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<table class="wysiwyg-macro" data-macro-name="info" data-macro-parameters="icon=false|title=Table of Contents" style="background-image: url(/plugins/servlet/confluence/placeholder/macro-heading?definition=e2luZm86aWNvbj1mYWxzZXx0aXRsZT1UYWJsZSBvZiBDb250ZW50c30&amp;locale=en_GB&amp;version=2); background-repeat: no-repeat;" data-macro-body-type="RICH_TEXT"><tr><td class="wysiwyg-macro-body"><p><img class="editor-inline-macro" src="/plugins/servlet/confluence/placeholder/macro?definition=e3RvYzptYXhMZXZlbD0yfQ&amp;locale=en_GB&amp;version=2" data-macro-name="toc" data-macro-parameters="maxLevel=2"></p></td></tr></table><h1>What is LCOM4?</h1><p>Cohesion is the degree to which the methods of a single class are tied together. When two methods in a class do not use a common attribute or a common method, it means that they share nothing and should probably not belong to the same class to respect the <a href="http://en.wikipedia.org/wiki/Single_responsibility_principle">Single Responsibility Principle</a>. In other words you can split your class into several new classes to gain modularity at class level.</p><p>Sonar computes the LCOM4 (Lack of Cohesion of Methods) metric based on Hitz &amp; Montazeri. Here is a good introduction: <a href="http://www.aivosto.com/project/help/pm-oo-cohesion.html#LCOM4">http://www.aivosto.com/project/help/pm-oo-cohesion.html#LCOM4</a>.</p><p>The best value for this metric is 1.</p><h1>How to Hunt for Bad LCOM4?</h1><p>Add the <strong>LCOM4</strong> widget (was <strong>Chidamber &amp; Kemerer</strong> widget for Sonar versions prior to 3.3) on your <a class="confluence-link" href="/display/SONAR/Dashboards" data-linked-resource-id="163872785" data-linked-resource-type="page" data-linked-resource-default-alias="Dashboards" data-base-url="http://docs.codehaus.org">dashboard</a>:</p><p><img class="confluence-embedded-image" confluence-query-params="effects=drop-shadow" src="/download/attachments/231079997/lcom4_widget.png?version=1&amp;modificationDate=1369217865844&amp;effects=drop-shadow" data-image-src="/download/attachments/231079997/lcom4_widget.png?version=1&amp;modificationDate=1369217865844&amp;effects=drop-shadow" data-linked-resource-id="231375053" data-linked-resource-type="attachment" data-linked-resource-default-alias="lcom4_widget.png" data-base-url="http://docs.codehaus.org" data-linked-resource-container-id="231079997" title="null > lcom4_widget.png"></p><p>Drill down:</p><p><img class="confluence-embedded-image" width="900" src="/download/attachments/231079997/lcom4-drilldown.png?version=1&amp;modificationDate=1369217865860" data-image-src="/download/attachments/231079997/lcom4-drilldown.png?version=1&amp;modificationDate=1369217865860" data-linked-resource-id="231375054" data-linked-resource-type="attachment" data-linked-resource-default-alias="lcom4-drilldown.png" data-base-url="http://docs.codehaus.org" data-linked-resource-container-id="231079997" title="null > lcom4-drilldown.png"></p><h1>Example</h1><h2>Bad LCOM4</h2><p>Project with one single class: <em>Client.</em></p><p> </p><table class="wysiwyg-macro" data-macro-name="code" data-macro-parameters="language=java|linenumbers=true|title=Client" style="background-image: url(/plugins/servlet/confluence/placeholder/macro-heading?definition=e2NvZGU6dGl0bGU9Q2xpZW50fGxpbmVudW1iZXJzPXRydWV8bGFuZ3VhZ2U9amF2YX0&amp;locale=en_GB&amp;version=2); background-repeat: no-repeat;" data-macro-body-type="PLAIN_TEXT"><tr><td class="wysiwyg-macro-body"><pre>package example;

public class Client {
  
  public String firstname;
  public String lastname;
  public String street;
  public String city;
  public String zipCode;

public String getFullName() {
    return firstname + " " + lastname;
  }

public String getFullAddress() {
    return street + " " + city + " " + zipCode;
  }
  
