In this article, we will discuss how to convert a modifiable HashMap entries into an unmodifiable HashMap entries using Collections.unmodifiableMap(); method
1. Convert HashMap to Read-only:
- HashMap is an implementation-class of Map interface which grows/shrink in size dynamically whenever entries is
- added to a HashMap using put() method
- deleted from existing HashMap using remove() method
- updated/modified from existing HashMap using replace() method
- For some business purpose/requirement, HashMap needs to be made read-only or immutable or unmodifiable
- To achieve this, use one of the utility method called unmodifiableMap() from Collections class
- Syntax : Collections.unmodifiableMap(modifiableMap); i.e.; pass modifiable map as input-argument
- Trying to add/remove/modify an unmodifiable Map throws UnsupportedOperationException as shown in the below example
- Note: however, adding/deleting/updating Map entries in the original HashMap is still possible
ConvertHashMapToReadOnly.java
package net.bench.resources.unmodifiable.collection;
import java.util.Collections;
import java.util.HashMap;
import java.util.Map;
public class ConvertHashMapToReadOnly {
public static void main(String[] args) {
// 1. HashMap
Map<String, String> map = new HashMap<>();
// 1.1 add Map entries
map.put("America", "Biden");
map.put("Russia", "Putin");
map.put("India", "Modi");
map.put("China", "Xi");
// 1.2 iterating/printing original List
System.out.println("1. Orginal Map entries : \n");
map.forEach((key, value) -> System.out.println(key + "\t" + value));
// 2. convert modifiable map to immutable map
Map<String, String> unmodifiableMap = Collections
.unmodifiableMap(map);
// 2.1 iterating/printing original Map
System.out.println("\n\n2. Read-only Map : \n");
unmodifiableMap.forEach((key, value) -> System.out.println(key + "\t" + value));
// 3. trying to modify unmodifiable map
System.out.println("\n\n3. Trying to modify unmodifiable map : \n");
try {
unmodifiableMap.put("British", "Boris");
}
catch(UnsupportedOperationException usopex) {
System.out.println("In Exception block : ");
usopex.printStackTrace();
}
}
}
Output:
1. Orginal Map entries :
China Xi
America Biden
Russia Putin
India Modi
2. Read-only Map :
China Xi
America Biden
Russia Putin
India Modi
3. Trying to modify unmodifiable map :
In Exception block :
java.lang.UnsupportedOperationException
at java.util.Collections$UnmodifiableMap.put(Collections.java:1457)
at net.bench.resources.unmodifiable.collection.ConvertHashMapToReadOnly
.main(ConvertHashMapToReadOnly.java:40)
Related Articles:
- Java – How to make List or ArrayList unmodifiable List or read-only ?
- Java – How to make Set or HashSet unmodifiable Set or read-only ?
- Java – How to make Map or HashMap unmodifiable Map or read-only ?
- Java – How to make any Collection class as unmodifiable Collection or read-only ?
References :
- https://docs.oracle.com/javase/8/docs/api/java/util/Collections.html#unmodifiableMap-java.util.Map-
- https://docs.oracle.com/javase/8/docs/api/java/util/Collection.html
- https://docs.oracle.com/javase/8/docs/api/java/util/Map.html
- https://docs.oracle.com/javase/8/docs/api/java/util/HashMap.html
Happy Coding !!
Happy Learning !!
