Unlocking the Power of Java’s HashMap Merge Method
When working with Java’s HashMap, merging data from multiple sources can be a daunting task. However, with the merge() method, you can effortlessly combine data while handling duplicate keys with ease.
Understanding the Merge Method
The merge() method takes three parameters: key, value, and remappingFunction. It allows you to associate a value with a key, and if the key already exists, it applies the remappingFunction to determine the new value. The method returns the new value associated with the key, or null if no value is associated.
Inserting New Entries
In our first example, we create a HashMap named prices and use the merge() method to insert a new entry. The lambda expression (oldValue, newValue) -> oldValue + newValue serves as the remapping function. Since the key “Shirt” is not present, the method inserts the mapping “Shirt=100”.
Handling Duplicate Keys
In our second example, we create a HashMap named countries and use the merge() method to insert an entry with a duplicate key. The lambda expression (oldValue, newValue) -> oldValue + "/" + newValue serves as the remapping function. Since the key “Washington” is already present, the old value is replaced by the value returned by the remapping function, resulting in the mapping “Washington=America/USA”.
Merging Two HashMaps
In our third example, we create two HashMaps named prices1 and prices2 and use the merge() method to merge them. We utilize two lambda expressions: (key, value) -> prices.merge(...) accesses each entry of prices2 and passes it to the merge() method, while (oldValue, newValue) -> {...} serves as the remapping function. Since the key “Shoes” is present in both maps, the value is replaced by the result of the remapping function.
HashMap Merge vs. PutAll
While the putAll() method can also be used to merge two HashMaps, it lacks the flexibility of the merge() method. With putAll(), if a key is present in both maps, the old value is replaced by the new value without allowing for custom remapping. In contrast, the merge() method provides a remapping function, giving you control over how duplicate keys are handled.
By mastering the merge() method, you can efficiently combine data from multiple sources and handle duplicate keys with precision, making your Java applications more robust and efficient.