1003: Implement JSONObject.fromJson() with unit tests

src/main/java/org/json/JSONBuilder.java

@@ -0,0 +1,122 @@
+package org.json;
+
+import java.util.Map;
+import java.util.HashMap;
+import java.util.List;
+import java.util.ArrayList;
+import java.util.Set;
+import java.util.HashSet;
+import java.util.Collection;
+import java.util.function.Function;
+import java.util.function.Supplier;
+
+/**
+ * The {@code JSONBuilder} class provides a configurable mechanism for
+ * defining how different Java types are handled during JSON serialization
+ * or deserialization.
+ *
+ * <p>This class maintains two internal mappings:
+ * <ul>
+ *   <li>A {@code classMapping} which maps Java classes to functions that convert
+ *   an input {@code Object} into an appropriate instance of that class.</li>
+ *   <li>A {@code collectionMapping} which maps collection interfaces (like {@code List}, {@code Set}, {@code Map})
+ *   to supplier functions that create new instances of concrete implementations (e.g., {@code ArrayList} for {@code List}).</li>
+ * </ul>
+ *
+ * <p>The mappings are initialized with default values for common primitive wrapper types
+ * and collection interfaces, but they can be modified at runtime using setter methods.
+ *
+ * <p>This class is useful in custom JSON serialization/deserialization frameworks where
+ * type transformation and collection instantiation logic needs to be flexible and extensible.
+ */
+public class JSONBuilder {
+
+  /**
+   * A mapping from Java classes to functions that convert a generic {@code Object}
+   * into an instance of the target class.
+   *
+   * <p>Examples of default mappings:
+   * <ul>
+   *   <li>{@code int.class} or {@code Integer.class} -> Converts a {@code Number} to {@code int}</li>
+   *   <li>{@code boolean.class} or {@code Boolean.class} -> Identity function</li>
+   *   <li>{@code String.class} -> Identity function</li>
+   * </ul>
+   */
+  private static final Map<Class<?>, Function<Object, ?>> classMapping = new HashMap<>();
+
+  /**
+   * A mapping from collection interface types to suppliers that produce
+   * instances of concrete collection implementations.
+   *
+   * <p>Examples of default mappings:
+   * <ul>
+   *   <li>{@code List.class} -> {@code ArrayList::new}</li>
+   *   <li>{@code Set.class} -> {@code HashSet::new}</li>
+   *   <li>{@code Map.class} -> {@code HashMap::new}</li>
+   * </ul>
+   */
+  private static final Map<Class<?>, Supplier<?>> collectionMapping = new HashMap<>();
+  
+   // Static initializer block to populate default mappings
+   static {
+    classMapping.put(int.class, s -> ((Number) s).intValue());
+    classMapping.put(Integer.class, s -> ((Number) s).intValue());
+    classMapping.put(double.class, s -> ((Number) s).doubleValue());
+    classMapping.put(Double.class, s -> ((Number) s).doubleValue());
+    classMapping.put(float.class, s -> ((Number) s).floatValue());
+    classMapping.put(Float.class, s -> ((Number) s).floatValue());
+    classMapping.put(long.class, s -> ((Number) s).longValue());
+    classMapping.put(Long.class, s -> ((Number) s).longValue());
+    classMapping.put(boolean.class, s -> s);
+    classMapping.put(Boolean.class, s -> s);
+    classMapping.put(String.class, s -> s);
+
+    collectionMapping.put(List.class, ArrayList::new);
+    collectionMapping.put(Set.class, HashSet::new);
+    collectionMapping.put(Map.class, HashMap::new);
+   }
+
+  /**
+   * Returns the current class-to-function mapping used for type conversions.
+   *
+   * @return a map of classes to functions that convert an {@code Object} to that class
+   */
+   public Map<Class<?>, Function<Object, ?>> getClassMapping() {
+     return this.classMapping;
+   }
+
+  /**
+   * Returns the current collection-to-supplier mapping used for instantiating collections.
+   *
+   * @return a map of collection interface types to suppliers of concrete implementations
+   */
+   public Map<Class<?>, Supplier<?>> getCollectionMapping() {
+     return this.collectionMapping;
+   }
+
+  /**
+   * Adds or updates a type conversion function for a given class.
+   *
+   * <p>This allows users to customize how objects are converted into specific types
+   * during processing (e.g., JSON deserialization).
+   *
+   * @param clazz the target class for which the conversion function is to be set
+   * @param function a function that takes an {@code Object} and returns an instance of {@code clazz}
+   */
+   public void setClassMapping(Class<?> clazz, Function<Object, ?> function) {
+     classMapping.put(clazz, function);
+   }
+
+  /**
+   * Adds or updates a supplier function for instantiating a collection type.
+   *
+   * <p>This allows customization of which concrete implementation is used for
+   * interface types like {@code List}, {@code Set}, or {@code Map}.
+   *
+   * @param clazz the collection interface class (e.g., {@code List.class})
+   * @param function a supplier that creates a new instance of a concrete implementation
+   */
+   public void setCollectionMapping(Class<?> clazz, Supplier<?> function) {
+     collectionMapping.put(clazz, function);
+   }
+}

