Add "Unknown Error" fallback for title resolution

In order to ensure non-null constraint on title field, some default
value must be decided for it, as the builder tries to resolve missing
title based on status code, matching codes that originate from
ProblemStatus enum. Current draft implementation consists of adding
"Unknown Error" string, stored in constant of Problem interface. This
still might change in the future.

Author: damianmalczewski

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

@@ -53,7 +53,7 @@ public abstract class AbstractProblem implements Problem, Serializable {
   private static final long serialVersionUID = 1L;
 
   private final URI type;
-  private final @Nullable String title;
+  private final String title;
   private final int status;
   private final @Nullable String detail;
   private final @Nullable URI instance;
@@ -73,7 +73,7 @@ public abstract class AbstractProblem implements Problem, Serializable {
    */
   public AbstractProblem(
       @Nullable URI type,
-      @Nullable String title,
+      String title,
       int status,
       @Nullable String detail,
       @Nullable URI instance,
@@ -98,7 +98,7 @@ public URI getType() {
    * @return a short, human-readable title describing the problem
    */
   @Override
-  public @Nullable String getTitle() {
+  public String getTitle() {
     return this.title;
   }
 
@@ -233,9 +233,7 @@ public int hashCode() {
   public String toString() {
     List<String> entries = new ArrayList<>();
     entries.add("type=" + getType());
-    if (getTitle() != null) {
-      entries.add("title=" + getTitle());
-    }
+    entries.add("title=" + getTitle());
     entries.add("status=" + getStatus());
     if (getDetail() != null) {
       entries.add("detail=" + getDetail());

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

@@ -266,9 +266,7 @@ public Problem build() {
     String title = this.title;
     if (title == null) {
       Optional<ProblemStatus> status = ProblemStatus.findValue(this.status);
-      if (status.isPresent()) {
-        title = status.get().getTitle();
-      }
+      title = status.map(ProblemStatus::getTitle).orElse(Problem.UNKNOWN_TITLE);
     }
     return new ProblemImpl(type, title, status, detail, instance, extensions);
   }

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

@@ -47,6 +47,12 @@ public interface Problem {
   /** Default type URI for generic problems. */
   URI BLANK_TYPE = URI.create("about:blank");
 
+  /**
+   * Fallback to resolution of {@code title} field while calling {@link ProblemBuilder#build()}
+   * method.
+   */
+  String UNKNOWN_TITLE = "Unknown Error";
+
   /** MIME content type for this problem. */
   String CONTENT_TYPE = "application/problem+json";
 
@@ -82,7 +88,7 @@ static Extension extension(@Nullable String key, @Nullable Object value) {
   /**
    * @return a short, human-readable title describing the problem
    */
-  @Nullable String getTitle();
+  String getTitle();
 
   /**
    * @return the HTTP status code generated for this problem

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

@@ -136,10 +136,7 @@ protected ProblemException(
    */
   private static @Nullable String produceExceptionMessage(Problem problem) {
     StringBuilder builder = new StringBuilder();
-
-    if (problem.getTitle() != null) {
-      builder.append(problem.getTitle());
-    }
+    builder.append(problem.getTitle());
 
     if (problem.getDetail() != null) {
       if (builder.length() > 0) {

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

@@ -30,7 +30,7 @@ final class ProblemImpl extends AbstractProblem {
 
   ProblemImpl(
       @Nullable URI type,
-      @Nullable String title,
+      String title,
       int status,
       @Nullable String detail,
       @Nullable URI instance,

src/test/java/io/github/problem4j/core/ProblemBuilderTest.java

@@ -77,7 +77,7 @@ void givenNullProblemStatus_shouldNotSetTitleOrStatus() {
     Problem problem = Problem.builder().status(null).build();
 
     assertThat(problem.getStatus()).isZero();
-    assertThat(problem.getTitle()).isNull();
+    assertThat(problem.getTitle()).isEqualTo(Problem.UNKNOWN_TITLE);
   }
 
