The challenge with learning a language is less about the grammar, which you can learn in a matter of months, but gobs and gobs of vocabulary which you must learn, and internalize. This is especially hard when you have to consider nuances: when things are said, how word X differs slightly from word Y and so on.
However, if you’re studying for a language exam, such as the JLPT certification exam for Japanese, this means you have to learn a lot of vocabulary in a shorter, more compressed span of time, and chances are you’ll be rushed and unable to really learn the word properly, in context.
For example, while building up my flash cards for the N1 JLPT exam, I learned the word 迫力 (hakuryoku) meaning force or impact. That kind of makes sense, but even in English that’s a bit vague.
Then, later, while watching a certain Japanese documentary on Buddhist temples, the host said in Japanese while visiting the famous Nio-mon Gate (仁王門) at one temple: 「迫力のある仁王門ですね」
This sentence is pretty basic, but shows a nice native example, in context no less, how the word 迫力 is used. I then looked up the word in my favorite Japanese dictionary, and sure enough other sentences used the same pattern: 迫力のあるX (“X has a lot of impact, or intensity”).
The point here is that if I had not seen an actual example of this being used, I would not have learned the proper usage and context of this word. So, memorizing just isn’t enough. Even though it takes longer, you will not really master a language, including Japanese, if you don’t take the time to absorb words through media and most important in context. Then, you can make flash cards using the sentences you saw, in order to properly practice new information.