java.util.Collections类—emptyList()方法的使用

   /**
     * The empty list (immutable).  This list is serializable.
     *
     * @see #emptyList()
     */
    public static final List EMPTY_LIST = new EmptyList();
    /**
     * Returns the empty list (immutable).  This list is serializable.
     *
     * <p>This example illustrates the type-safe way to obtain an empty list:
     * <pre>
     *     List&lt;String&gt; s = Collections.emptyList();
     * </pre>
     * Implementation note:  Implementations of this method need not
     * create a separate <tt>List</tt> object for each call.   Using this
     * method is likely to have comparable cost to using the like-named
     * field.  (Unlike this method, the field does not provide type safety.)
     *
     * @see #EMPTY_LIST
     * @since 1.5
     */
    public static final <T> List<T> emptyList() {
    return (List<T>) EMPTY_LIST;
    }
    /**
     * @serial include
     */
    private static class EmptyList extends AbstractList<Object> implements RandomAccess,Serializable {
    // use serialVersionUID from JDK 1.2.2 for interoperability
    private static final long serialVersionUID = 8842843931221139166L;
        public int size() {return 0;}
        public boolean contains(Object obj) {return false;}
        public Object get(int index) {
            throw new IndexOutOfBoundsException("Index: "+index);
        }
        // Preserves singleton property
        private Object readResolve() {
            return EMPTY_LIST;
        }
    }

public class CollectionsTest {
    public static void main(String[] a) {
        List<Integer> list = new ArrayList<Integer>();
        list.add(1);
        list.add(2);
        System.out.println(list);
        list = Collections.emptyList();
        System.out.println(list);
        list.add(3);
    }
}
//执行结果
[1, 2]
Exception in thread "main" java.lang.UnsupportedOperationException
    at java.util.AbstractList.add(AbstractList.java:131)
    at java.util.AbstractList.add(AbstractList.java:91)[]
    at com.jiuqi.pay.importfile.test.CollectionsTest.main(CollectionsTest.java:22)

 @Test
    public void test1() {
        String str = "";
        List<String> list = getList(str);
        System.out.println(list.size());
    }
    private static List<String> getList(String str) {
        if (StringUtils.isBlank(str)) {
            // 使用时不会报空指针
            return Collections.emptyList();
            // 使用null报空指针异常
//            return null; 
        }
        List<String> list = new ArrayList<String>();
        list.add(str);
        return list;
    }

 @Test
    public void test2() {
        String str = "abc";
        List<String> list = Collections.emptyList();
        list.add(str);
        System.out.println(list.size());
    }

 @Test
    public void test2() {
        String str = "abc";
        List<String> list = Collections.emptyList();
        List<String> resultList = new ArrayList<>(list);
        resultList.add(str);
        System.out.println(resultList.size());
        System.out.println(resultList);
    }