Collections.singletonList() 源码与用法

Collections.singletonList() 源码与用法

方法注释： 

 /**
     * Returns an immutable list containing only the specified object.
     * The returned list is serializable.
     *
     * @param  <T> the class of the objects in the list
     * @param o the sole object to be stored in the returned list.
     * @return an immutable list containing only the specified object.
     * @since 1.3
     */

这个方法主要用于只有一个元素的优化，减少内存分配，无需分配额外的内存，可以从SingletonList内部类看得出来,由于只有一个element,因此可以做到内存分配最小化，相比之下ArrayList的DEFAULT_CAPACITY=10个。

//SingletonList类的源码
    private static class SingletonList<E>
        extends AbstractList<E>
        implements RandomAccess, Serializable {

        private static final long serialVersionUID = 3093736618740652951L;

        private final E element;

        SingletonList(E obj)                {element = obj;}

        public Iterator<E> iterator() {
            return singletonIterator(element);
        }

        public int size()                   {return 1;}

        public boolean contains(Object obj) {return eq(obj, element);}

        public E get(int index) {
            if (index != 0)
              throw new IndexOutOfBoundsException("Index: "+index+", Size: 1");
            return element;
        }

        // Override default methods for Collection
        @Override
        public void forEach(Consumer<? super E> action) {
            action.accept(element);
        }
        @Override
        public boolean removeIf(Predicate<? super E> filter) {
            throw new UnsupportedOperationException();
        }
        @Override
        public void replaceAll(UnaryOperator<E> operator) {
            throw new UnsupportedOperationException();
        }
        @Override
        public void sort(Comparator<? super E> c) {
        }
        @Override
        public Spliterator<E> spliterator() {
            return singletonSpliterator(element);
        }
    }

//普通写法
    List<MyBean> beans= MyService.getInstance().queryBean(param);
    if (CollectionUtils.isEmpty(beans)) {
      beans= new ArrayList<>();
      MyBean bean= new MyBean(param);
      beans.add(bean);
    }

//优化写法
    List<MyBean> beans= MyService.getInstance().queryBean(param);
    if (CollectionUtils.isEmpty(beans)) {
      MyBean bean= new MyBean(param);
      beans= Collections.singletonList(bean);
    }

其他特殊容器类

public static <T> Set<T> singleton(T o);

public static <T> List<T> singletonList(T o);

public static <K,V> Map<K,V> singletonMap(K key, V value);
 
// 或者直接调用常量 EMPTY_LIST
public static final <T> List<T> emptyList();

//或者直接调用常量 EMPTY_MAP
public static final <K,V> Map<K,V> emptyMap();

//或者直接调用常量 EMPTY_SET
public static final <T> Set<T> emptySet()

需要注意的是，以上6个方法返回的容器类均是immutable，即只读的，如果调用修改接口，将会抛出UnsupportedOperationException