One of the first principles you will hear from anyone hoping to impart software development wisdom on young disciples is DRY. Don't Repeat Yourself. If software best practices all lived in a little nation, the Honourable Commonwealth of Softwaria, its king would be DRY. A good king he was too.
Over the years, DRY took on further battles for the people of Softwaria. In the Thirty Year War against Visual Basic programmers, DRY brought forth the mighty powers of composition. Now large segments of functionality and data, not just individual methods, could be transported and duplicated to be used anywhere. A method to format an integer into currency? How about a currency class instead that can not only format itself to a string but also do any other logic you want to on currency all bundled nicely in a little package. Besides removing duplicate code, composition also provided encapsulation, higher cohesion and lowered coupling (the bad kind not the good kind!).
As Lord Action once said though, power tends to corrupt, and absolute power corrupts absolutely. For in his zealous fervor to ban any duplicate code or logic he was tempted into delving into powerful and dark arts. DRY called upon his most powerful sorcerers who gave him the power of implementation inheritance. Yes he did eke out yet more removal of duplicated code. But in the process he sacrificed most of the benefits that encapsulation had provided.
Yet more dissidents were found in the kingdom of Softwaria though and DRY delved into yet darker arts to combat them. This time he tapped into the power of reflection. With this power he could make mountains move (even if they were declared private!). Giddy with power, the king annihilated the duplicated code. Yes he had to sacrifice readability, but it seemed a small price to pay to eliminate those treasonous and heretical duplicate code bastards.
Finally, in the dark recesses of the kingdom, DRY found the last remaining resistance movements of duplicated code. They didn't bother people much, they were largely just simple workers tilling away at their small plot of program. By this time the good king had become fanatical though. ALL DUPLICATED CODE MUST DIE! He called on his sorcerers one final time and asked for the power to remove the remaining resistance. He offered anything in his kingdom just for this one final grant of power. And the sorcerers delivered for they offered the power of metaprogramming. Now the King was no longer bound by the traditional physics of the land, instead he could reshape it in whatever likeness he desired. In the land of Java the king created code generators that created wide areas of forbidden code, code never to be touched or even looked at. XML spread like a disease, soon used to store the most important program logic. In the land of Ruby, programs created other programs and whole new languages sprang forth from nowhere to solve the most trivial of problems. Ruby on Rails came riding high with instant magical scaffolds. And the last of the duplicated code died...
The king was satisfied with himself for he had finally fulfilled his ultimate goal. Old King DRY stepped out to view his wondrous kingdom and no longer recognized what he saw. It was incomprehensible. Nothing made sense anymore, nothing was predictable and nothing worked as it seemed. Nobody, not even King DRY who created this world, understood it anymore. The sorcerers laughed knowing the land was now ruled by magic and the good king wept.