Java Collections的emptyList、EMPTY_LIST详解与使用说明

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return Collections.EMPTY_LIST;

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public void add(int index, E element) {
        throw new UnsupportedOperationException();
    }

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public E get(int index) {
            throw new IndexOutOfBoundsException("Index: "+index);
        }

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List<String> list1 = Collections.emptyList();
        for(String s:list1) {
        }
        for(int i=0;i<list1.size();i++) {
        }

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public int size() {return 0;}

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/**
     * The empty list (immutable).  This list is serializable.
     *
     * @see #emptyList()
     */
    @SuppressWarnings("unchecked")
    public static final List EMPTY_LIST = new EmptyList<>();

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/**
     * 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
     */
    @SuppressWarnings("unchecked")
    public static final <T> List<T> emptyList() {
        return (List<T>) EMPTY_LIST;
    }

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public void run() {
    ......
    List list = buildList(param);
    ......
    Object newNode = getNode(...);
    list.add(newNode);
    ......
}
 
public List buildList(Object param) {
    if (isInValid(param)) {
        return Collections.emptyList();
    } else {
        ......
    }
}

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public static final <T> List<T> emptyList() {
    return (List<T>) EMPTY_LIST;
}

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public static final List EMPTY_LIST = new EmptyList<>();

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private static class EmptyList<E>
        extends AbstractList<E>
        implements RandomAccess, Serializable {
        private static final long serialVersionUID = 8842843931221139166L; 
        public Iterator<E> iterator() {
            return emptyIterator();
        }
        public ListIterator<E> listIterator() {
            return emptyListIterator();
        }
 
        public int size() {return 0;}
        public boolean isEmpty() {return true;} 
        public boolean contains(Object obj) {return false;}
        public boolean containsAll(Collection<?> c) { return c.isEmpty(); } 
        public Object[] toArray() { return new Object[0]; } 
        public <T> T[] toArray(T[] a) {
            if (a.length > 0)
                a[0] = null;
            return a;
        }
 
        public E get(int index) {
            throw new IndexOutOfBoundsException("Index: "+index);
        }
 
        public boolean equals(Object o) {
            return (o instanceof List) && ((List<?>)o).isEmpty();
        }
 
        public int hashCode() { return 1; }
 
        // Preserves singleton property
        private Object readResolve() {
            return EMPTY_LIST;
        }
    }

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public boolean add(E e) {
    add(size(), e);
    return true;
}
 
public void add(int index, E element) {
    throw new UnsupportedOperationException();
}