java 如何检查 Iterator.next() 是否为 == null？
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How to check if Iterator.next() is == null?
提问by Lazlow
How do I check if the next element in the list is null ?
如何检查列表中的下一个元素是否为 null ？
while(it.hasNext()){
System.out.println(it.next()+"\n");
}
this is how I tried, but when the last element is null it prints it as null.
这就是我尝试的方式，但是当最后一个元素为空时，它会将其打印为空。
I tried to change it
我试图改变它
while(it.hasNext()){
if(it.next()==null){
}else
System.out.println(it.next()+"\n");
}
but this just makes it worst because some of the elements don't even print!
但这只会让情况变得更糟，因为有些元素甚至无法打印！
This is my Iteration method/anonymous class
这是我的迭代方法/匿名类
public Iterator<Filmi> iterator3DFilms ()throws FilmiException{
if(filmList.isEmpty())
throw new FilmiException("No Films on the list");
return new Iterator<Filmi>(){
private int index=0;
public boolean hasNext(){
return index <filmList.size();
}
public Filmi next(){
Filmi lb = filmList.get(index++);
if(lb.is3D()== true)
return lb;
if(hasNext())
return next();
return null;
}
public void remove(){}
};
}
The null print only happens at the last element Thank you.
空打印只发生在最后一个元素谢谢。
回答by Bathsheba
Naturally, code like
自然地，代码像
if (it.next() == null){
} else {
System.out.println(it.next()+"\n");
}
will consume every other non-nullelement, as you are observing. Plus calling it.next()without checking it.hasNext()is a recipe for disaster.
null正如您所观察到的那样，将消耗所有其他非元素。加上it.next()不检查it.hasNext()就打电话是灾难的秘诀。
Why not write
为什么不写
Foo/*ToDo - use the correct type here*/ foo = it.next()
if (foo != null){
/*ToDo*/
}
instead?
反而？
回答by Nicolas Filotto
No it cannot work this way because if it.next()is not nullyou call it.next()twice which will make you skip a value that could not even be available.
不，它不能以这种方式工作，因为如果it.next()不是，null您将调用it.next()两次，这将使您跳过一个甚至不可用的值。
Use a variable instead as next:
使用变量代替如下：
Object o = it.next();
if (o != null) {
...
}
回答by sab
you should use stream instead of iterator.
您应该使用流而不是迭代器。
filmList.stream().filter(film->film!=null).filter(film->film.is3D())
Edit: or, if you'r not in Java 8 :
编辑：或者，如果您不在 Java 8 中：
Predicate<Film> isNotNullAnd3D = new Predicate<Person>() {
public boolean apply(Film f) {
return f != null && f.is3D();
}
};
Collection2.filter(filmList, isNotNullAnd3D)
回答by Ivan Matavulj
You never mentioned why you use iterators explicitly in the first place. Why not use implicit iterator notation like this ? :
您一开始从未提到为什么要明确使用迭代器。为什么不使用这样的隐式迭代器符号？：
for (Film film : filmList) {
if (film != null ){
....
}
}
