Every time I have to iterate over a collection I end up checking for null, just before the iteration of the for-each loop starts. Like this:
if( list1 != null ){
for(Object obj : list1){
}
}
Is there a shorter way, so that we can avoid writing the "if" block ? Note: I am using Java 5, and will be stuck with it for sometime.
Another Java 8+ way would be to create a forEach method that does not crash when using a null
value:
public static <T> void forEach(Iterable<T> set, Consumer<T> action) {
if (set != null) {
set.forEach(action);
}
}
The usage of the own defined foreach
is close to the native Java 8 one:
ArrayList<T> list = null;
//java 8+ (will throw a NullPointerException)
list.forEach(item -> doSomething(...) );
//own implementation
forEach(list, item -> doSomething(...) );
How much shorter do you want it to be? It is only an extra 2 lines AND it is clear and concise logic.
I think the more important thing you need to decide is if null
is a valid value or not. If they are not valid, you should write you code to prevent it from happening. Then you would not need this kind of check. If you go get an exception while doing a foreach
loop, that is a sign that there is a bug somewhere else in your code.
1) if list1 is a member of a class, create the list in the constructor so it's there and non-null though empty.
2) for (Object obj : list1 != null ? list1 : new ArrayList())
If possible, you should design your code such that the collections aren't null
in the first place.
null
collections are bad practice (for this reason); you should use empty collections instead. (eg, Collections.emptyList()
)
Alternatively, you could make a wrapper class that implements Iterable
and takes a collections, and handles a null
collection.
You could then write foreach(T obj : new Nullable<T>(list1))
public <T extends Iterable> T nullGuard(T item) {
if (item == null) {
return Collections.EmptyList;
} else {
return item;
}
}
or, if saving lines of text is a priority (it shouldn't be)
public <T extends Iterable> T nullGuard(T item) {
return (item == null) ? Collections.EmptyList : item;
}
would allow you to write
for (Object obj : nullGuard(list)) {
...
}
Of course, this really just moves the complexity elsewhere.
It's already 2017, and you can now use Apache Commons Collections4
The usage:
for(Object obj : CollectionUtils.emptyIfNull(list1)){
// Do your stuff
}
Null check in an enhanced for loop
public static <T> Iterable<T> emptyIfNull(Iterable<T> iterable) {
return iterable == null ? Collections.<T>emptyList() : iterable;
}
Then use:
for (Object object : emptyIfNull(someList)) { ... }
Apache Commons
for (String code: ListUtils.emptyIfNull(codes)) {
}
Google Guava
for (String code: Optional.of(codes).get()) {
}
In Java 8 there is another solution available by using java.util.Optional
and the ifPresent
-method.
Optional.ofNullable(list1).ifPresent(l -> l.forEach(item -> {/* do stuff */}));
So, not a solution for the exact problem but it is a oneliner and possibly more elegant.
Source: Stackoverflow.com