Add omitEmpty flag setter to toStringHelper

[YutaHiguchi-bsn]

1. What are you trying to do?

This is similar to #1404 adding support of omitting empty values from toStringHelper output.

What I noticed is that the current code already has the feature,
just that there is no public method like omitNullValues() to enable the omitEmptyValues flag.

guava/guava/src/com/google/common/base/MoreObjects.java

Line 155 in 5da71a7

private boolean omitEmptyValues = false;

It supports various types detecting empty-ness

guava/guava/src/com/google/common/base/MoreObjects.java

Lines 334 to 356 in 5da71a7

	private static boolean isEmpty(Object value) {
	// Put types estimated to be the most frequent first.
	if (value instanceof CharSequence) {
	return ((CharSequence) value).length() == 0;
	} else if (value instanceof Collection) {
	return ((Collection<?>) value).isEmpty();
	} else if (value instanceof Map) {
	return ((Map<?, ?>) value).isEmpty();
	} else if (value instanceof java.util.Optional) {
	return !((java.util.Optional<?>) value).isPresent();
	} else if (value instanceof OptionalInt) {
	return !((OptionalInt) value).isPresent();
	} else if (value instanceof OptionalLong) {
	return !((OptionalLong) value).isPresent();
	} else if (value instanceof OptionalDouble) {
	return !((OptionalDouble) value).isPresent();
	} else if (value instanceof Optional) {
	return !((Optional) value).isPresent();
	} else if (value.getClass().isArray()) {
	return Array.getLength(value) == 0;
	}
	return false;
	}

Can omitEmptyValues() be added to ToStringHelper methods, so that the implemented feature can be exercised?

2. What's the best code you can write to accomplish that without the new feature?

        return MoreObjects.toStringHelper(this)
            .omitNullValues()
            .add("optional", optional.orElse(null))
            .add("array", Array.getLength(array) == 0 ? null : array)
            .add("collection", collection.isEmpty() ? null : collection)
            .toString();

3. What would that same code look like if we added your feature?

        return MoreObjects.toStringHelper(this)
            .omitEmptyValues()
            .add("optional", optional)
            .add("array", array)
            .add("collection", collection)
            .toString();

(Optional) What would the method signatures for your feature look like?

No response

Concrete Use Cases

Exclude empty Optional, Collection, etc. from toString() output.

Packages

com.google.common.base

Checklist

• I agree to follow the code of conduct.

• I have read and understood the contribution guidelines.

• I have read and understood Guava's philosophy, and I strongly believe that this proposal aligns with it.

• I have visited the idea graveyard, and did not see anything similar to this idea.

[kluever]

Good eye --- we have this exact feature available internally (which is why some of the implementation bits have "leaked" into Guava). I'm not opposed to open-sourcing it, given that "emptiness" seems to be a fairly well-understood (but not officially documented or statically encoded) concept.

I think we'll need to beef up the javadocs on our API a bit --- we may want to explicitly list the current types we check for emptiness, and also give ourselves some wiggle room for expanding (or shrinking) that list in the future.