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@Test
public void changingImmutableMapCausesException() {
    var messages = Map.of("message", "not nice");
    assertThrows(UnsupportedOperationException.class,
            () -> messages.put("message", "nice")
    );
}

A modification of an umodifiable Map causes:

java.lang.UnsupportedOperationException
    at java.base/java.util.ImmutableCollections.uoe(ImmutableCollections.java:142)
    at java.base/java.util.ImmutableCollections$AbstractImmutableMap.put(ImmutableCollections.java:1072)    

To modify the Map, create a new Map, then modify (add, remove, change):

@Test
public void copyMapAndModify() {
    var messages = Map.of("message", "not nice");
    var copiedMessages = new HashMap<String,String>(messages);
    copiedMessages.put("message", "nice");
    assertEquals(copiedMessages.get("message"),"nice");
}