Collections.EMPTY_LIST 和 Collections.emptyList()的区别

Collections中的EMPTY_LIST 和 emptyList 其实通过源码查看，他们最终都是同一个final的不变的实例对象

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

public static final <T> List<T> emptyList() {

    return (List<T>) EMPTY_LIST;
}

在Collections中自定义了一个内部类EmptyList，该内部类继承了AbstractList的抽象类，拥有了list的数据结构

并重写了get方法

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;
    }

    //重写该get方法，禁止该方法去获取对象，使用时抛出下标越界
    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; }

    @Override
    public boolean removeIf(Predicate<? super E> filter) {
        Objects.requireNonNull(filter);
        return false;
    }
    @Override
    public void replaceAll(UnaryOperator<E> operator) {
        Objects.requireNonNull(operator);
    }
    @Override
    public void sort(Comparator<? super E> c) {
    }

    // Override default methods in Collection
    @Override
    public void forEach(Consumer<? super E> action) {
        Objects.requireNonNull(action);
    }

    @Override
    public Spliterator<E> spliterator() { return Spliterators.emptySpliterator(); }

    // Preserves singleton property
    private Object readResolve() {
        return EMPTY_LIST;
    }
}