src/main/java/org/json/JSONObject.java

@@ -17,6 +17,10 @@ import java.math.BigInteger;
 import java.util.*;
 import java.util.Map.Entry;
 import java.util.regex.Pattern;
+import java.util.function.Function;
+import java.util.function.Supplier;
+import java.lang.reflect.ParameterizedType;
+import java.lang.reflect.Type;
 
 /**
  * A JSONObject is an unordered collection of name/value pairs. Its external
@@ -119,6 +123,12 @@ public class JSONObject {
      */
     static final Pattern NUMBER_PATTERN = Pattern.compile("-?(?:0|[1-9]\\d*)(?:\\.\\d+)?(?:[eE][+-]?\\d+)?");
 
+
+    /**
+     * A Builder class for handling the conversion of JSON to Object.
+     */
+    private JSONBuilder builder;
+
     /**
      * The map where the JSONObject's properties are kept.
      */
@@ -212,6 +222,25 @@ public class JSONObject {
         }
     }
 
+    /**
+     * Construct a JSONObject with JSONBuilder for conversion from JSON to POJO
+     *
+     * @param builder builder option for json to POJO
+     */
+    public JSONObject(JSONBuilder builder) {
+      this();
+      this.builder = builder;
+    }
+
+    /**
+     * Method to set JSONBuilder.
+     *
+     * @param builder
+     */
+    public void setJSONBuilder(JSONBuilder builder) {
+      this.builder = builder;
+    }
+
     /**
      * Parses entirety of JSON object
      *
@@ -3207,4 +3236,121 @@ public class JSONObject {
             "JavaBean object contains recursively defined member variable of key " + quote(key)
         );
     }
+
+    /**
+     * Deserializes a JSON string into an instance of the specified class.
+     *
+     * <p>This method attempts to map JSON key-value pairs to the corresponding fields
+     * of the given class. It supports basic data types including int, double, float,
+     * long, and boolean (as well as their boxed counterparts). The class must have a 
+     * no-argument constructor, and the field names in the class must match the keys 
+     * in the JSON string.
+     *
+     * @param clazz the class of the object to be returned
+     * @param <T> the type of the object
+     * @return an instance of type T with fields populated from the JSON string
+     */
+    public <T> T fromJson(Class<T> clazz) {
+      try {
+        T obj = clazz.getDeclaredConstructor().newInstance();
+        if (this.builder == null) {
+          this.builder = new JSONBuilder();
+        }
+        Map<Class<?>, Function<Object, ?>> classMapping = this.builder.getClassMapping();
+
+        for (Field field: clazz.getDeclaredFields()) {
+          field.setAccessible(true);
+          String fieldName = field.getName();
+          if (this.has(fieldName)) {
+            Object value = this.get(fieldName);
+            Class<?> pojoClass = field.getType();
+            if (classMapping.containsKey(pojoClass)) {
+              field.set(obj, classMapping.get(pojoClass).apply(value));
+            } else {
+              if (value.getClass() == JSONObject.class) {
+                field.set(obj, fromJson((JSONObject) value, pojoClass));
+              } else if (value.getClass() == JSONArray.class) {
+                if (Collection.class.isAssignableFrom(pojoClass)) {
+
+                  Collection<?> nestedCollection = fromJsonArray((JSONArray) value,
+                      (Class<? extends Collection>) pojoClass,
+                      field.getGenericType());
+
+                  field.set(obj, nestedCollection);
+                }
