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<p>Groovy is a powerful tool. Like other powerful tools (think of a chainsaw) it requires a certain amount of user expertise and attention. Otherwise, results may be fatal.</p>

<p>Following code fragments are allowed in Groovy but usually result in unintended behaviour or incomprehensible problems. Simply don't do this and spare yourself some of the frustration that new and unskilled Groovy programmers sometimes experience.</p>

<p><strong>1. Accessing an object's type like a property</strong></p>

<p>Using <code>.class</code> instead of <code>.getClass()</code> is ok - as long as you know exactly what kind of object you have. But then you don't need that at all. Otherwise, you run in the risk of getting <code>null</code> or something else, but not the class of the object.</p>
<table class="wysiwyg-macro" data-macro-name="code" data-macro-default-parameter="java" style="background-image: url(/plugins/servlet/confluence/placeholder/macro-heading?definition=e2NvZGU6amF2YX0&amp;locale=en_GB&amp;version=2); background-repeat: no-repeat;" data-macro-body-type="PLAIN_TEXT"><tr><td class="wysiwyg-macro-body"><pre>
a = [:]
println a.class.simpleName    // NullPointerException, because a.class is null.
</pre></td></tr></table>
<p><strong>2. Omitting parentheses around method arguments when not appropriate</strong></p>

<p>Better use parentheses for argument lists unless you are sure.<br />
For instance, a left parenthesis after a method name must always enclose the parameter list.</p>
<table class="wysiwyg-macro" data-macro-name="code" data-macro-default-parameter="java" style="background-image: url(/plugins/servlet/confluence/placeholder/macro-heading?definition=e2NvZGU6amF2YX0&amp;locale=en_GB&amp;version=2); background-repeat: no-repeat;" data-macro-body-type="PLAIN_TEXT"><tr><td class="wysiwyg-macro-body"><pre>
println a*(b+c)     // is OK
println (a+b)*c     // NullPointer exception, because println (a+b) results in null.
</pre></td></tr></table>
<p><strong>3. Putting a newline at the wrong place into the code</strong></p>

<p>This may cause the compiler to to assume a complete statement although it is meant as continuing in the next line. It is not always as obvious as in the following example.</p>
<table class="wysiwyg-macro" data-macro-name="code" data-macro-default-parameter="java" style="background-image: url(/plugins/servlet/confluence/placeholder/macro-heading?definition=e2NvZGU6amF2YX0&amp;locale=en_GB&amp;version=2); background-repeat: no-repeat;" data-macro-body-type="PLAIN_TEXT"><tr><td class="wysiwyg-macro-body"><pre>
myVariable = "This is a very long statement continuing in the next line. Result is="
    + 42             // this line has no effect
</pre></td></tr></table>
<p><strong>4. Forgetting to write the second equals sign of the equals operator</strong></p>

<p>As a result, a comparison expression turns into an assignment.</p>
<table class="wysiwyg-macro" data-macro-name="code" data-macro-default-parameter="java" style="background-image: url(/plugins/servlet/confluence/placeholder/macro-heading?definition=e2NvZGU6amF2YX0&amp;locale=en_GB&amp;version=2); background-repeat: no-repeat;" data-macro-body-type="PLAIN_TEXT"><tr><td class="wysiwyg-macro-body"><pre>
while (a=b) { ... }                  // will be executed as long as b is true (Groovy-truth)
String s = "A"; boolean b = s = "B"  // b becomes true
</pre></td></tr></table>
<p>(One measure is to put constant expressions in comparisons always before the equals operator.)</p>

<p><strong>5. Specifying the wrong return type when overriding a method</strong> (Groovy version &lt; 1.5.1 only)</p>

<p>Such a method may be considered as overloaded (a different method with the same name) and not be called as expected.</p>
<table class="wysiwyg-macro" data-macro-name="code" data-macro-default-parameter="java" style="background-image: url(/plugins/servlet/confluence/placeholder/macro-heading?definition=e2NvZGU6amF2YX0&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 toString() { ... }  // does not override toString() inherited from Object
</pre></td></tr></table>
<p><strong>6. Disregarding other objects' privacy</strong></p>

<p>When accessing methods, fields, or properties of other classes, make sure that you do not interfere with private or protected members. Currently Groovy doesn't distinguish properly between public, private, and protected members, so watch out yourself.</p>
<table class="wysiwyg-macro" data-macro-name="code" data-macro-default-parameter="java" style="background-image: url(/plugins/servlet/confluence/placeholder/macro-heading?definition=e2NvZGU6amF2YX0&amp;locale=en_GB&amp;version=2); background-repeat: no-repeat;" data-macro-body-type="PLAIN_TEXT"><tr><td class="wysiwyg-macro-body"><pre>
z = [1,2,3]
z.size = 10  // size is private; errors when trying to access z[3] to z[9], although z.size()==10

'more'.value = 'less'     // results in "more"=="less"
</pre></td></tr></table>
<p><strong>7. Thoughtless dynamic programming</strong></p>

<p>Check for conflicts with intended class functionality before adding or changing methods or properties using Groovy's dynamic facilities.</p>
<table class="wysiwyg-macro" data-macro-name="code" data-macro-default-parameter="java" style="background-image: url(/plugins/servlet/confluence/placeholder/macro-heading?definition=e2NvZGU6amF2YX0&amp;locale=en_GB&amp;version=2); background-repeat: no-repeat;" data-macro-body-type="PLAIN_TEXT"><tr><td class="wysiwyg-macro-body"><pre>
String.metaClass.class = Integer  // results in 'abc'.getClass()==java.lang.Integer
</pre></td></tr></table>
<p><strong>8. String concatenation</strong><br />
As in Java, you can concatenate Strings with the &quot;+&quot; symbol. But Java only needs that one of the two items of a &quot;+&quot; expression to be a String, no matter if it's in the first place or in the last one. Java will use the <code>toString()</code> method in the non-String object of your &quot;+&quot; expression. But in Groovy, you just should be safe the first item of your &quot;+&quot; expression implements the <code>plus()</code> method in the right way, because Groovy will search and use it. In Groovy GDK, only the Number and String/StringBuffer/Character classes have the <code>plus()</code> method implemented to concatenate strings. To avoid surprises, always use GStrings.</p>

<table class="wysiwyg-macro" data-macro-name="code" data-macro-default-parameter="java" style="background-image: url(/plugins/servlet/confluence/placeholder/macro-heading?definition=e2NvZGU6amF2YX0&amp;locale=en_GB&amp;version=2); background-repeat: no-repeat;" data-macro-body-type="PLAIN_TEXT"><tr><td class="wysiwyg-macro-body"><pre>
// Java code, it works
boolean v = true;
System.out.println(" foo "+v);
System.out.println(v+" foo ");
</pre></td></tr></table>
<table class="wysiwyg-macro" data-macro-name="code" data-macro-default-parameter="java" style="background-image: url(/plugins/servlet/confluence/placeholder/macro-heading?definition=e2NvZGU6amF2YX0&amp;locale=en_GB&amp;version=2); background-repeat: no-repeat;" data-macro-body-type="PLAIN_TEXT"><tr><td class="wysiwyg-macro-body"><pre>
// Groovy code
boolean v = true
println " foo "+v   // It works
println v+" foo "   // It fails with MissingMethodException: No signature of method: java.lang.Boolean.plus()
</pre></td></tr></table>