完整代码示例 下面是一个完整的代码示例,展示了如何使用上述步骤来判断集合是否为空或者为 null: publicbooleanisCollectionEmptyOrNull(Collection<?>collection){if(collection==null){// 集合为空returntrue;}if(collection.isEmpty()){// 集合为空returntrue;}else{// 集合不为空returnfalse;}} 1. 2. 3. ...
null判断是判断有没有对list集合分配内存空间,而不是list里面内容是否为空。 比如,new一个user对象,判断user的list内容是否为空,出现异常。这是因为, 使用isEmpty()和size()的前提是,list是一个空集合,而不是null,否则会抛异常。 所有在判断集合不为空的时候常采用: if(list!=null && !list.isEmpty()){un...
public void nullCollectionUnderRawTest() { List<String> list1 = null; if (null == list1) { (">>>lsit1 is null"); } else if (list1.isEmpty()) { (">>>list1 is empty"); } else { (">>>list1 is:{}", list1); } } 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12...
*@returnthe number of elements in this queue*/publicintsize() {intcount = 0;for(Node<E> p = first(); p !=null; p =succ(p))if(p.item !=null)//Collection.size() spec says to max outif(++count ==Integer.MAX_VALUE)break;returncount; } 总结 通过不同源码的对比,isEmpty()方法时...
private static final Object[] EMPTY_ELEMENTDATA = {}; /** * The array buffer into which the elements of the ArrayList are stored. * DEFAULT_CAPACITY when the first element is added. */ private transient Object[] elementData; /**
(); /** * 判断集合是否为空 */ public boolean isEmpty() { return size() == 0; } /** * 查找当前集合中是否存在等价于参数的元素(通过 equals 方法) * 通过迭代器遍历集合并比对元素实现 */ public boolean contains(Object o) { Iterator<E> it = iterator(); if (o==null) { while (it...
return Optional.ofNullable(collection) .map(Collection::stream) .orElseGet(Stream::empty); } Optional.ofNullable(collection)creates anOptionalobject from the passed-in collection. An emptyOptionalobject is created if the collection isnull. map(Collection::stream)extracts the value contained in theOpti...
the number of elements in this collection isEmpty boolean isEmpty() Returnstrueif this collection contains no elements. Returns: trueif this collection contains no elements contains boolean contains(Objecto) Returnstrueif this collection contains the specified element. More formally, returnstrueif and...
Collection.isEmpty()”报错 java.lang.NullPointerException: element cannot be mapped to a null ...
Java8的Optional是不是鸡肋？使用Optional的一个隐含条件是，optional永远不为null。 既然optional可以永远...