Does Java have a wrapper type for Null?

user_925549 0
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I need to distinguish between a property having a null value, and a property not existing at all. I could use a map, but for performance reasons, I am trying to use a fixed size array.

In the array, I could use null to indicate when a property does not exist at all. But, for a property that does exist and is null valued, is there a standard way to represent it in an array?

I thought of keeping a static member, e.g.

class MyClass {

    private static final Object NULL = new Object();   // null wrapper
    
    private Object[] m_arr = new Object[10];

    // 'i' represents the index of a property in the array

    boolean exists(int i) {
        return m_arr[i] != null;
    }

    Object value(int i) {
        if( !exists(i) ) throw new NullPointerException();   // does not exist
        if( m_arr[i] == NULL ) return null;

        // ... handling for other data types ...
    }
}

Another possibility for representing null might be an enum?

class MyClass {
      ...
      enum Holder {
            NULL
      }
      ...
      // to check for a null value use m_arr[i] == Holder.NULL
}
wrapper null java
4 Answers

Use Optional, eg

private Optional<String> myField;

There are three states. Here's how to handle them:

myfield = Optional.of("foo"); // attribute has non-null value
myfield = Optional.empty();   // attribute is present, but null
myfield = null;               // attribute is not present

Jackson (ie Spring boot) json deserialization supports this out of the box, which is very handy for handling PATCH methods that require the distinction between a json key being specified but null and not specified.

user_2561960

Does Java have a wrapper type for Null?

No.

But how I'd solve this problem means you don't need to a wrapper: just maintain a set of the indices which represent "explicit" null values.

class MyClass {
    private Object[] m_arr = new Object[10];
    private Set<Integer> presentButNullIndices = new HashSet<>();

    // 'i' represents the index of a property in the array

    Object value(int i) {
        if (m_arr[i] == null && !presentButNullIndices.contains(i)) {
            throw new NullPointerException();
        }
        // ... handling for other data types ...
    }

    // just an example of how to maintain the set
    void insert(int i, Object value) {
        if (value == null) {
             presentButNullIndices.add(i);
        }
        else {
             presentButNullIndices.remove(i);
        }
        m_arr[i] = value;
    }
}

Worst case, the space complexity doubles, but that's only for clients which make heavy use of null values. contains on a set is O(1)

I'd also consider just prohibiting null values in the first place. Some map implementations do that and I've never found myself in a situation where I wish they didn't.

user_18985630
user_28513110

Apache commons ObjectUtils has a NULL field for this purpose, if you don't want to define your own.

user_38007820
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