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。
1 import java.util.ArrayList; 2 import java.util.Arrays; 3 import java.util.Collection; 4 import java.util.Collections; 5 import java.util.List; 6 7 class Unsupported { 8 static void test(String msg, List<String> list) { 9 System.out.println("--- " + msg + " ---"); 10 Collection<String> c = list; 11 Collection<String> subList = list.subList(1, 8); 12 Collection<String> c2 = new ArrayList<String>(subList); 13 try { 14 c.retainAll(c2); 15 System.out.println("retainAll(): SUCCESS!"); 16 } catch (Exception e) { 17 System.out.println("retainAll(): " + e); 18 } 19 try { 20 c.removeAll(c2); 21 System.out.println("removeAll(): SUCCESS!"); 22 } catch (Exception e) { 23 System.out.println("removeAll(): " + e); 24 } 25 try { 26 c.clear(); 27 System.out.println("clear(): SUCCESS!"); 28 } catch (Exception e) { 29 System.out.println("clear(): " + e); 30 } 31 try { 32 c.add("X"); 33 System.out.println("add(): SUCCESS!"); 34 } catch (Exception e) { 35 System.out.println("add(): " + e); 36 } 37 try { 38 c.addAll(c2); 39 System.out.println("addAll(): SUCCESS!"); 40 } catch (Exception e) { 41 System.out.println("addAll(): " + e); 42 } 43 try { 44 c.remove("C"); 45 System.out.println("remove(): SUCCESS!"); 46 } catch (Exception e) { 47 System.out.println("remove(): " + e); 48 } 49 try { 50 list.set(0, "X"); 51 System.out.println("List.set(): SUCCESS!"); 52 } catch (Exception e) { 53 System.out.println("List.set(): " + e); 54 } 55 } 56 } 57 58 public class Test6 { 59 public static void main(String[] args) { 60 List<String> list = Arrays.asList("A B C D E F G H I J K L".split(" ")); 61 Unsupported.test("Modifiable Copy", new ArrayList<String>(list)); 62 Unsupported.test("Arrays.asList()", list); 63 Unsupported.test("unmodifiableList()", Collections.unmodifiableList(new ArrayList<String>(list))); 64 // Output: 65 // --- Modifiable Copy --- 66 // retainAll(): SUCCESS! 67 // removeAll(): SUCCESS! 68 // clear(): SUCCESS! 69 // add(): SUCCESS! 70 // addAll(): SUCCESS! 71 // remove(): SUCCESS! 72 // List.set(): SUCCESS! 73 // --- Arrays.asList() --- 74 // retainAll(): java.lang.UnsupportedOperationException 75 // removeAll(): java.lang.UnsupportedOperationException 76 // clear(): java.lang.UnsupportedOperationException 77 // add(): java.lang.UnsupportedOperationException 78 // addAll(): java.lang.UnsupportedOperationException 79 // remove(): java.lang.UnsupportedOperationException 80 // List.set(): SUCCESS! 81 // --- unmodifiableList() --- 82 // retainAll(): java.lang.UnsupportedOperationException 83 // removeAll(): java.lang.UnsupportedOperationException 84 // clear(): java.lang.UnsupportedOperationException 85 // add(): java.lang.UnsupportedOperationException 86 // addAll(): java.lang.UnsupportedOperationException 87 // remove(): java.lang.UnsupportedOperationException 88 // List.set(): java.lang.UnsupportedOperationException 89 } 90 }