   @Test
@@ -266,7 +266,7 @@ void givenUnknownNumericStatus_shouldNotDeriveTitle() {
     Problem problem = Problem.builder().status(999).build();
 
     assertThat(problem.getStatus()).isEqualTo(999);
-    assertThat(problem.getTitle()).isNull();
+    assertThat(problem.getTitle()).isEqualTo(Problem.UNKNOWN_TITLE);
   }
 
   @Test

src/test/java/io/github/problem4j/core/ProblemExceptionTest.java

@@ -98,7 +98,7 @@ void givenProblemWithDetail_whenCreatingException_thenMessageWithDetail() {
 
     ProblemException exception = new ProblemException(problem);
 
-    assertEquals("Something went wrong", exception.getMessage());
+    assertEquals("Unknown Error: Something went wrong", exception.getMessage());
   }
 
   @Test
@@ -144,7 +144,7 @@ void givenProblemWithLargeStatus_whenCreatingException_thenStatusIncludedInMessa
 
     ProblemException exception = new ProblemException(problem);
 
-    assertEquals("Custom protocol error (code: 999)", exception.getMessage());
+    assertEquals("Unknown Error: Custom protocol error (code: 999)", exception.getMessage());
   }
 
   @Test

src/test/java/io/github/problem4j/core/ProblemImplTest.java

@@ -67,11 +67,11 @@ void givenAllFieldsPopulated_whenToString_thenContainsAllFields() {
   @Test
   void givenNullExtensionsAndNullableFields_whenToString_thenOmitsNulls() {
     Map<String, Object> extensions = new HashMap<>();
-    Problem problem = new ProblemImpl(null, null, 200, null, null, extensions);
+    Problem problem = new ProblemImpl(null, ProblemStatus.OK_TITLE, 200, null, null, extensions);
 
     String result = problem.toString();
 
-    assertThat(result).isEqualTo("Problem{type=about:blank, status=200}");
+    assertThat(result).isEqualTo("Problem{type=about:blank, title=OK, status=200}");
   }
 
   @Test
@@ -80,7 +80,8 @@ void givenOnlyNumberExtensions_whenToString_thenContainsNumbers() {
     extensions.put("ext1", 123);
     extensions.put("ext2", 456.78);
 
-    Problem problem = new ProblemImpl(null, null, 0, null, null, extensions);
+    Problem problem =
+        new ProblemImpl(null, ProblemStatus.INTERNAL_SERVER_ERROR_TITLE, 0, null, null, extensions);
 
     String result = problem.toString();
 
@@ -94,7 +95,8 @@ void givenOnlyBooleanExtensions_whenToString_thenContainsBooleans() {
     extensions.put("flag1", true);
     extensions.put("flag2", false);
 
-    Problem problem = new ProblemImpl(null, null, 0, null, null, extensions);
+    Problem problem =
+        new ProblemImpl(null, ProblemStatus.INTERNAL_SERVER_ERROR_TITLE, 0, null, null, extensions);
 
     String result = problem.toString();
 
@@ -107,7 +109,8 @@ void givenNonPrimitiveExtension_whenToString_thenUsesItsToString() {
     Map<String, Object> extensions = new HashMap<>();
     extensions.put("obj", new DummyObject("biz\tbar"));
 
-    Problem problem = new ProblemImpl(null, null, 0, null, null, extensions);
+    Problem problem =
+        new ProblemImpl(null, ProblemStatus.INTERNAL_SERVER_ERROR_TITLE, 0, null, null, extensions);
 
     String result = problem.toString();
 
@@ -119,7 +122,8 @@ void givenStringWithSpecialCharacters_whenToString_thenProperlyHandlesIt() {
     Map<String, Object> extensions = new HashMap<>();
     extensions.put("ext", "a\"b\\c\nd");
 
-    Problem problem = new ProblemImpl(null, null, 0, null, null, extensions);
+    Problem problem =
+        new ProblemImpl(null, ProblemStatus.INTERNAL_SERVER_ERROR_TITLE, 0, null, null, extensions);
 
     String result = problem.toString();