ARROW-6311: [Java] Make ApproxEqualsVisitor accept DiffFunction to make it more flexible

[tianchen92]

Related to ARROW-6311.

Currently ApproxEqualsVisitor will accept a epsilon for both float and double compare, and the difference calculation is always Math.abs(f1-f2)

For some cases like Validator it is not very suitable as:
i. it has different epsilon values for float/double
ii. it difference function is not Math.abs(f1-f2)

To resolve these, make this visitor accept both float/double epsilons and diff functions.

[tianchen92]

cc @pravindra

[pravindra]

the change lgtm. some minor questions

• is it useful to have different epsilons for floats and doubles (i guess a double epsilon can cover both)?
• did you consider using a Java.Util.Comparator instead of the diff functions ?

[tianchen92]

the change lgtm. some minor questions

• is it useful to have different epsilons for floats and doubles (i guess a double epsilon can cover both)?
• did you consider using a Java.Util.Comparator instead of the diff functions ?

Thanks for your comments,
for 1, most cases an epsilon can cover both, but in some cases they may be different, such as Validator.java(1.0E-6f and 1.0E-12f for each). And we have different constructor(with one epsilon for both or have two epsilon) for different cases.
for 2, the diff functions are used to calculate differences between values like Math.abs(f1-f2) by default or something as below in Validator.java, and maybe other implementations.

float average = Math.abs((f1 + f2) / 2);
float differenceScaled = Math.abs(f1 - f2) / (average == 0.0f ? 1f : average);

java.util.comparator seems only compare two values which is larger i think can’t handle this?
Thanks!