+              } 
+            }
+          }
+        }
+        return obj;
+      } catch (NoSuchMethodException | InstantiationException | IllegalAccessException | InvocationTargetException e) {
+        throw new JSONException(e);
+      }
+    }
+
+    private <T> Collection<T> fromJsonArray(JSONArray jsonArray, Class<?> collectionType, Type elementType) throws JSONException {
+      try {
+        Map<Class<?>, Function<Object, ?>> classMapping = this.builder.getClassMapping();
+        Map<Class<?>, Supplier<?>> collectionMapping = this.builder.getCollectionMapping();
+        Collection<T> collection = (Collection<T>) (collectionMapping.containsKey(collectionType) ? 
+            collectionMapping.get(collectionType).get() 
+            : collectionType.getDeclaredConstructor().newInstance());
+
+
+        Class<?> innerElementClass = null;
+        Type innerElementType = null;
+        if (elementType instanceof ParameterizedType) {
+          ParameterizedType pType = (ParameterizedType) elementType;
+          innerElementType = pType.getActualTypeArguments()[0];
+          innerElementClass = (innerElementType instanceof Class) ? 
+              (Class<?>) innerElementType 
+            : (Class<?>) ((ParameterizedType) innerElementType).getRawType();
+        } else {
+          innerElementClass = (Class<?>) elementType;
+        }
+
+        for (int i = 0; i < jsonArray.length(); i++) {
+          Object jsonElement = jsonArray.get(i);
+          if (classMapping.containsKey(innerElementClass)) {
+            collection.add((T) classMapping.get(innerElementClass).apply(jsonElement));
+          } else if (jsonElement.getClass() == JSONObject.class) {
+            collection.add((T) ((JSONObject) jsonElement).fromJson(innerElementClass));
+          } else if (jsonElement.getClass() == JSONArray.class) {
+            if (Collection.class.isAssignableFrom(innerElementClass)) {
+
+              Collection<?> nestedCollection = fromJsonArray((JSONArray) jsonElement,
+                  innerElementClass,
+                  innerElementType);
+
+              collection.add((T) nestedCollection);
+            } else {
+              throw new JSONException("Expected collection type for nested JSONArray, but got: " + innerElementClass);
+            }
+          } else {
+            collection.add((T) jsonElement.toString());
+          }
+        }
+        return collection;
+      } catch (NoSuchMethodException | InstantiationException | IllegalAccessException | InvocationTargetException e) {
+        throw new JSONException(e);
+      }
+    }
+
+    /**
+     * Deserializes a JSON string into an instance of the specified class.
+     *
+     * <p>This method attempts to map JSON key-value pairs to the corresponding fields
+     * of the given class. It supports basic data types including int, double, float,
+     * long, and boolean (as well as their boxed counterparts). The class must have a 
+     * no-argument constructor, and the field names in the class must match the keys 
+     * in the JSON string.
+     *
+     * @param object JSONObject of internal class
+     * @param clazz the class of the object to be returned
+     * @param <T> the type of the object
+     * @return an instance of type T with fields populated from the JSON string
+     */
+    private <T> T fromJson(JSONObject object, Class<T> clazz) {
+      return object.fromJson(clazz);
+    }
 }

