Since the map created by this method is a view of the original map, modifying the original will reflect the changes in it:
package com.logicbig.example.collections;
import java.util.Collections;
import java.util.HashMap;
import java.util.Map;
public class UnmodifiableMapExample {
public static void main(String... args) {
Map<Integer, String> map = new HashMap<>();
map.put(1, "one");
map.put(2, "two");
System.out.println("Original Map: " + map);
Map<Integer, String> map2 = Collections.unmodifiableMap(map);
System.out.println("unmodifiableMap: " + map2);
//modifying the original
map.put(3, "three");
map.put(4, "four");
map.remove(1);
System.out.println("unmodifiableMap: " + map2);
}
}
Output
Original Map: {1=one, 2=two}
unmodifiableMap: {1=one, 2=two}
unmodifiableMap: {2=two, 3=three, 4=four}
Modifying itself will throw the exception:
package com.logicbig.example.collections;
import java.util.Collections;
import java.util.HashMap;
import java.util.Map;
public class UnmodifiableMapExample2 {
public static void main(String... args) {
Map<Integer, String> map = new HashMap<>();
map.put(1, "one");
map.put(2, "two");
Map<Integer, String> map2 = Collections.unmodifiableMap(map);
map2.put(3, "three");
}
}
Output
Caused by: java.lang.UnsupportedOperationException
at java.base/java.util.Collections$UnmodifiableMap.put(Collections.java:1453)
at com.logicbig.example.collections.UnmodifiableMapExample2.main(UnmodifiableMapExample2.java:20)
... 6 more