}</pre></td></tr></table><p> </p><p>The project has an average LCOM4 value of 2:</p><p style="margin-left: 30.0px;"><img class="confluence-embedded-image" src="/download/attachments/231079997/lcom4-bad.png?version=1&amp;modificationDate=1369217865864" data-image-src="/download/attachments/231079997/lcom4-bad.png?version=1&amp;modificationDate=1369217865864" data-linked-resource-id="231375059" data-linked-resource-type="attachment" data-linked-resource-default-alias="lcom4-bad.png" data-base-url="http://docs.codehaus.org" data-linked-resource-container-id="231079997" title="null > lcom4-bad.png"></p><p>The <em>Client </em>class has an LCOM4 value of 2:</p><p><img class="confluence-embedded-image" src="/download/attachments/231079997/lcom4-bad-client.png?version=1&amp;modificationDate=1369217865863" data-image-src="/download/attachments/231079997/lcom4-bad-client.png?version=1&amp;modificationDate=1369217865863" data-linked-resource-id="231375058" data-linked-resource-type="attachment" data-linked-resource-default-alias="lcom4-bad-client.png" data-base-url="http://docs.codehaus.org" data-linked-resource-container-id="231079997" title="null > lcom4-bad-client.png"></p><p>It shows that there should be two different objects: <em>Client</em> and <em>Address.</em></p><h2>Good LCOM4 after Refactoring</h2><p>After creating a new <em>Address</em> class and creating an <em>Address</em> object in <em>Client:</em></p><table class="wysiwyg-macro" data-macro-name="code" data-macro-parameters="language=java|linenumbers=true|title=Address" style="background-image: url(/plugins/servlet/confluence/placeholder/macro-heading?definition=e2NvZGU6dGl0bGU9QWRkcmVzc3xsaW5lbnVtYmVycz10cnVlfGxhbmd1YWdlPWphdmF9&amp;locale=en_GB&amp;version=2); background-repeat: no-repeat;" data-macro-body-type="PLAIN_TEXT"><tr><td class="wysiwyg-macro-body"><pre>package example;

public class Address {
  
  public String street;
  public String city;
  public String zipCode;

public String getFullAddress() {
    return street + " " + city + " " + zipCode;
  }
</pre></td></tr></table><table class="wysiwyg-macro" data-macro-name="code" data-macro-parameters="language=java|linenumbers=true|title=Client" style="background-image: url(/plugins/servlet/confluence/placeholder/macro-heading?definition=e2NvZGU6dGl0bGU9Q2xpZW50fGxpbmVudW1iZXJzPXRydWV8bGFuZ3VhZ2U9amF2YX0&amp;locale=en_GB&amp;version=2); background-repeat: no-repeat;" data-macro-body-type="PLAIN_TEXT"><tr><td class="wysiwyg-macro-body"><pre>package example;

public class Client {
  
  public String firstname;
  public String lastname;
  public Address address;

public String getFullName() {
    return firstname + " " + lastname;
  }

public String getFullAddress() {
    return address.getFullAddress();
  }
  
}</pre></td></tr></table><p>The LCOM4 of the project is down to the best value:</p><p style="margin-left: 30.0px;"><img class="confluence-embedded-image" src="/download/attachments/231079997/lcom4-good.png?version=1&amp;modificationDate=1369217865862" data-image-src="/download/attachments/231079997/lcom4-good.png?version=1&amp;modificationDate=1369217865862" data-linked-resource-id="231375057" data-linked-resource-type="attachment" data-linked-resource-default-alias="lcom4-good.png" data-base-url="http://docs.codehaus.org" data-linked-resource-container-id="231079997" title="null > lcom4-good.png"></p><p>Thanks to the fact that <em>Client</em> and <em>Address</em> have an LCOM4 value of 1:</p><p style="margin-left: 30.0px;"><img class="confluence-embedded-image" src="/download/attachments/231079997/lcom4-good-address.png?version=1&amp;modificationDate=1369217865862" data-image-src="/download/attachments/231079997/lcom4-good-address.png?version=1&amp;modificationDate=1369217865862" data-linked-resource-id="231375056" data-linked-resource-type="attachment" data-linked-resource-default-alias="lcom4-good-address.png" data-base-url="http://docs.codehaus.org" data-linked-resource-container-id="231079997" title="null > lcom4-good-address.png"></p><p style="margin-left: 30.0px;"><img class="confluence-embedded-image" src="/download/attachments/231079997/lcom4-good-client.png?version=1&amp;modificationDate=1369217865861" data-image-src="/download/attachments/231079997/lcom4-good-client.png?version=1&amp;modificationDate=1369217865861" data-linked-resource-id="231375055" data-linked-resource-type="attachment" data-linked-resource-default-alias="lcom4-good-client.png" data-base-url="http://docs.codehaus.org" data-linked-resource-container-id="231079997" title="null > lcom4-good-client.png"></p><h1>Related Topics</h1><p>See the following blog post: <a href="http://www.sonarsource.org/clean-up-design-at-class-level-with-sonar/">Clean Up Design at Class Level with Sonar</a>.</p><p> </p>