There are two ways to look at this:
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Integer.toBinaryString(n)produces a string of lengthceiling(log2(n)). That computation will take time that is roughly proportional tolog(n).
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The maximum value for nis 2^31 – 1, and that computation will always take less than some fixed constant time.
In fact, the question of the big O complexity of Integer.toBinaryString(n) is not mathematically meaningful.  By definition, big O complexity is about  a function’s behavior as some variable tends to infinity.  And the only variable ( n ) cannot do that in this context.  
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solved Time complexity of Integer.toBinaryString() in Java