增强的 for 循环中的空检查

Question

What is the best way to guard against null in a for loop in Java?

This seems ugly :

if (someList != null) {
    for (Object object : someList) {
        // do whatever
    }
}

Or

if (someList == null) {
    return; // Or throw ex
}
for (Object object : someList) {
    // do whatever
}

There might not be any other way. Should they have put it in the for construct itself, if it is null then don't run the loop?

Answer 1

You should better verify where you get that list from.

An empty list is all you need, because an empty list won't fail.

If you get this list from somewhere else and don't know if it is ok or not you could create a utility method and use it like this:

for( Object o : safe( list ) ) {
   // do whatever 
 }

And of course safe would be:

public static List safe( List other ) {
    return other == null ? Collections.EMPTY_LIST : other;
}

Answer 2

You could potentially write a helper method which returned an empty sequence if you passed in null:

public static <T> Iterable<T> emptyIfNull(Iterable<T> iterable) {
    return iterable == null ? Collections.<T>emptyList() : iterable;
}

Then use:

for (Object object : emptyIfNull(someList)) {
}

I don't think I'd actually do that though - I'd usually use your second form. In particular, the "or throw ex" is important - if it really shouldn't be null, you should definitely throw an exception. You know that something has gone wrong, but you don't know the extent of the damage. Abort early.

Answer 3

It's already 2017, and you can now use Apache Commons Collections4

The usage:

for(Object obj : ListUtils.emptyIfNull(list1)){
    // Do your stuff
}

You can do the same null-safe check to other Collection classes with CollectionUtils.emptyIfNull .

Answer 4

With Java 8 Optional :

for (Object object : Optional.ofNullable(someList).orElse(Collections.emptyList())) {
    // do whatever
}

Answer 5

Use ArrayUtils.nullToEmpty from the commons-lang library for Arrays

for( Object o : ArrayUtils.nullToEmpty(list) ) {
   // do whatever 
}

This functionality exists in the commons-lang library, which is included in most Java projects.

// ArrayUtils.nullToEmpty source code 
public static Object[] nullToEmpty(final Object[] array) {
    if (isEmpty(array)) {
        return EMPTY_OBJECT_ARRAY;
    }
    return array;
}

// ArrayUtils.isEmpty source code
public static boolean isEmpty(final Object[] array) {
    return array == null || array.length == 0;
}

This is the same as the answer given by @OscarRyz, but for the sake of the DRY mantra, I believe it is worth noting. See the commons-lang project page. Here is the nullToEmpty API documentation and source

Maven entry to include commons-lang in your project if it is not already.

<dependency>
    <groupId>org.apache.commons</groupId>
    <artifactId>commons-lang3</artifactId>
    <version>3.4</version>
</dependency>

Unfortunately, commons-lang doesn't provide this functionality for List types. In this case you would have to use a helper method as previously mentioned.

public static <E> List<E> nullToEmpty(List<E> list)
{
    if(list == null || list.isEmpty())
    {
        return Collections.emptyList();
    }
    return list;
}

Answer 6

If you are getting that List from a method call that you implement, then don't return null , return an empty List .

If you can't change the implementation then you are stuck with the null check. If it should't be null , then throw an exception.

I would not go for the helper method that returns an empty list because it may be useful some times but then you would get used to call it in every loop you make possibly hiding some bugs.

Answer 7

I have modified the above answer, so you don't need to cast from Object

public static <T> List<T> safeClient( List<T> other ) {
            return other == null ? Collections.EMPTY_LIST : other;
}

and then simply call the List by

for (MyOwnObject ownObject : safeClient(someList)) {
    // do whatever
}

Explaination: MyOwnObject: If List<Integer> then MyOwnObject will be Integer in this case.

Answer 8

在 for 循环中有效防止空值的另一种方法是使用 Google Guava 的Optional<T>包装您的集合，因为人们希望这样可以明确有效地清空集合的可能性，因为客户端需要检查集合是否与Optional.isPresent() 。

Answer 9

For anyone uninterested in writing their own static null safety method you can use: commons-lang's org.apache.commons.lang.ObjectUtils.defaultIfNull(Object, Object) . For example:

    for (final String item : 
    (List<String>)ObjectUtils.defaultIfNull(items, Collections.emptyList())) { ... }

ObjectUtils.defaultIfNull JavaDoc

Answer 10

Use, CollectionUtils.isEmpty(Collection coll) method which is Null-safe check if the specified collection is empty.

for this import org.apache.commons.collections.CollectionUtils .

Maven dependency

<dependency>
    <groupId>org.apache.commons</groupId>
    <artifactId>commons-collections4</artifactId>
    <version>4.0</version>
</dependency>

Answer 11

The "||" or the "??" comes in handy here

Best choice and IE compatible is the ||

for (Object object : someList || []) {
    // undefined and null gets defaulted to an empty array []
}

Nullish coalescing operator : Not IE compatible

for (Object object : someList ?? []) {
    // undefined and null gets defaulted to an empty array []
}

Answer 12

for (Object object : someList) {

   // do whatever
}  throws the null pointer exception.