Collections. unmodifiablecollection

Collections. unmodifiablecollection this can get an image of a collection, and its return results cannot be changed directly, otherwise it will prompt

java.lang.UnsupportedOperationException      at java.util.Collections$UnmodifiableCollection.add(Collections.java:1018)

To prevent data changes. In addition, when you modify the original collections, the corresponding image will be modified at the same time.

public class CollectionsTest {            @Test      public void test(){          Collection<String> c = new ArrayList<String>();                    Collection<String> s = Collections.unmodifiableCollection(c);                    c.add("str");                    System.out.println(s);      }    }

In this case, the C set is modified, and the s set is also changed.

Collections. unmodifiablecollection