Add toString to builder

It probably won't be used anyway, but it will be usefull in debug mode.

Author: damianmalczewski

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

@@ -20,6 +20,8 @@
  */
 package io.github.problem4j.core;
 
+import static io.github.problem4j.core.JsonEscape.escape;
+
 import java.io.Serializable;
 import java.net.URI;
 import java.util.ArrayList;
@@ -209,17 +211,17 @@ public int hashCode() {
   public String toString() {
     List<String> lines = new ArrayList<>();
     if (getType() != null) {
-      lines.add("\"type\" : \"" + quote(getType().toString()) + "\"");
+      lines.add("\"type\" : \"" + escape(getType().toString()) + "\"");
     }
     if (getTitle() != null) {
-      lines.add("\"title\" : \"" + quote(getTitle()) + "\"");
+      lines.add("\"title\" : \"" + escape(getTitle()) + "\"");
     }
     lines.add("\"status\" : " + getStatus());
     if (getDetail() != null) {
-      lines.add("\"detail\" : \"" + quote(getDetail()) + "\"");
+      lines.add("\"detail\" : \"" + escape(getDetail()) + "\"");
     }
     if (getInstance() != null) {
-      lines.add("\"instance\" : \"" + quote(getInstance().toString()) + "\"");
+      lines.add("\"instance\" : \"" + escape(getInstance().toString()) + "\"");
     }
 
     getExtensionMembers()
@@ -230,7 +232,7 @@ public String toString() {
               }
 
               if (value instanceof String) {
-                lines.add("\"" + field + "\" : \"" + quote((String) value) + "\"");
+                lines.add("\"" + field + "\" : \"" + escape((String) value) + "\"");
               } else if (value instanceof Number || value instanceof Boolean) {
                 lines.add("\"" + field + "\" : " + value);
               } else {
@@ -241,13 +243,9 @@ public String toString() {
     return lines.stream().collect(Collectors.joining(", ", "{ ", " }"));
   }
 
-  private static String quote(String string) {
-    return JsonEscape.escape(string);
-  }
-
   private String getObjectLine(String field, Object value) {
     String className = value.getClass().getSimpleName();
-    return "\"" + field + "\" : \"" + className + ":" + quote(value.toString()) + "\"";
+    return "\"" + field + "\" : \"" + className + ":" + escape(value.toString()) + "\"";
   }
 
   /** Represents a single key-value extension in a {@link Problem}. */
@@ -321,9 +319,9 @@ public String toString() {
       if (getValue() instanceof Number || getValue() instanceof Boolean) {
         valueLine = "\"value\" : " + getValue();
       } else {
-        valueLine = "\"value\" : " + "\"" + quote(getValue().toString()) + "\"";
+        valueLine = "\"value\" : " + "\"" + escape(getValue().toString()) + "\"";
       }
-      return "{ \"key\" : \"" + quote(getKey()) + "\", " + valueLine + " }";
+      return "{ \"key\" : \"" + escape(getKey()) + "\", " + valueLine + " }";
     }
   }
 }

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

@@ -20,12 +20,17 @@
  */
 package io.github.problem4j.core;
 
+import static io.github.problem4j.core.JsonEscape.escape;
+
 import java.io.Serializable;
 import java.net.URI;
+import java.util.ArrayList;
 import java.util.Collection;
 import java.util.HashMap;
+import java.util.List;
 import java.util.Map;
 import java.util.Optional;
+import java.util.stream.Collectors;
 import java.util.stream.Stream;
 
 /**
@@ -267,7 +272,47 @@ public Problem build() {
     return new ProblemImpl(type, title, status, detail, instance, extensions);
   }
 
+  @Override
+  public String toString() {
+    List<String> lines = new ArrayList<>();
+    if (type != null) {
+      lines.add("\"type\" : \"" + escape(type.toString()) + "\"");
+    }
+    if (title != null) {
+      lines.add("\"title\" : \"" + escape(title) + "\"");
+    }
+    lines.add("\"status\" : " + status);
+    if (detail != null) {
+      lines.add("\"detail\" : \"" + escape(detail) + "\"");
+    }
+    if (instance != null) {
+      lines.add("\"instance\" : \"" + escape(instance.toString()) + "\"");
+    }
+
+    extensions.forEach(
+        (field, value) -> {
+          if (value == null) {
+            return;
+          }
+
+          if (value instanceof String) {
+            lines.add("\"" + field + "\" : \"" + escape((String) value) + "\"");
+          } else if (value instanceof Number || value instanceof Boolean) {
+            lines.add("\"" + field + "\" : " + value);
+          } else {
+            lines.add(getObjectLine(field, value));
+          }
+        });
+
+    return lines.stream().collect(Collectors.joining(", ", "{ ", " }"));
+  }
+
   private static boolean isExtensionValid(Problem.Extension extension) {
     return extension != null && extension.getKey() != null && extension.getValue() != null;
   }
+
+  private String getObjectLine(String field, Object value) {
+    String className = value.getClass().getSimpleName();
+    return "\"" + field + "\" : \"" + className + ":" + escape(value.toString()) + "\"";
+  }
 }

problem4j-core/src/test/java/io/github/problem4j/core/DummyObject.java

@@ -0,0 +1,38 @@
+/*
+ * Copyright (c) 2025 Damian Malczewski
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+package io.github.problem4j.core;
+
+/**
+ * Simple object used for testing situations where extension is not a primitive nor simple object.
+ */
+class DummyObject {
+
+  private final String value;
+
+  DummyObject(String value) {
+    this.value = value;
+  }
+
+  @Override
+  public String toString() {
+    return value;
+  }
+}

