There's no built-in API that will do this for you, so you'll have to write your own.
One idea is to use the mod (%) operator to help figure out what value you have... something like this
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1445
1445 %1000 = 1, so you know you have
one thousand (1445 - 1000) % 100 = 4, so you know you have
four hundred (445 - 400) % 10 = 4, so you know you have
forty (45 - 40) % 1 = 5, so you know you have
five which gives you something close to one thousand Four Hundred and Forty Five.
you'll just have to make a big mapping between what place you're in (thousands, hundreds, tens, ones, etc) and what the written value is for each number 0-9 in that value (thirty, forty, fifty, etc.
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I'm moving this to Java In General (Intermediate) as I don't think this is an advanced question.