Throwing JsonParseException when the user expected class doesn't match the JSON input during deserialization.

gson/src/main/java/com/google/gson/JsonArrayDeserializationVisitor.java

@@ -42,6 +42,9 @@ final class JsonArrayDeserializationVisitor<T> extends JsonDeserializationVisito
 
     TypeInfo typeInfo = new TypeInfo(targetType);
 
+    if (!json.isJsonArray()) {
+      throw new JsonParseException("Expecting array found: " + json); 
+    }
     JsonArray jsonArray = json.getAsJsonArray();
     if (typeInfo.isArray()) {
       TypeInfoArray arrayTypeInfo = TypeInfoFactory.getTypeInfoForArray(targetType);
@@ -55,6 +58,9 @@ final class JsonArrayDeserializationVisitor<T> extends JsonDeserializationVisito
   }
 
   public void visitArray(Object array, Type arrayType) {
+    if (!json.isJsonArray()) {
+      throw new JsonParseException("Expecting array found: " + json); 
+    }
     JsonArray jsonArray = json.getAsJsonArray();
     TypeInfoArray arrayTypeInfo = TypeInfoFactory.getTypeInfoForArray(arrayType);
     for (int i = 0; i < jsonArray.size(); i++) {
@@ -82,18 +88,21 @@ final class JsonArrayDeserializationVisitor<T> extends JsonDeserializationVisito
   // instead.
 
   public void startVisitingObject(Object node) {
-    throw new UnsupportedOperationException();
+    throw new JsonParseException("Expecting array but found object: " + node);
   }
 
   public void visitArrayField(Field f, Type typeOfF, Object obj) {
-    throw new UnsupportedOperationException();
+    throw new JsonParseException("Expecting array but found array field " + f.getName() + ": "
+        + obj);
   }
 
   public void visitObjectField(Field f, Type typeOfF, Object obj) {
-    throw new UnsupportedOperationException();
+    throw new JsonParseException("Expecting array but found object field " + f.getName() + ": " 
+        + obj);
   }
 
   public boolean visitFieldUsingCustomHandler(Field f, Type actualTypeOfField, Object parent) {
-    throw new UnsupportedOperationException();
+    throw new JsonParseException("Expecting array but found field " + f.getName() + ": " 
+        + parent);
   }
 }

gson/src/main/java/com/google/gson/JsonObjectDeserializationVisitor.java

@@ -47,11 +47,14 @@ final class JsonObjectDeserializationVisitor<T> extends JsonDeserializationVisit
 
   public void visitArray(Object array, Type componentType) {
     // should not be called since this case should invoke JsonArrayDeserializationVisitor
-    throw new IllegalStateException();
+    throw new JsonParseException("Expecting object but found array: " + array);
   }
 
   public void visitObjectField(Field f, Type typeOfF, Object obj) {
     try {
+      if (!json.isJsonObject()) {
+        throw new JsonParseException("Expecting object found: " + json); 
+      }
       JsonObject jsonObject = json.getAsJsonObject();
       String fName = getFieldName(f);
       JsonElement jsonChild = jsonObject.get(fName);
@@ -68,6 +71,9 @@ final class JsonObjectDeserializationVisitor<T> extends JsonDeserializationVisit
 
   public void visitArrayField(Field f, Type typeOfF, Object obj) {
     try {
+      if (!json.isJsonObject()) {
+        throw new JsonParseException("Expecting object found: " + json); 
+      }
       JsonObject jsonObject = json.getAsJsonObject();
       String fName = getFieldName(f);
       JsonArray jsonChild = (JsonArray) jsonObject.get(fName);
@@ -90,6 +96,9 @@ final class JsonObjectDeserializationVisitor<T> extends JsonDeserializationVisit
   public boolean visitFieldUsingCustomHandler(Field f, Type actualTypeOfField, Object parent) {
     try {
       String fName = getFieldName(f);
+      if (!json.isJsonObject()) {
+        throw new JsonParseException("Expecting object found: " + json); 
+      }
       JsonElement child = json.getAsJsonObject().get(fName);
       if (child == null) {
         return true;

gson/src/test/java/com/google/gson/functional/JsonParserTest.java

@@ -62,7 +62,7 @@ public class JsonParserTest extends TestCase {
     try {
       gson.fromJson(array, BagOfPrimitives.class);
       fail("BagOfPrimitives is not an array");
-    } catch (UnsupportedOperationException expected) { }
+    } catch (JsonParseException expected) { }
   }
   
   public void testBadFieldTypeForCustomDeserializerCustomTree() {
@@ -93,7 +93,7 @@ public class JsonParserTest extends TestCase {
     try {
       gson.fromJson(obj, Nested.class);
       fail("Nested has field BagOfPrimitives which is not an array");
-    } catch (IllegalStateException expected) { }
+    } catch (JsonParseException expected) { }
   }
 
   public void testChangingCustomTreeAndDeserializing() {