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<p>The <a href="http://en.wikipedia.org/wiki/Null_Object_pattern">Null Object Pattern</a> involves using a special object place-marker object representing null. Typically, if you have a reference to null, you can't invoke <code>reference.field</code> or <code>reference.method()</code>. You receive the dreaded <code>NullPointerException</code>. The null object pattern uses a special object representing null, instead of using an actual <code>null</code>. This allows you to invoke field and method references on the null object. The result of using the null object should semantically be equivalent to <em>doing nothing</em>.</p>

<h3>Simple Example</h3>

<p>Suppose we have the following system:</p>

<table class="wysiwyg-macro" data-macro-name="code" style="background-image: url(/plugins/servlet/confluence/placeholder/macro-heading?definition=e2NvZGV9&amp;locale=en_GB&amp;version=2); background-repeat: no-repeat;" data-macro-body-type="PLAIN_TEXT"><tr><td class="wysiwyg-macro-body"><pre>
class Job {
    def salary
}

class Person {
    def name
    def Job job
}

def people = [
    new Person(name:'Tom', job:new Job(salary:1000)),
    new Person(name:'Dick', job:new Job(salary:1200)),
]

def biggestSalary = people.collect{ p -> p.job.salary }.max()
println biggestSalary
</pre></td></tr></table>

<p>When run, this prints out <code>1200</code>. Suppose now that we now invoke:</p>

<p>If we now try to calculate <code>biggestSalary</code> again, we receive a null pointer exception.</p>

<p>To overcome this problem, we can introduce a <code>NullJob</code> class and change the above statement to become:</p>

people &lt;&lt; new Person(name:'Harry', job:new NullJob())
biggestSalary = people.collect{ p -> p.job.salary }.max()
println biggestSalary
</pre></td></tr></table>

<p>This works as we require but it's not always the best way to do this with Groovy. Groovy's safe-dereference operator (<code>?.</code>) operator and null aware closures often allow Groovy to avoid the need to create a special null object or null class. This is illustrated by examining a groovier way to write the above example:</p>

<p>Two things are going on here to allow this to work. First of all, <code>max()</code> is <em>'null aware'</em> so that <code>[300, null, 400].max() == 400</code>. Secondly, with the <code>?.</code> operator, an expression like <code>p?.job?.salary</code> will be equal to null if <code>salary</code> is equal to null, or if <code>job</code> is equal to null or if <code>p</code> is equal to null. You don't need to code a complex nested <code>if ... then ... else</code> to avoid a <code>NullPointerException</code>.</p>

<h3>Tree Example</h3>

<p>Consider the following example (inspired by <a href="http://wiki.rubygarden.org/Ruby/page/show/NullObjectPattern">this</a>) where we want to calculate size, cumulative sum and cumulative product of all the values in a tree structure.</p>

<p>Our first attempt has special logic within the calculation methods to handle null values.</p>

def sum() {
       value + (left ? left.sum() : 0) + (right ? right.sum() : 0)
    }

def product() {
       value * (left ? left.product() : 1) * (right ? right.product() : 1)
    }
}

def root = new NullHandlingTree(
    value:2,
    left: new NullHandlingTree(
        value:3,
        right: new NullHandlingTree(value:4),
        left: new NullHandlingTree(value:5)
    )
)

println root.size()
println root.sum()
println root.product()
</pre></td></tr></table>

<p>If we introduce the null object pattern (here by defining the <code>NullTree</code> class), we can now simplify the logic in the <code>size()</code>, <code>sum()</code> and <code>product()</code> methods. These methods now much more clearly represent the logic for the normal (and now universal) case. Each of the methods within <code>NullTree</code> returns a value which represents doing nothing.</p>

def sum() {
       value + left.sum() + right.sum()
    }

def product() {
       value * left.product() * right.product()
    }
}

class NullTree {
    def size() { 0 }
    def sum() { 0 }
    def product() { 1 }
}

def root = new Tree(
    value:2,
    left: new Tree(
        value:3,
        right: new Tree(value:4),
        left: new Tree(value:5)
    )
)

println root.size()
println root.sum()
println root.product()
</pre></td></tr></table>

<p>The result of running either of these examples is:</p>

<p>Note: a slight variation with the null object pattern is to combine it with the singleton pattern. So, we wouldn't write <code>new NullTree()</code> wherever we needed a null object as shown above. Instead we would have a single null object instance which we would place within our data structures as needed.</p>