from a map?
HashMap. get() method of HashMap class is used to retrieve or fetch the value mapped by a particular key mentioned in the parameter. It returns NULL when the map contains no such mapping for the key.
What is ConcurrentHashMap example?
Java ConcurrentHashMap Example The ConcurrentHashMap class is similar to HashMap, except that it's thread-safe and allows modification while iteration. It's clear from the output that ConcurrentHashMap takes care of the new entry in the map while iteration whereas HashMap throws ConcurrentModificationException .
What is difference between HashMap and ConcurrentHashMap?
HashMap is non-Synchronized in nature i.e. HashMap is not Thread-safe whereas ConcurrentHashMap is Thread-safe in nature. HashMap performance is relatively high because it is non-synchronized in nature and any number of threads can perform simultaneously.
Why is ConcurrentHashMap thread safe?
Concurrent. ConcurrentHashMap class achieves thread-safety by dividing the map into segments, the lock is required not for the entire object but for one segment, i.e one thread requires a lock of one segment. In ConcurrentHashap the read operation doesn't require any lock.
What is the difference between putIfAbsent and computeIfAbsent?
computeIfAbsent returns “the current (existing or computed) value associated with the specified key, or null if the computed value is null”. putIfAbsent returns “the previous value associated with the specified key, or null if there was no mapping for the key”.
Is putIfAbsent thread safe?
HashMap is Hash table based implementation of the Map interface. HashMap provides constant-time performance for the basic operations (get and put). HashMap based implementation is not thread safe.
What is map size in Java?
HashMap size() Method in Java HashMap. size() method of HashMap class is used to get the size of the map which refers to the number of the key-value pair or mappings in the Map. Syntax: Hash_Map.size() Parameters: The method does not take any parameters.
Can putIfAbsent return null?
The putIfAbsent() method returns the previous value associated with the specified key, or null if there was no mapping for the key. (A null return can also indicate that the map previously associated null with the key, if the implementation supports null values.)
What is putIfAbsent flutter?
putIfAbsent method Null safety Returns the value associated to key , if there is one. Otherwise calls ifAbsent to get a new value, associates key to that value, and then returns the new value.
What is map entry?
A map entry (key-value pair). The Map. entrySet method returns a collection-view of the map, whose elements are of this class. The only way to obtain a reference to a map entry is from the iterator of this collection-view.
What is the return type of values () method in map?
Map Values() method returns the Collection view of the values contained in this map.
2.1. Key Related to a Non-Null Value
First, it checks if the key is present in the map. If the key is present, and a non-null value is related to the key, then it returns that value:
2.2. Using the Mapping Function to Compute the Value
Furthermore, if the key isn't present in the map, or the null value is related to the key, then it attempts to compute the value using the given mappingFunction. It also enters the calculated value into the map unless the calculated value is null.
2.3. Mapping Function Returns null
Also, if the mappingFunction returns null, the map records no mapping:
2.4. Mapping Function Throws an Exception
Finally, if the mappingFunction throws an unchecked exception, then the exception is re-thrown, and the map records no mapping:
What is computeIfAbsent in HashMap?from geeksforgeeks.org
The computeIfAbsent (Key, Function) method of HashMap class is used to compute value for a given key using the given mapping function, if key is not already associated with a value (or is mapped to null) and enter that computed value in Hashmap else null.
How does John compute the value?from baeldung.com
Since the key “John” is not present, it computes the value by passing the key as a parameter to the mappingFunction.
What is key -> 280?
key -> 280 is a lambda expression. It returns the value 280. To learn more about the lambda expression , visit Java Lambda Expressions.
Does computeIfAbsent compute the new value for shoes?
In the above example, the mapping for Shoes is already present in the hashmap. Hence, the computeIfAbsent () method does not compute the new value for Shoes.
What is computeIfAbsent in HashMap?from geeksforgeeks.org
The computeIfAbsent (Key, Function) method of HashMap class is used to compute value for a given key using the given mapping function, if key is not already associated with a value (or is mapped to null) and enter that computed value in Hashmap else null.
How does John compute the value?from baeldung.com
Since the key “John” is not present, it computes the value by passing the key as a parameter to the mappingFunction.
Syntax
public V computeIfAbsent(K key, Function<? super K,? extends V> mappingFunction)
Return value
If the key is not present and a new entry is added when the computeIfAbsent () method is called, then the new value mapped with the specified key is returned.
What is computeifabsent in Java?from concretepage.com
The computeIfAbsent is the default method of java.util.Map and has been introduced in Java 8. The computeIfAbsent method works when the value associated with specified key is not available or null and in this case the computeIfAbsent method put the new value for that key computed by given mapping function. Find the method declaration from Java doc.
How does John compute the value?from baeldung.com
Since the key “John” is not present, it computes the value by passing the key as a parameter to the mappingFunction.
What happens if the key is already associated with a value and the new value calculated by mapping function is not nu?from concretepage.com
If the specified key is already associated with a value and new value calculated by mapping function is not null, in this case the computeIfAbsent method will not put the new value for the specified key.
What is key -> 280?from programiz.com
key -> 280 is a lambda expression. It returns the value 280. To learn more about the lambda expression , visit Java Lambda Expressions.
Does computeIfAbsent compute the new value for shoes?from programiz.com
In the above example, the mapping for Shoes is already present in the hashmap. Hence, the computeIfAbsent () method does not compute the new value for Shoes.
What is computeifabsent in Java?from concretepage.com
The computeIfAbsent is the default method of java.util.Map and has been introduced in Java 8. The computeIfAbsent method works when the value associated with specified key is not available or null and in this case the computeIfAbsent method put the new value for that key computed by given mapping function. Find the method declaration from Java doc.
How does John compute the value?from baeldung.com
Since the key “John” is not present, it computes the value by passing the key as a parameter to the mappingFunction.
What happens if the key is already associated with a value and the new value calculated by mapping function is not nu?from concretepage.com
If the specified key is already associated with a value and new value calculated by mapping function is not null, in this case the computeIfAbsent method will not put the new value for the specified key.
What is key -> 280?from programiz.com
key -> 280 is a lambda expression. It returns the value 280. To learn more about the lambda expression , visit Java Lambda Expressions.
Does computeIfAbsent compute the new value for shoes?from programiz.com
In the above example, the mapping for Shoes is already present in the hashmap. Hence, the computeIfAbsent () method does not compute the new value for Shoes.
Example 1 – Computeifabsent() – Key Is Absent
- In this example, we will initialize a HashMap with mappings from String to Integer. Using computeIfAbsent() function, we will compute a value for key "M". Since the key "M"is not already present in the HashMap, this new mapping will be inserted to HashMap. Java Program Output
Example 2 – Computeifabsent() – Key Is Present Already
- In this example, we will initialize a HashMap with mappings from String to Integer. Using computeIfAbsent() function, we will compute a value for key "C". Since the key "C"is already present in the HashMap, computeIfAbsent() does nothing, and the HashMap is unaffected. Java Program Output
Example 3 – Computeifabsent() – Return Value
- In this example, we will store the value returned by computeIfAbsent(), and print to console. The datatype of return value is the datatype of values of mappings in the HashMap. In this example, the datatype of the the return value is Integer. Java Program Output
Example 4 – computeIfAbsent
- In this example, we will initialize a HashMap with mappings from String to Integer. Using computeIfAbsent() function, we will compute a value for key "M" and provide a null value for the mappingFunctionparameter. computeIfAbsent() throws java.lang.NullPointerException. Java Program Output
Conclusion
- In this Java Tutorial, we have learnt the syntax of Java HashMap.computeIfAbsent() function, and also learnt how to use this function with the help of examples.