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

@@ -312,4 +312,88 @@ void givenAssigningTheSameExtensionLater_shouldOverwriteEarlierValues() {
     assertThat(problem.getExtensions()).containsExactly("k");
     assertThat(problem.getExtensionMembers()).isEqualTo(mapOf("k", "v2"));
   }
+
+  @Test
+  void givenAllFieldsPopulated_whenToString_thenContainsAllFieldsProperly() {
+    URI type = URI.create("https://example.com/problem");
+    String title = "Test Problem";
+    int status = 400;
+    String detail = "Something went wrong";
+    URI instance = URI.create("https://example.com/instance");
+    Map<String, Object> extensions = new HashMap<>();
+    extensions.put("stringExt", "value");
+    extensions.put("numberExt", 42);
+    extensions.put("booleanExt", true);
+    extensions.put("objectExt", new DummyObject("foo"));
+
+    ProblemBuilder builder =
+        Problem.builder().type(type).title(title).status(status).detail(detail).instance(instance);
+    extensions.forEach(builder::extension);
+
+    String result = builder.toString();
+
+    assertThat(result).contains("\"type\" : \"" + JsonEscape.escape(type.toString()) + "\"");
+    assertThat(result).contains("\"title\" : \"" + JsonEscape.escape(title) + "\"");
+    assertThat(result).contains("\"status\" : " + status);
+    assertThat(result).contains("\"detail\" : \"" + JsonEscape.escape(detail) + "\"");
+    assertThat(result)
+        .contains("\"instance\" : \"" + JsonEscape.escape(instance.toString()) + "\"");
+    assertThat(result).contains("\"stringExt\" : \"" + JsonEscape.escape("value") + "\"");
+    assertThat(result).contains("\"numberExt\" : 42");
+    assertThat(result).contains("\"booleanExt\" : true");
+    assertThat(result).contains("\"objectExt\" : \"DummyObject:" + JsonEscape.escape("foo") + "\"");
+  }
+
+  @Test
+  void givenNullExtensionsAndNullableFields_whenToString_thenOmitsNulls() {
+    ProblemBuilder builder = Problem.builder().status(200);
+    String result = builder.toString();
+    assertThat(result).isEqualTo("{ \"status\" : 200 }");
+  }
+
+  @Test
+  void givenOnlyStringExtensions_whenToString_thenContainsQuotedStrings() {
+    ProblemBuilder builder = Problem.builder();
+    builder.extension("ext1", "value1");
+    builder.extension("ext2", "value2");
+    String result = builder.toString();
+    assertThat(result).contains("\"ext1\" : \"" + JsonEscape.escape("value1") + "\"");
+    assertThat(result).contains("\"ext2\" : \"" + JsonEscape.escape("value2") + "\"");
+  }
+
+  @Test
+  void givenOnlyNumberExtensions_whenToString_thenContainsNumbers() {
+    ProblemBuilder builder = Problem.builder();
+    builder.extension("ext1", 123);
+    builder.extension("ext2", 456.78);
+    String result = builder.toString();
+    assertThat(result).contains("\"ext1\" : 123");
+    assertThat(result).contains("\"ext2\" : 456.78");
+  }
+
+  @Test
+  void givenOnlyBooleanExtensions_whenToString_thenContainsBooleans() {
+    ProblemBuilder builder = Problem.builder();
+    builder.extension("flag1", true);
+    builder.extension("flag2", false);
+    String result = builder.toString();
+    assertThat(result).contains("\"flag1\" : true");
+    assertThat(result).contains("\"flag2\" : false");
+  }
+
+  @Test
+  void givenNonPrimitiveExtension_whenToString_thenUsesClassNamePrefix() {
+    ProblemBuilder builder = Problem.builder();
+    builder.extension("obj", new DummyObject("bar"));
+    String result = builder.toString();
+    assertThat(result).contains("\"obj\" : \"DummyObject:" + JsonEscape.escape("bar") + "\"");
+  }
+
+  @Test
+  void givenStringWithSpecialCharacters_whenToString_thenProperlyEscapes() {
+    ProblemBuilder builder = Problem.builder();
+    builder.extension("ext", "a\"b\\c\nd");
+    String result = builder.toString();
+    assertThat(result).contains("\"ext\" : \"" + JsonEscape.escape("a\"b\\c\nd") + "\"");
+  }
 }

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

@@ -244,18 +244,4 @@ void givenTwoDifferentExtensions_shouldHaveDifferentHashCodes() {
 
     assertThat(ext1.hashCode()).isNotEqualTo(ext2.hashCode());
   }
-
-  private static class DummyObject {
-
-    private final String value;
-
-    DummyObject(String value) {
-      this.value = value;
-    }
-
-    @Override
-    public String toString() {
-      return value;
-    }
-  }
 }