Learn English – When would “mom/dad” require capital letter in writing?

"Dad" is a specific reference (when you say it you mean somebody different from when I say it), so it gets capitalized like any proper noun. On the other hand, "dad" is a common noun meaning "father" (anybody's). You only use disambiguators like "my" or "a" with common nouns ("my dad", but not "my Dad" just like you wouldn't say "my John Smith").

So the passage you quoted has it right. (3) is not capitalized because it's using a common noun ("a dad"), not a proper one ("Dad").

As far as Java and Android SDK are concerned, they're proper nouns.

Object-oriented programming is often written as OOP. Perhaps the author of the sentence thought it should be capitalised because the abbreviated form OOP has capitalised letters. The proper way of writing it would be 'object-oriented programming', since it is in the middle of a sentence.

Design patterns should not have capitalised letters - it isn't a proper noun, neither does it have an abbreviation; the author may have capitalised it for emphasis. Or he just isn't familiar with the rules of this language that well!