Create rule S7125

Author: kaufco

rules/S7125/java/metadata.json

@@ -1,5 +1,5 @@
 {
-  "title": "FIXME",
+  "title": "Null values should not be used in non-nullable input positions",
   "type": "CODE_SMELL",
   "status": "ready",
   "remediation": {
@@ -11,14 +11,12 @@
   "defaultSeverity": "Major",
   "ruleSpecification": "RSPEC-7125",
   "sqKey": "S7125",
-  "scope": "All",
+  "scope": "Main",
   "defaultQualityProfiles": ["Sonar way"],
   "quickfix": "unknown",
   "code": {
     "impacts": {
-      "MAINTAINABILITY": "HIGH",
-      "RELIABILITY": "MEDIUM",
-      "SECURITY": "LOW"
+      "RELIABILITY": "HIGH"
     },
     "attribute": "CONVENTIONAL"
   }

rules/S7125/java/rule.adoc

@@ -1,44 +1,123 @@
-FIXME: add a description
-
-// If you want to factorize the description uncomment the following line and create the file.
-//include::../description.adoc[]
-
 == Why is this an issue?
 
-FIXME: remove the unused optional headers (that are commented out)
+Using `null` in a non-nullable input position (e.g., as the right-hand side of an assignment, a function call argument, or a return statement argument) can lead to a NullPointerException (NPE) at runtime. This occurs because the receiving code typically assumes the value is non-null and omits null checks.
 
-//=== What is the potential impact?
+Formally, non-nullable and nullable versions of a type are distinct, with different domains.
+The domain of a non-nullable type is _D_, while the domain of a nullable type is _D ∪ null_, a superset of _D_.
+Thus, a non-null value can be used wherever a nullable type is expected, but not vice versa.
+The only reason it's allowed by the compiler is that null-safety is not a built-in Java language feature, and it's therefore handled via nullability annotations by external tools bypassing the regular typing system.
 
 == How to fix it
-//== How to fix it in FRAMEWORK NAME
+
+Depending on the use-case, there are different strategies to fix this problem
 
 === Code examples
 
+**1. Change the input position type from non-nullable to nullable:** This resolves the issue at the reported location but may propagate it elsewhere. Note: you should avoid declaring everything nullable; only do so where it aligns with your data and state models. Otherwise, consider the other approaches.
+
 ==== Noncompliant code example
 
 [source,java,diff-id=1,diff-type=noncompliant]
 ----
-FIXME
+@NonNull String title = null;
 ----
 
 ==== Compliant solution
 
 [source,java,diff-id=1,diff-type=compliant]
 ----
-FIXME
+String title = null;
+----
+
+==== Noncompliant code example
+
+[source,java,diff-id=2,diff-type=noncompliant]
+----
+@NullMarked
+class Collector {
+  void collectData(List<Entity> entities) {
+    // ...
+  }
+}
+
+void process() {
+  collector.collectData(null);
+}
 ----
 
-//=== How does this work?
+==== Compliant solution
 
-//=== Pitfalls
+[source,java,diff-id=2,diff-type=compliant]
+----
+class Collector {
+  void collectData(List<Entity> entities) {
+    // ...
+  }
+}
 
-//=== Going the extra mile
+void process() {
+  collector.collectData(null);
+}
+----
 
+**2. Replace `null` with a Guard Element:** This is particularly effective for array and collection types, where `null` can easily be replaced with an empty array or collection instance.
 
-//== Resources
-//=== Documentation
-//=== Articles & blog posts
-//=== Conference presentations
-//=== Standards
-//=== External coding guidelines
-//=== Benchmarks
+==== Noncompliant code example
+
+[source,java,diff-id=3,diff-type=noncompliant]
+----
+@NullMarked
+class Collector {
+  void collectData(List<Entity> entities) {
+    // ...
+  }
+}
+
+void process() {
+  collector.collectData(null);
+}
+----
+
+==== Compliant solution
+
+[source,java,diff-id=3,diff-type=compliant]
+----
+@NullMarked
+class Collector {
+  void collectData(List<Entity> entities) {
+    // ...
+  }
+}
+
+void process() {
+  collector.collectData(List.of());
+}
+----
+
+**3. Throw an Exception:** For unexpected or uninitialized values or unspecified behavior, throw an exception instead of returning `null`. This reports the issue at its origin, not somewhere else in the source code where the unexpected `null` value suddenly becomes a problem. This makes debugging easier.
+
+==== Noncompliant code example
+
+[source,java,diff-id=4,diff-type=noncompliant]
+----
+@NonNull State getNextState() {
+  return switch (state) {
+    case State.PENDING -> State.PROCESSING;
+    case State.PROCESSING -> State.PENDING;
+    default -> null;
+  };
+}
+----
+
+==== Compliant solution
+
+[source,java,diff-id=4,diff-type=compliant]
+----
+@NonNull State getNextState() {
+  return switch (state) {
+    case State.PENDING -> State.PROCESSING;
+    case State.PROCESSING -> State.PENDING;
+    default -> throw new IllegalStateException();
+  };
+}
+----