Create rule S6888: "case null" should be preferred over "if" pre-condition (#3574)

rules/S6888/java/metadata.json

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+{
+  "title": "\"case null\" should be preferred over \"if\" pre-condition",
+  "type": "CODE_SMELL",
+  "status": "ready",
+  "remediation": {
+    "func": "Constant\/Issue",
+    "constantCost": "5min"
+  },
+  "tags": [
+    "java21"
+  ],
+  "defaultSeverity": "Minor",
+  "ruleSpecification": "RSPEC-6888",
+  "sqKey": "S6888",
+  "scope": "All",
+  "defaultQualityProfiles": ["Sonar way"],
+  "quickfix": "unknown",
+  "code": {
+    "impacts": {
+      "MAINTAINABILITY": "MEDIUM"
+    },
+    "attribute": "CONVENTIONAL"
+  }
+}

rules/S6888/java/rule.adoc

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+== Why is this an issue?
+
+Java 21 introduces `case null` for `switch`. It is a more concise and readable way to handle nullability compared to an `if` statement before a `switch`.
+[source,java]
+----
+switch (s) {
+  case null: /* code if null */
+  // ...
+}
+----
+When the selector expression evaluates to null and the `case null` is present, switch statements don't throw a NullPointerException anymore.
+It is not mandatory anymore to test for `null` before a `switch`, and  it is more readable to use the `case null` form.
+
+This rule identifies `if` statements that could be replaced by a more readable `case null` label.
+[source,java]
+----
+if (s == null) {
+  /* code if null */
+}
+switch (s) {
+  // ...
+}
+----
+
+== How to fix it
+
+Move the null check inside the switch statement.
+
+=== Code examples
+
+==== Noncompliant code example
+
+[source,java,diff-id=1,diff-type=noncompliant]
+----
+static void f(String s) {
+  if (s == null) { // Noncompliant, we can move the null check in the switch statement.
+    System.out.println("Oops!");
+    return;
+  }
+  switch (s) {
+    case "Foo", "Bar" -> System.out.println("Great");
+    default           -> System.out.println("Ok");
+   }
+}
+----
+
+==== Compliant solution
+
+[source,java,diff-id=1,diff-type=compliant]
+----
+static void f(String s) {
+  switch (s) {
+    case null         -> System.out.println("Oops!");
+    case "Foo", "Bar" -> System.out.println("Great");
+    default           -> System.out.println("Ok");
+   }
+}
+----
+
+
+
+== Resources
+* https://openjdk.org/jeps/441[JEP 441: Pattern Matching for Switch]

rules/S6888/metadata.json

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+{
+}