Null Object

About 2 min

Also known as

Active Nothing
Stub

Intent

To provide a default behavior for an object, avoiding the need for null checks and enhancing code readability.

Explanation

Real world example

We are building a binary tree from nodes. There are ordinary nodes and "empty" nodes. Traversing the tree normally should not cause errors, so we use null object pattern where necessary.

In plain words

Null Object pattern handles "empty" objects gracefully.

Wikipedia says

In object-oriented computer programming, a null object is an object with no referenced value or with defined neutral ("null") behavior. The null object design pattern describes the uses of such objects and their behavior (or lack thereof).

Programmatic Example

Here's the definition of Node interface.

public interface Node {

  String getName();

  int getTreeSize();

  Node getLeft();

  Node getRight();

  void walk();
}

We have two implementations of Node. The normal implementation NodeImpl and NullNode for empty nodes.

@Slf4j
public class NodeImpl implements Node {

  private final String name;
  private final Node left;
  private final Node right;

  /**
   * Constructor.
   */
  public NodeImpl(String name, Node left, Node right) {
    this.name = name;
    this.left = left;
    this.right = right;
  }

  @Override
  public int getTreeSize() {
    return 1 + left.getTreeSize() + right.getTreeSize();
  }

  @Override
  public Node getLeft() {
    return left;
  }

  @Override
  public Node getRight() {
    return right;
  }

  @Override
  public String getName() {
    return name;
  }

  @Override
  public void walk() {
    LOGGER.info(name);
    if (left.getTreeSize() > 0) {
      left.walk();
    }
    if (right.getTreeSize() > 0) {
      right.walk();
    }
  }
}

public final class NullNode implements Node {

  private static final NullNode instance = new NullNode();

  private NullNode() {
  }

  public static NullNode getInstance() {
    return instance;
  }

  @Override
  public int getTreeSize() {
    return 0;
  }

  @Override
  public Node getLeft() {
    return null;
  }

  @Override
  public Node getRight() {
    return null;
  }

  @Override
  public String getName() {
    return null;
  }

  @Override
  public void walk() {
    // Do nothing
  }
}

Then we can construct and traverse the binary tree without errors as follows.

    var root = new NodeImpl("1",
            new NodeImpl("11",
                new NodeImpl("111", NullNode.getInstance(), NullNode.getInstance()),
                NullNode.getInstance()
            ),
            new NodeImpl("12",
                NullNode.getInstance(),
                new NodeImpl("122", NullNode.getInstance(), NullNode.getInstance())
            )
        );
    root.walk();

Program output:

Class diagram

Applicability

When you need to provide a default behavior in place of a null object.
To simplify the client code by eliminating null checks.
When a default action is preferable to handling a null reference.

Known Uses

Logging systems where a NullLogger can be used to avoid null checks.
Collections that use a NullIterator to handle empty collections gracefully.
GUI systems where a NullComponent can be used to represent a component that does nothing.

Consequences

Benefits:

Eliminates the need for null checks, reducing the risk of NullPointerException.
Simplifies the client code and enhances readability.
Promotes the use of polymorphism by handling default behavior through a common interface.

Trade-offs:

May introduce additional classes, potentially increasing the overall complexity of the system.
The default behavior might mask potential issues that would otherwise be caught by explicit null handling.

Strategyopen in new window: Null Object can be seen as a special case of the Strategy Pattern where the strategy is to do nothing.
Stateopen in new window: Similar in that both patterns can handle different states or behaviors; Null Object is like a state that does nothing.
Factoryopen in new window: Often used to provide Null Objects in place of actual objects.