feat: Add CollectionsEmptyConstantsProcessor (SonarSource rule 1596)

[fermadeiral]

This PR adds a processor for the rule "Collections.EMPTY_LIST", "EMPTY_MAP", and "EMPTY_SET" should not be used.

Example repair:

- List<String> collection1 = Collections.EMPTY_LIST;  // Noncompliant
- Map<String, String> collection2 = Collections.EMPTY_MAP;  // Noncompliant
- Set<String> collection3 = Collections.EMPTY_SET;  // Noncompliant
+ List<String> collection1 = Collections.emptyList();
+ Map<String, String> collection2 = Collections.emptyMap();
+ Set<String> collection3 = Collections.emptySet();

[slarse]

One comment on the code that I think could be simpler/less hard-coded.

Apart from my comment about simplifying the processor a bit, there's one other thing that crossed my mind. Right now, the target of the method invocation is computed by directly getting a reference to Collections. But what if Collections isn't actually imported, and it's used with a fully qualified name? That would lead to this repair, which could cause a compile error:

// original
List<Integer> list = java.util.Collections.EMPTY_LIST;
// repair
List<Integer> list = Collections.emptyList();

However, the check in Sonar is a bit lazily implemented and only checks if the full name of the target is Collections, thus it doesn't flag qualified accesses to java.util.Collections. But perhaps it will in the future, so you could perhaps add a line like so in the .java and .expected files.

List<Integer> list = java.util.Collections.EMPTY_LIST; // Compliant due to false positive

That way we catch it if they fix the problem (which we should potentially report).

[slarse]

How about simplifying this a bit? The pattern is "camel case the name and make method invocation".

Suggested change

	CtType<?> collectionsType = getFactory().Class().get(Collections.class);
	CtMethod<?> methodToBeCalled;
	if (element.getVariable().getSimpleName().equals("EMPTY_LIST")) {
	methodToBeCalled = collectionsType.getMethodsByName("emptyList").get(0);
	} else if (element.getVariable().getSimpleName().equals("EMPTY_MAP")) {
	methodToBeCalled = collectionsType.getMethodsByName("emptyMap").get(0);
	} else {
	methodToBeCalled = collectionsType.getMethodsByName("emptySet").get(0);
	}
	String[] loweredNameParts = element.getVariable().getSimpleName().toLowerCase().split("_");
	String camelCasedName = loweredNameParts[0] + StringUtils.capitalize(loweredNameParts[1]);
	CtMethod<?> methodToBeCalled =
	getFactory().Class().get(Collections.class).getMethod(camelCasedName);

StringUtils is from org.apache.common.lang (so, import org.apache.common.lang.StringUtils).

[fermadeiral]

That's a great suggestion! Applied!

[slarse]

On a side note, this violates the rule you implemented a processor for in #562 :)

(sorald-buildbreaker does not catch it because it uses Sorald 0.3.0, which doesn't have that processor)

[fermadeiral]

:X

[fermadeiral]

@slarse, suggestions applied.

[slarse]

Nice, short and simple processor :)