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方法注释

/**     * Returns an immutable list containing only the specified object.     * The returned list is serializable.     *     * @param  
   
     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     */
   

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。

//SingletonList类的源码    private static class SingletonList
   
            extends AbstractList
    
             implements RandomAccess, Serializable {        private static final long serialVersionUID = 3093736618740652951L;        private final E element;        SingletonList(E obj)                {element = obj;}        public Iterator
     
       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
       action) {            action.accept(element);        }        @Override        public boolean removeIf(Predicate
       filter) {            throw new UnsupportedOperationException();        }        @Override        public void replaceAll(UnaryOperator
      
        operator) {            throw new UnsupportedOperationException();        }        @Override        public void sort(Comparator
        c) {        }        @Override        public Spliterator
       
         spliterator() {            return singletonSpliterator(element);        }    }
       
      
     
    
   

//普通写法    List
   
     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
   
     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 
   
     Set
    
      singleton(T o);public static 
     
       List
      
        singletonList(T o);public static 
       
         Map
        
          singletonMap(K key, V value); // 或者直接调用常量 EMPTY_LISTpublic static final 
         
           List
          
            emptyList();//或者直接调用常量 EMPTY_MAPpublic static final 
           
             Map
            
              emptyMap();//或者直接调用常量 EMPTY_SETpublic static final 
             
               Set
              
                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

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