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Sep 6, 2009 · If you build the map in your own code, try putting the key and value in the map together: public class KeyValue { public Object key; public Object value; public KeyValue(Object key, Object value) { ... } } map.put(key, new KeyValue(key, value)); Then when you have a value, you also have the key.
- Introduction
- An Iterative Approach
- A Functional Approach
- Using Apache Commons Collections
- Using Google Guava
- Conclusion
In this quick tutorial, we’re going to demonstrate three different approaches for retrieving the key from a map for a given value. We’ll also discuss the positives and negatives of the various solutions. To learn more about the Map interface, you can check out this article.
The Map interface of Java Collections offers a method called entrySet(). It returns all the entries or key-value pairs of the map in a Set. The idea is to iterate over this entry-set and return the key for which the value matches the supplied value: However, there might be a possibility that multiple keys are pointing to the same value. In that cas...
With the introduction of Lambda Expressions in Java 8, we can do it in a more flexible and readable way. We convert the entry-set to a Streamand supply a lambda to filter only those entries with the given value. Then we use the map method to return a Streamof the keys from the filtered entries: The advantage of returning a stream is that it can cat...
The above ideas wouldn’t be very helpful if we need to call the functions very frequently for a particular map. It will unnecessarily iterate the set of its keys again and again. In this scenario, maintaining another map of value to the keys would make more sense as it will take constant time to retrieve the key for a value. The Commons Collections...
We may use another bi-directional Map called BiMap found in Guava developed by Google. This class provides a method named inverse() to get the value-key Map or the reverse Mapto fetch the key based on a given value: Like BidiMap, BiMap also doesn’t allow multiple keys referring to the same value. If we try to make such an attempt, it throws a java....
In this brief article, we’ve discussed some methods of retrieving a Map’skey given the value. Each approach has its own pros and cons. We should always consider the use-cases and choose the most appropriate one based on the situation. The complete source code for the above tutorial is available over on GitHub.
May 7, 2023 · The get() method of Map interface in Java 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.
Feb 9, 2024 · In this article, we will learn how to retrieve a specific value from a HashMap using its key in Java. Syntax: HasMap.get(object key_element) Program to Retrieve a Specific Value From a HashMap Using Key in Java. Below is the implementation of retrieving a specific value from a HashMap using its key: Java
Nov 25, 2024 · In Java, Map Interface is present in java.util package represents a mapping between a key and a value. Java Map interface is not a subtype of the Collection interface. Therefore it behaves a bit differently from the rest of the collection types. A map contains unique keys.
The Map defines a member interface: Map.Entry to model a key-value pair. This interface defines three methods to access the key and the values: getKey(): to read the key; getValue() and setValue(value): to read and update the value bound to that key. The Map.Entry objects you can get from a given map are views on the content of the map ...
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May 11, 2024 · A map is a key-value mapping, which means that every key is mapped to exactly one value and that we can use the key to retrieve the corresponding value from a map. One might ask why not simply add the value to a list.
