In Java, the Map.of() factory method can be used to create an unmodifiable map of specified key-value pairs. This method is available in Java 9 and later versions.
Creating a map is a bit more complicated than creating lists or sets. Because we need to provide keys and values when creating a map. When using the Map.of() factory method we set the content of the map by alternating between the keys and values of the map.
Consider the following example:
package org.kodejava.util;
import java.util.Map;
public class MapOfExample {
public static void main(String[] args) {
Map<String, Integer> map = Map.of("John", 25, "Mary", 30, "Alice", 27, "Rosa", 22);
for (Map.Entry<String, Integer> entry : map.entrySet()) {
System.out.println(entry.getKey() + " : " + entry.getValue());
}
}
}
Output:
Rosa : 22
Mary : 30
John : 25
Alice : 27
In the example above, the Map.of("John", 25, "Mary", 30, "Alice", 27, "Rosa", 22) statement creates an unmodifiable map with three key-value pairs. After the map is created, any attempt to modify the map (add, update or remove elements) will throw an UnsupportedOperationException.
Note that Map.of() doesn’t accept null keys or values. If a null key or value is provided, then a NullPointerException is thrown. Besides, if duplicate keys are provided, an IllegalArgumentException is thrown.
The Map.of() method is overloaded to accept up to 10 key-value pairs. If there are more than 10 pairs, you can use Map.ofEntries() factory method to create a map. This is how we use it:
Map<String, Integer> map = Map.ofEntries(
Map.entry("John", 25),
Map.entry("Mary", 30),
Map.entry("Alice", 27),
Map.entry("Bob", 32),
// ...
);
for (Map.Entry<String, Integer> entry : map.entrySet()) {
System.out.println(entry.getKey() + " : " + entry.getValue());
}
Map.entry() is another factory method provided to create Map.Entry object.
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