17.4 可选操作

未获支持的操作都是由固定尺寸的数据结构支持的容器产生的。
1，Arrays.asList 只支持只读操作
2，Collections包装的容器

public 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<>(subList);
        try { c.retainAll(c2); } catch(Exception e) {
            System.out.println("retainAll(): " + e);
        }
        try { c.removeAll(c2); } catch(Exception e) {
            System.out.println("removeAll(): " + e);
        }
        try { c.clear(); } catch(Exception e) {
            System.out.println("clear(): " + e);
        }
        try { c.add("X"); } catch(Exception e) {
            System.out.println("add(): " + e);
        }
        try { c.addAll(c2); } catch(Exception e) {
            System.out.println("addAll(): " + e);
        }
        try { c.remove("C"); } catch(Exception e) {
            System.out.println("remove(): " + e);
        }
        // The List.set() method modifies the value but
        // doesn't change the size of the data structure:
        try {
            list.set(0, "X");
        } catch(Exception e) {
            System.out.println("List.set(): " + e);
        }
    }
    public static void main(String[] args) {
        List<String> list =  Arrays.asList("A B C D E F G H I J K L".split(" "));
        //经过ArrayList包裹的类 支持全部的操作
        ArrayList<String> arrayList = new ArrayList<>(list);
        //未包装的list 仅支持只读和修改的操作
        //test("Arrays.asList()", list);
        //System.out.println(list);
        List<String> unmodifiableList = Collections.unmodifiableList(new ArrayList<String>(list));
        //仅支持只读操作，修改也不支持
        test("unmodifiableList()", unmodifiableList);
        System.out.println(unmodifiableList);
    }
}