src/test/java/org/json/junit/JSONObjectTest.java

@@ -33,6 +33,7 @@ import org.json.JSONObject;
 import org.json.JSONPointerException;
 import org.json.JSONParserConfiguration;
 import org.json.JSONString;
+import org.json.JSONBuilder;
 import org.json.JSONTokener;
 import org.json.ParserConfiguration;
 import org.json.XML;
@@ -4095,4 +4096,219 @@ public class JSONObjectTest {
         assertTrue("JSONObject should be empty", jsonObject.isEmpty());
     }
 
+
+    @Test
+    public void jsonObjectParseFromJson_0() {
+      JSONObject object = new JSONObject();
+      object.put("number", 12);
+      object.put("name", "Alex");
+      object.put("longNumber", 1500000000L);
+      String jsonObject = object.toString();
+      CustomClass customClass = object.fromJson(CustomClass.class);
+      CustomClass compareClass = new CustomClass(12, "Alex", 1500000000L);
+      assertEquals(customClass, compareClass);
+    }
+
+    public static class CustomClass {
+      public int number;
+      public String name;
+      public Long longNumber;
+
+      public CustomClass() {}
+      public CustomClass (int number, String name, Long longNumber) {
+        this.number = number;
+        this.name = name;
+        this.longNumber = longNumber;
+      }
+      @Override
+      public boolean equals(Object o) {
+        CustomClass customClass = (CustomClass) o;
+
+        return (this.number == customClass.number
+          && this.name.equals(customClass.name)
+          && this.longNumber.equals(customClass.longNumber));
+      }
+    }
+
+    @Test
+    public void jsonObjectParseFromJson_1() {
+      JSONBuilder builder = new JSONBuilder();
+      builder.setClassMapping(java.time.LocalDateTime.class, s -> java.time.LocalDateTime.parse((String)s));
+      JSONObject object = new JSONObject(builder);
+      java.time.LocalDateTime localDateTime = java.time.LocalDateTime.now();
+      object.put("localDate", localDateTime.toString());
+      CustomClassA customClassA = object.fromJson(CustomClassA.class);
+      CustomClassA compareClassClassA = new CustomClassA(localDateTime);
+      assertEquals(customClassA, compareClassClassA);
+    }
+
+    public static class CustomClassA {
+      public java.time.LocalDateTime localDate;
+
+      public CustomClassA() {}
+      public CustomClassA(java.time.LocalDateTime localDate) {
+        this.localDate = localDate;
+      }
+
+      @Override
+      public boolean equals(Object o) {
+        CustomClassA classA = (CustomClassA) o;
+        return this.localDate.equals(classA.localDate);
+      }
+    }
+    
+    @Test
+    public void jsonObjectParseFromJson_2() {
+      JSONObject object = new JSONObject();
+      object.put("number", 12);
+
+      JSONObject classC = new JSONObject();
+      classC.put("stringName", "Alex");
+      classC.put("longNumber", 123456L);
+
+      object.put("classC", classC);
+
+      CustomClassB customClassB = object.fromJson(CustomClassB.class);
+      CustomClassC classCObject = new CustomClassC("Alex", 123456L);
+      CustomClassB compareClassB = new CustomClassB(12, classCObject);
+      assertEquals(customClassB, compareClassB);
+    }
+
+    public static class CustomClassB {
+      public int number;
+      public CustomClassC classC;
+
+      public CustomClassB() {}
+      public CustomClassB(int number, CustomClassC classC) {
+        this.number = number;
+        this.classC = classC;
+      }
+
+      @Override
+      public boolean equals(Object o) {
+        CustomClassB classB = (CustomClassB) o;
+        return this.number == classB.number
+          && this.classC.equals(classB.classC);
+      }
+    }
+
+    public static class CustomClassC {
+      public String stringName;
+      public Long longNumber;
