BigDecimal.valueOf(double d) uses canonical String representation of double value, internally Double.toString(double) is used, that's why you are getting 0.9 in second case.

Note: This is generally the preferred way to convert a double (or float) into a BigDecimal, as the value returned is equal to that resulting from constructing a BigDecimal from the result of using Double.toString(double).

While with new BigDecimal(0.9) it converts value to exact floating point representation of double value without using String representation,

Translates a double into a BigDecimal which is the exact decimal representation of the double's binary floating-point value.

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NOTES :

1. The results of this constructor can be somewhat unpredictable.

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FOR EXAMPLE :

BigDecimal bd1 = new BigDecimal(Double.toString(0.9));
BigDecimal bd2 = new BigDecimal(0.9);
System.out.println(bd1);
System.out.println(bd2);

OUTPUT :

0.9
0.90000000000000002220446049250313080847263336181640625