Null object

Null Object PatternAvoids null pointer exceptions.

Replaces conditionals with polymorphism.

The Problem

print $house->getTenant()->getName();

Error!

Cannot call method on non-object.

“Name”

The Wrong Solution

$tenant = $house->getTenant();if ($tenant) { print $tenant->getName();} else { print “no tenant”;}

The Null Object Solution

class House { private $tenant; function getTenant() { return $this.tenant ?: NullTenant::getInstance(); }

class NullTenant implements Tenant { function getName() { return “no tenant”; }

print $house->getTenant()->getName();

Salient Points

• Null Objects are Singletons

• Null Objects keep no state.

• Null Objects implement the same interface as the “real” objects.

• Null Objects “do the right thing” when a null is expected.

Salient Points

More generally the rule is: never return a nullwhen your client is expecting an object, returna Null Object instead.

Real Worldclass Node { private $left, $right; function copy() { $leftcopy = $this->left ? $this->left->copy(); : null; $rightcopy = $this->right ? $this->right->copy(); : null; return new self($leftcopy, $rightcopy); }

Real Worldclass RealNode implements Node { private $left, $right; function copy() { return new self($left->copy(), $right->copy()); }

class NullNode implements Node { function copy() { return $this; }

Real World

• Objective C has the Null Object Pattern built-in!

• Calling a method on null in Objective C results in null.

Real World

• Fowler calls this the “Special Case Object” pattern.

• We can have alternative null objects for different circumstances, i.e.

• NoTennant

• UnknownTennant