Java集合(4)：未获支持的操作及UnsupportedOperationException

执行各种添加和移除的方法在Collection中都是可选操作的，这意味着实现类并不需要为这些方法提供实现。当我们调用这些方法时，将不会执行有意义的行为，而是通常抛出UnsupportedOperationException。

下面例子我们可以看出Collection的各种实现类中：

(1) java.util.ArrayList实现了所有Collection接口。

(2) java.util.Arrays.ArrayList不支持对对象的add，remove，clear，但是支持set修改元素值。这是因为Arrays.asList返回固定尺寸的List。

(3) java.util.Collections.UnmodifiableList产生的是read-only的List，所以不能做任何修改。

 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collection;
 import java.util.Collections;
 import java.util.List;
 
 class Unsupported {
     static void test(String msg, List<String> list) {
         System.out.println("--- " + msg + " ---");
         Collection<String> c = list;
         Collection<String> subList = list.subList(1, 8);
         Collection<String> c2 = new ArrayList<String>(subList);
         try {
             c.retainAll(c2);
             System.out.println("retainAll(): SUCCESS!");
         } catch (Exception e) {
             System.out.println("retainAll(): " + e);
         }
         try {
             c.removeAll(c2);
             System.out.println("removeAll(): SUCCESS!");
         } catch (Exception e) {
             System.out.println("removeAll(): " + e);
         }
         try {
             c.clear();
             System.out.println("clear(): SUCCESS!");
         } catch (Exception e) {
             System.out.println("clear(): " + e);
         }
         try {
             c.add("X");
             System.out.println("add(): SUCCESS!");
         } catch (Exception e) {
             System.out.println("add(): " + e);
         }
         try {
             c.addAll(c2);
             System.out.println("addAll(): SUCCESS!");
         } catch (Exception e) {
             System.out.println("addAll(): " + e);
         }
         try {
             c.remove("C");
             System.out.println("remove(): SUCCESS!");
         } catch (Exception e) {
             System.out.println("remove(): " + e);
         }
         try {
             list.set(0, "X");
             System.out.println("List.set(): SUCCESS!");
         } catch (Exception e) {
             System.out.println("List.set(): " + e);
         }
     }
 }
 
 public class Test6 {
     public static void main(String[] args) {
         List<String> list = Arrays.asList("A B C D E F G H I J K L".split(" "));
         Unsupported.test("Modifiable Copy", new ArrayList<String>(list));
         Unsupported.test("Arrays.asList()", list);
         Unsupported.test("unmodifiableList()", Collections.unmodifiableList(new ArrayList<String>(list)));
         // Output:
         // --- Modifiable Copy ---
         // retainAll(): SUCCESS!
         // removeAll(): SUCCESS!
         // clear(): SUCCESS!
         // add(): SUCCESS!
         // addAll(): SUCCESS!
         // remove(): SUCCESS!
         // List.set(): SUCCESS!
         // --- Arrays.asList() ---
         // retainAll(): java.lang.UnsupportedOperationException
         // removeAll(): java.lang.UnsupportedOperationException
         // clear(): java.lang.UnsupportedOperationException
         // add(): java.lang.UnsupportedOperationException
         // addAll(): java.lang.UnsupportedOperationException
         // remove(): java.lang.UnsupportedOperationException
         // List.set(): SUCCESS!
         // --- unmodifiableList() ---
         // retainAll(): java.lang.UnsupportedOperationException
         // removeAll(): java.lang.UnsupportedOperationException
         // clear(): java.lang.UnsupportedOperationException
         // add(): java.lang.UnsupportedOperationException
         // addAll(): java.lang.UnsupportedOperationException
         // remove(): java.lang.UnsupportedOperationException
         // List.set(): java.lang.UnsupportedOperationException
     }
 }