Collections.singletonList使用方法

方法注释


应用：
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。

rivate 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<My> beans= MyServiceImpl.queryBean(param);
    if (CollectionUtils.isEmpty(beans)) {
      beans= new ArrayList<>();
      My bean= new My(param);
      beans.add(bean);
    }

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

其他特殊容器类

private static class SingletonSet<E>
        extends AbstractSet<E>
        implements Serializable
    {
        private static final long serialVersionUID = 3193687207550431679L;

        private final E element;

        SingletonSet(E e) {element = e;}

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

        public int size() {return 1;}

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

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

    private static class SingletonMap<K,V>
          extends AbstractMap<K,V>
          implements Serializable {
        private static final long serialVersionUID = -6979724477215052911L;

        private final K k;
        private final V v;

        SingletonMap(K key, V value) {
            k = key;
            v = value;
        }

        public int size()                                           {return 1;}
        public boolean isEmpty()                                {return false;}
        public boolean containsKey(Object key)             {return eq(key, k);}
        public boolean containsValue(Object value)       {return eq(value, v);}
        public V get(Object key)              {return (eq(key, k) ? v : null);}

        private transient Set<K> keySet;
        private transient Set<Map.Entry<K,V>> entrySet;
        private transient Collection<V> values;

        public Set<K> keySet() {
            if (keySet==null)
                keySet = singleton(k);
            return keySet;
        }

        public Set<Map.Entry<K,V>> entrySet() {
            if (entrySet==null)
                entrySet = Collections.<Map.Entry<K,V>>singleton(
                    new SimpleImmutableEntry<>(k, v));
            return entrySet;
        }

        public Collection<V> values() {
            if (values==null)
                values = singleton(v);
            return values;
        }

        // Override default methods in Map
        @Override
        public V getOrDefault(Object key, V defaultValue) {
            return eq(key, k) ? v : defaultValue;
        }

        @Override
        public void forEach(BiConsumer<? super K, ? super V> action) {
            action.accept(k, v);
        }

        @Override
        public void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) {
            throw new UnsupportedOperationException();
        }

        @Override
        public V putIfAbsent(K key, V value) {
            throw new UnsupportedOperationException();
        }

        @Override
        public boolean remove(Object key, Object value) {
            throw new UnsupportedOperationException();
        }

        @Override
        public boolean replace(K key, V oldValue, V newValue) {
            throw new UnsupportedOperationException();
        }

        @Override
        public V replace(K key, V value) {
            throw new UnsupportedOperationException();
        }

        @Override
        public V computeIfAbsent(K key,
                Function<? super K, ? extends V> mappingFunction) {
            throw new UnsupportedOperationException();
        }

        @Override
        public V computeIfPresent(K key,
                BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
            throw new UnsupportedOperationException();
        }

        @Override
        public V compute(K key,
                BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
            throw new UnsupportedOperationException();
        }

        @Override
        public V merge(K key, V value,
                BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
            throw new UnsupportedOperationException();
        }
    }

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(注意看一开始的returns)。

借鉴：https://www.cnblogs.com/oreo/p/9761940.html