Polish interpolation and javadocs

Author: damianmalczewski

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

@@ -37,6 +37,11 @@ final class DefaultProblemContext implements ProblemContext, Serializable {
     return context.get(key);
   }
 
+  @Override
+  public boolean containsKey(String key) {
+    return context.containsKey(key);
+  }
+
   @Override
   public ProblemContext put(String key, @Nullable String value) {
     if (value == null) {

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

@@ -94,6 +94,10 @@ public DefaultProblemMapper() {}
    * annotation. Such builder can be further extended or executed to create {@link Problem}
    * response.
    *
+   * <p>This method is {@code final} and always delegates to {@link #toProblemBuilder(Throwable,
+   * ProblemContext)} with a {@code null} context. Subclasses should override {@link
+   * #toProblemBuilder(Throwable, ProblemContext)} to customize mapping behavior.
+   *
    * @param t {@link Throwable} to convert (may be {@code null})
    * @return a {@link ProblemBuilder} instance
    * @throws ProblemMappingException when something goes wrong while building the Problem
@@ -368,8 +372,8 @@ protected String interpolate(String template, Throwable t, @Nullable ProblemCont
         replacement = t.getMessage() == null ? "" : String.valueOf(t.getMessage());
       } else if (key.startsWith(CONTEXT_LABEL_PREFIX)) {
         String contextKey = key.substring(CONTEXT_LABEL_PREFIX.length());
-        replacement =
-            (context == null || !context.containsKey(contextKey)) ? "" : context.get(contextKey);
+        String contextValue = context != null ? context.get(contextKey) : null;
+        replacement = contextValue != null ? contextValue : "";
       } else {
         Object v = resolvePlaceholderSource(t, key);
         replacement = (v == null) ? "" : String.valueOf(v);

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

@@ -42,6 +42,10 @@
  * exception class is not annotated with {@link ProblemMapping}, and may throw {@link
  * ProblemMappingException} if an error occurs during problem creation.
  *
+ * <p>An empty builder is indistinguishable from a builder produced by an annotation with all blank
+ * fields; callers should use {@link #isMappingCandidate(Throwable)} to determine whether an
+ * exception is eligible for mapping before calling {@code toProblemBuilder}.
+ *
  * @since 1.3.0
  */
 public interface ProblemMapper {