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<p>The <a href="http://www.c2.com/cgi/wiki?BouncerPattern">Bouncer Pattern</a> describes usage of a method whose sole purpose is to either throw an exception (when particular conditions hold) or do nothing. Such methods are often used to defensively guard pre-conditions of a method.</p>

<p>When writing utility methods, you should always guard against faulty input arguments. When writing internal methods, you may be able to ensure that certain pre-conditions always hold by having sufficient unit tests in place. Under such circumstances, you may reduce the desirability to have guards on your methods.</p>

<p>Groovy differs from other languages in that you frequently use the <code>assert</code> method within your methods rather than having a large number of utility checker methods or classes.</p>

<h3>Null Checking Example</h3>

<p>We might have a utility method such as:</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 NullChecker {
    static check(name, arg) {
        if (arg == null)
            throw new IllegalArgumentException(name + " is null")
    }
}
</pre></td></tr></table>
<p>And we would use it like this:</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>
public void doStuff(String name, Object value) {
    NullChecker.check("name", name);
    NullChecker.check("value", value);
    // do stuff
}
</pre></td></tr></table>
<p>But a more Groovy way to do this would simply be like this:</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>
public void doStuff(String name, Object value) {
    assert name != null, 'name should not be null'
    assert value != null, 'value should not be null'
    // do stuff
}
</pre></td></tr></table>

<h3>Validation Example</h3>

<p>As an alternative example, we might have this utility method:</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>
public class NumberChecker {
    static final NUMBER_PATTERN = /\d+(\.\d+(E-?\d+)?)?/
    static isNumber(str) {
        if (!str ==~ NUMBER_PATTERN)
            throw new IllegalArgumentException(/Argument '$str' must be a number/)
    }
    static isNotZero(number) {
        if (number == 0)
            throw new IllegalArgumentException('Argument must not be 0')
    }
}
</pre></td></tr></table>
<p>And we would use it like this:</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>
def stringDivide(String dividendStr, String divisorStr) {
    NumberChecker.isNumber(dividendStr)
    NumberChecker.isNumber(divisorStr)
    def dividend = dividendStr.toDouble()
    def divisor = divisorStr.toDouble()
    NumberChecker.isNotZero(divisor)
    dividend / divisor
}

println stringDivide('1.2E2', '3.0')
// => 40.0
</pre></td></tr></table>
<p>But with Groovy we could just as easily use:</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>
def stringDivide(String dividendStr, String divisorStr) {
    assert dividendStr =~ NumberChecker.NUMBER_PATTERN
    assert divisorStr =~ NumberChecker.NUMBER_PATTERN
    def dividend = dividendStr.toDouble()
    def divisor = divisorStr.toDouble()
    assert divisor != 0, 'Divisor must not be 0'
    dividend / divisor
}
</pre></td></tr></table>