+
+      public CustomClassC() {}
+      public CustomClassC(String stringName, Long longNumber) {
+        this.stringName = stringName;
+        this.longNumber = longNumber;
+      }
+
+      public JSONObject toJSON() {
+        JSONObject object = new JSONObject();
+        object.put("stringName", this.stringName);
+        object.put("longNumber", this.longNumber);
+        return object;
+      }
+
+      @Override
+      public boolean equals(Object o) {
+        CustomClassC classC = (CustomClassC) o;
+        return this.stringName.equals(classC.stringName)
+          && this.longNumber.equals(classC.longNumber);
+      }
+
+      @Override
+      public int hashCode() {
+        return java.util.Objects.hash(stringName, longNumber);
+      }
+    }
+
+    @Test
+    public void jsonObjectParseFromJson_3() {
+      JSONObject object = new JSONObject();
+      JSONArray array = new JSONArray();
+      array.put("test1");
+      array.put("test2");
+      array.put("test3");
+      object.put("stringList", array);
+
+      CustomClassD customClassD = object.fromJson(CustomClassD.class);
+      CustomClassD compareClassD = new CustomClassD(Arrays.asList("test1", "test2", "test3"));
+      assertEquals(customClassD, compareClassD);
+    }
+
+    public static class CustomClassD {
+      public List<String> stringList;
+
+      public CustomClassD() {}
+      public CustomClassD(List<String> stringList) {
+        this.stringList = stringList;
+      }
+
+      @Override
+      public boolean equals(Object o) {
+        CustomClassD classD = (CustomClassD) o;
+        return this.stringList.equals(classD.stringList);
+      }
+    }
+
+    @Test
+    public void jsonObjectParseFromJson_4() {
+      JSONObject object = new JSONObject();
+      JSONArray array = new JSONArray();
+      array.put(new CustomClassC("test1", 1L).toJSON());
+      array.put(new CustomClassC("test2", 2L).toJSON());
+      object.put("listClassC", array);
+
+      CustomClassE customClassE = object.fromJson(CustomClassE.class);
+      CustomClassE compareClassE = new CustomClassE(java.util.Arrays.asList(
+      new CustomClassC("test1", 1L),
+      new CustomClassC("test2", 2L)));
+      assertEquals(customClassE, compareClassE);
+    }
+
+    public static class CustomClassE {
+      public List<CustomClassC> listClassC;
+
+      public CustomClassE() {}
+      public CustomClassE(List<CustomClassC> listClassC) {
+        this.listClassC = listClassC;
+      }
+
+      @Override
+      public boolean equals(Object o) {
+        CustomClassE classE = (CustomClassE) o;
+        return this.listClassC.equals(classE.listClassC);
+      }
+    }
+
+    @Test
+    public void jsonObjectParseFromJson_5() {
+      JSONObject object = new JSONObject();
+      JSONArray array = new JSONArray();
+      array.put(Arrays.asList("A", "B", "C"));
+      array.put(Arrays.asList("D", "E"));
+      object.put("listOfString", array);
+
+      CustomClassF customClassF = object.fromJson(CustomClassF.class);
+      List<List<String>> listOfString = new ArrayList<>();
+      listOfString.add(Arrays.asList("A", "B", "C"));
+      listOfString.add(Arrays.asList("D", "E"));
+      CustomClassF compareClassF = new CustomClassF(listOfString);
+      assertEquals(customClassF, compareClassF);
+    }
+
+    public static class CustomClassF {
+      public List<List<String>> listOfString;
+
+      public CustomClassF() {}
+      public CustomClassF(List<List<String>> listOfString) {
+        this.listOfString = listOfString;
+      }
+
+      @Override
+      public boolean equals(Object o) {
+        CustomClassF classF = (CustomClassF) o;
+        return this.listOfString.equals(classF.listOfString);
+      }
+    }
 }