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:

“`java
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:

“`text
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:

“`java
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());
}
“`

The `Map.entry()` is another factory method provided to create `Map.Entry` object.