Best practice to validate null and empty collection in Java

249
votes

I want to verify whether a collection is empty and null. Could anyone please let me know the best practice.

Currently, I am checking as below:

if (null == sampleMap || sampleMap.isEmpty()) {
  // do something
} 
else {
  // do something else
}
9
javacollections
Aside from anything else, think about why you use null == sampleMap rather than sampleMap == null. Most people find the latter more readable - the former is a holdover from other languages. - Jon Skeet
By the way, null collection is bad. If possible, make it empty collection instead. See Effective Java: Item 43 - Return empty arrays or collections, not nulls. - 卢声远 Shengyuan Lu
@JonSkeet people use null == sampleMap in case they write = instead of ==. if you use sampleMap == null, when you forget one =, it becomes sampleMap = null, which will not raise error thus writing it the other way helps the developer see it - Uri Loya
@UriLoya: If you write if (null = sampleMap) in Java you'll get a compilation error. That's precisely the point of my first comment. The "reason" for it is language-specific, but has been propagated to other languages despite the readability penalty because people haven't considered why they're doing that. - Jon Skeet
You can use CollectionUtils class which is present in org.apache.commons.collections4.CollectionUtils package. There have many utility methods to find empty or null. - vikash

9 Answers

380
votes

If you use the Apache Commons Collections library in your project, you may use the CollectionUtils.isEmpty and MapUtils.isEmpty() methods which respectively check if a collection or a map is empty or null (i.e. they are "null-safe").

The code behind these methods is more or less what user @icza has written in his answer.

Regardless of what you do, remember that the less code you write, the less code you need to test as the complexity of your code decreases.

79
votes

That is the best way to check it. You could write a helper method to do it:

public static boolean isNullOrEmpty( final Collection< ? > c ) {
    return c == null || c.isEmpty();
}

public static boolean isNullOrEmpty( final Map< ?, ? > m ) {
    return m == null || m.isEmpty();
}
43
votes

If you use Spring frameworks, then you can use CollectionUtils to check against both Collections (List, Array) and Map etc.

if(CollectionUtils.isEmpty(...)) {...}
21
votes

Personally, I prefer to use empty collections instead of null and have the algorithms work in a way that for the algorithm it does not matter if the collection is empty or not.

12
votes

When you use spring then you can use

boolean isNullOrEmpty = org.springframework.util.ObjectUtils.isEmpty(obj);

where obj is any [map,collection,array,aything...]

otherwise: the code is:

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

public static boolean isEmpty(Object obj) {
    if (obj == null) {
        return true;
    }

    if (obj.getClass().isArray()) {
        return Array.getLength(obj) == 0;
    }
    if (obj instanceof CharSequence) {
        return ((CharSequence) obj).length() == 0;
    }
    if (obj instanceof Collection) {
        return ((Collection) obj).isEmpty();
    }
    if (obj instanceof Map) {
        return ((Map) obj).isEmpty();
    }

    // else
    return false;
}

for String best is:

boolean isNullOrEmpty = (str==null || str.trim().isEmpty());
5
votes

We'll check a Collection object is empty, null or not. these all methods which are given below, are present in org.apache.commons.collections4.CollectionUtils package.

Check on List or set type of collection Objects.

CollectionUtils.isEmpty(listObject);
CollectionUtils.isNotEmpty(listObject);

Check on Map type of Objects.

MapUtils.isEmpty(mapObject);
MapUtils.isNotEmpty(mapObject);

The return type of all methods is boolean.

3
votes

If you need to check for null, that is the way. However, if you have control on this, just return empty collection, whenever you can, and check only for empty later on.

This thread is about the same thing with C#, but the principles applies equally well to java. Like mentioned there, null should be returned only if

  • null might mean something more specific;
  • your API (contract) might force you to return null.
3
votes

You can use org.apache.commons.lang.Validate's "notEmpty" method:

Validate.notEmpty(myCollection) -> Validate that the specified argument collection is neither null nor a size of zero (no elements); otherwise throwing an exception.

-2
votes

For all the collections including map use: isEmpty method which is there on these collection objects. But you have to do a null check before:

Map<String, String> map;

........
if(map!=null && !map.isEmpty())
......