Method returning collection/array type returns null instead JAVA-W1066

null should not be returned from methods that are supposed to return collection instances or arrays.

Returning null from such methods results in uglier code because now every caller has to check if the method has returned null. If the caller fails to do this, there's a chance that executing such code will result in a NullPointerException.

When dealing with collections or arrays, the absence of values can usually be communicated by returning an empty instance instead of null. This is much more convenient because this rids the caller from the responsibility of checking for null every time this method is called.

Bad Practice

public List<Integer> method() {
    // ... method body

    if (/* some condition */)
        return null;
}

public int[] m2() {
    // ... method body

    if (/* some condition */)
        return null;
}

Recommended

Consider returning empty collection instances or arrays instead of null.

public List<Integer> method() {
    // ... method body

    if (/* some condition */)
        return Collections.emptyList();
}

public int[] m2() {
    // ... method body

    if (/* some condition */)
        return new int[];
}

References

• StackOverflow - Is it better to return null or empty collection?
• CMU - Return an empty array or collection instead of null