In a grammar textbook I have, there is this phrase:

幸い日本で日本語を勉強して、かなり話せるようになりました。日本に行っただけのことはありました。

【さいわい　にほんで　 にほんごを　 べんきょうして、　 かなり　 はなせるように　 なりました。　 にほんに　いった　 だけのことは　 ありました。】

The translation given is:

Fortunately, I studied Japanese in Japan and now I can speak it fairly well. I didn't go to Japan for nothing.

The thing that confuses me is in that last part, where the translation is "I didn't go to Japan for nothing".

It seems to me that 「だけのことがありました」 literally translates to something like "there was just that thing". 「だけ」, to me, implies exclusion of other things.

So to me, the sentence should be something like "That's all I went to Japan for", or perhaps "That's all I got from being in Japan". The way I read it has more of a negative implication than the given translation.

What am I not understanding about this phrase in order to see how the given translation makes sense? Or perhaps is the given translation not as good as it could be?

Please phrase answers in a way that all can understand, with no technical linguistic terminology. Thanks!

In a grammar textbook I have, there is this phrase:

幸{さいわ}い日本{にほん}で日本語{にほんご}を勉強{べんきょう}して、かなり話{はな}せるようになりました。日本{にほん}に行{い}っただけのことはありました。

The translation given is:

Fortunately, I studied Japanese in Japan and now I can speak it fairly well. I didn't go to Japan for nothing.

The thing that confuses me is in that last part, where the translation is "I didn't go to Japan for nothing".

It seems to me that だけのことがありました literally translates to something like "there was just that thing". だけ, to me, implies exclusion of other things.

So to me, the sentence should be something like "That's all I went to Japan for", or perhaps "That's all I got from being in Japan". The way I read it has more of a negative implication than the given translation.

What am I not understanding about this phrase in order to see how the given translation makes sense? Or perhaps is the given translation not as good as it could be?