Resolve issues with covariant arguments

The most troublesome issue was the behaviour of NullAway while passing
Map<String, Object> to an argument of type Map<String, @nullable Object>.

Author: damianmalczewski

CHANGELOG.md

@@ -6,6 +6,10 @@ The format is based on [Keep a Changelog][keepachangelog], and this project adhe
 
 ## [Unreleased]
 
+### Fixed
+
+- Change type from `Object` to `? extends Object` for generics in method arguments to support covariant arguments.
+
 ## [1.4.0] - 2026-02-16
 
 ### Added

src/main/java/io/github/problem4j/core/AbstractProblem.java

@@ -142,7 +142,7 @@ public AbstractProblem(
       int status,
       @Nullable String detail,
       @Nullable URI instance,
-      @Nullable Map<String, Object> extensions) {
+      @Nullable Map<String, ? extends Object> extensions) {
     this.type = type;
     this.title = title;
     this.status = status;

src/main/java/io/github/problem4j/core/AbstractProblemBuilder.java

@@ -191,8 +191,8 @@ public ProblemBuilder extension(@Nullable String name, @Nullable Object value) {
    * @return this builder instance for chaining
    */
   @Override
-  public ProblemBuilder extensions(@Nullable Map<String, @Nullable Object> extensions) {
-    if (extensions != null && !extensions.isEmpty()) {
+  public ProblemBuilder extensions(@Nullable Map<String, ? extends @Nullable Object> extensions) {
+    if (extensions != null) {
       extensions.forEach(
           (key, value) -> {
             if (value != null) {

src/main/java/io/github/problem4j/core/ProblemBuilder.java

@@ -115,7 +115,7 @@ public interface ProblemBuilder {
    * @param extensions map of extension keys and values
    * @return this builder instance for chaining
    */
-  ProblemBuilder extensions(@Nullable Map<String, @Nullable Object> extensions);
+  ProblemBuilder extensions(@Nullable Map<String, ? extends @Nullable Object> extensions);
 
   /**
    * Adds single custom extension from {@link Problem.Extension}.
@@ -178,7 +178,7 @@ default ProblemBuilder extension(Problem.@Nullable Extension extension) {
    * @deprecated use {@link #extensions(Map)} instead
    */
   @Deprecated
-  default ProblemBuilder extension(@Nullable Map<String, @Nullable Object> extensions) {
+  default ProblemBuilder extension(@Nullable Map<String, ? extends @Nullable Object> extensions) {
     return extensions(extensions);
   }
 

src/main/java/io/github/problem4j/core/ProblemImpl.java

@@ -29,12 +29,15 @@ final class ProblemImpl extends AbstractProblem {
 
   private static final long serialVersionUID = 1L;
 
-  ProblemImpl(String title, int status, @Nullable Map<String, Object> extensions) {
+  ProblemImpl(String title, int status, @Nullable Map<String, ? extends Object> extensions) {
     super(BLANK_TYPE, title, status, null, null, extensions);
   }
 
   ProblemImpl(
-      String title, int status, @Nullable String detail, @Nullable Map<String, Object> extensions) {
+      String title,
+      int status,
+      @Nullable String detail,
+      @Nullable Map<String, ? extends Object> extensions) {
     super(BLANK_TYPE, title, status, detail, null, extensions);
   }
 
@@ -44,7 +47,7 @@ final class ProblemImpl extends AbstractProblem {
       int status,
       @Nullable String detail,
       @Nullable URI instance,
-      Map<String, Object> extensions) {
+      @Nullable Map<String, ? extends Object> extensions) {
     super(type, title, status, detail, instance, extensions);
   }