Get to know java-15.2 collection's regular usage from scratch (2)-Watch out.

In the previous section we talked about the regular usage of collection. And the previous chapters, we introduced the AddAll method of collection, which we introduce to the attention point of this chapter.

Note that, in the regular usage, the basic is optional.

What is an optional operation?

Unfortunately, the method of operation is not set for all implementations. But only for the realization of a certain class set.

Like what:

Package Com.ray.ch15;import Java.util.arrays;import Java.util.collection;import Java.util.list;public class Test { public static void Test (list<string> List, String msg) {System.out.println ("---------" + msg + "--------"); list<string> sublist = list.sublist (1, 4); Collection<string> Collection = list;try {collection.retainall (sublist);} catch (Exception e) { System.out.println ("Retainall:" + E);}} public static void Main (string[] args) {list<string> list = Arrays.aslist ("1", "2", "3", "4", "5", "6"), Test (list, " Start1 ");}}

Output:

---------Start1--------
RetainAll:java.lang.UnsupportedOperationException

Why does it throw an exception?

Because Arrays.aslist is the lowest-level data structure of this method is an array, the array in Java is not modified. When Retainall this method to change the data structure inside the array, it is not agreed.

Assuming we output a bit more exception information above, we will locate the Abstractlist remove method. When it is called. Exceptions that are not supported will be thrown.

Public E Remove (int index) {throw new unsupportedoperationexception ();    }

Summary: This chapter focuses on the optional operation of collection's regular usage.

This chapter is here, thank you.

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Get to know java-15.2 collection's regular usage from scratch (2)-Watch out.