A nice table:
<table cellpadding="5" width="50%">
<tbody>
<tr>
<td valign="top"><strong>Do</strong>
</td>
<td valign="top"><strong>Don't</strong>
</td>
</tr>

<tr>
<td valign="top">1. Focus on interfaces.</td>
<td valign="top">2. Be worried about the implementation.</td>
</tr>

<tr>
<td valign="top">3. Mention semantic contract using interfaces.</td>
<td valign="top">4. Let subtypes break this semantic contract of their parent types.</td>
</tr>

<tr>
<td valign="top">5. Service decoupling using interfaces.</td>
<td valign="top">6. Be coupled with specific concrete implementation.</td>
</tr>

<tr>
<td valign="top">7. Family extension using interfaces.</td>
<td valign="top">8. Break the family relationship using concrete implementation.</td>
</tr>

<tr>
<td valign="top">9. Establish a family rule by abstract classes.</td>
<td valign="top">10. Impose your own rule through concrete classes.</td>
</tr>

<tr>
<td valign="top">11. Let interfaces answer all "what" about the system.</td>
<td valign="top">12. Forget to mention "how" are you answering all "what" from interfaces.</td>
</tr>
</tbody>
</table>
Also a good introduction of Strategy pattern<br><br>

http://today.java.net/pub/a/today/2006/03/28/testing-java-object-oriented.html

How to determine the levels of abstraction - http://programmers.stackexchange.com/questions/110933/how-to-determine-the-levels-of-abstraction

Simple and clear, good design -> Correct, clear, easy to change - http://randomartifacts.blogspot.com.au/2013/01/correctness-clarity-and-changeability.html

Data Oriented Programming in Java - https://www.infoq.com/articles/data-oriented-programming-java/