Do not leak out a ClassCastException to our clients.

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

@@ -402,7 +402,6 @@ public final class Gson {
    * @throws JsonParseException if json is not a valid representation for an object of type typeOfT
    * @since 1.3
    */
-  @SuppressWarnings("unchecked")
   public <T> T fromJson(JsonElement json, Class<T> classOfT) throws JsonParseException {
     T target = classOfT.cast(fromJson(json, (Type) classOfT));
     return target;

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

@@ -73,10 +73,14 @@ public abstract class JsonElement {
    * first.
    *
    * @return get this element as a {@link JsonObject}.
-   * @throws ClassCastException if the element is of another type.
+   * @throws IllegalStateException if the element is of another type.
    */
   public JsonObject getAsJsonObject() {
-    return (JsonObject) this;
+    if (isJsonObject()) {
+      return (JsonObject) this;
+    } else {
+      throw new IllegalStateException("This is not a JSON Object.");
+    }
   }
 
   /**
@@ -86,10 +90,14 @@ public abstract class JsonElement {
    * first.
    *
    * @return get this element as a {@link JsonArray}.
-   * @throws ClassCastException if the element is of another type.
+   * @throws IllegalStateException if the element is of another type.
    */
   public JsonArray getAsJsonArray() {
-    return (JsonArray) this;
+    if (isJsonArray()) {
+      return (JsonArray) this;
+    } else {
+      throw new IllegalStateException("This is not a JSON Array.");
+    }
   }
 
   /**
@@ -99,10 +107,14 @@ public abstract class JsonElement {
    * first.
    *
    * @return get this element as a {@link JsonPrimitive}.
-   * @throws ClassCastException if the element is of another type.
+   * @throws IllegalStateException if the element is of another type.
    */
   public JsonPrimitive getAsJsonPrimitive() {
-    return (JsonPrimitive) this;
+    if (isJsonPrimitive()) {
+      return (JsonPrimitive) this;
+    } else {
+      throw new IllegalStateException("This is not a JSON Primitive.");
+    }
   }
 
   /**
@@ -112,11 +124,15 @@ public abstract class JsonElement {
    * first.
    *
    * @return get this element as a {@link JsonNull}.
-   * @throws ClassCastException if the element is of another type.
+   * @throws IllegalStateException if the element is of another type.
    * @since 1.2
    */
   public JsonNull getAsJsonNull() {
-    return (JsonNull) this;
+    if (isJsonNull()) {
+      return (JsonNull) this;
+    } else {
+      throw new IllegalStateException("This is not a JSON Null.");
+    }
   }
 
   /**

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

@@ -16,14 +16,18 @@
 
 package com.google.gson.functional;
 
-import com.google.gson.Gson;
+import java.io.StringReader;
-import com.google.gson.JsonObject;
-import com.google.gson.JsonParser;
-import com.google.gson.common.TestTypes.BagOfPrimitives;
 
 import junit.framework.TestCase;
 
-import java.io.StringReader;
+import com.google.gson.Gson;
+import com.google.gson.JsonArray;
+import com.google.gson.JsonObject;
+import com.google.gson.JsonParseException;
+import com.google.gson.JsonParser;
+import com.google.gson.JsonPrimitive;
+import com.google.gson.common.TestTypes.BagOfPrimitives;
+import com.google.gson.common.TestTypes.Nested;
 
 /**
  * Functional tests for that use JsonParser and related Gson methods
@@ -49,6 +53,49 @@ public class JsonParserTest extends TestCase {
     assertEquals("foo", target.stringValue);
   }
   
+  public void testBadTypeForDeserializingCustomTree() {
+    JsonObject obj = new JsonObject();
+    obj.addProperty("stringValue", "foo");
+    obj.addProperty("intValue", 11);
+    JsonArray array = new JsonArray();
+    array.add(obj);
+    try {
+      gson.fromJson(array, BagOfPrimitives.class);
+      fail("BagOfPrimitives is not an array");
+    } catch (UnsupportedOperationException expected) { }
+  }
+  
+  public void testBadFieldTypeForCustomDeserializerCustomTree() {
+    JsonArray array = new JsonArray();
+    array.add(new JsonPrimitive("blah"));
+    JsonObject obj = new JsonObject();
+    obj.addProperty("stringValue", "foo");
+    obj.addProperty("intValue", 11);
+    obj.add("longValue", array);
+
+    try {
+      gson.fromJson(obj, BagOfPrimitives.class);
+      fail("BagOfPrimitives is not an array");
+    } catch (JsonParseException expected) { }
+  }
+
+  public void testBadFieldTypeForDeserializingCustomTree() {
+    JsonArray array = new JsonArray();
+    array.add(new JsonPrimitive("blah"));
+    JsonObject primitive1 = new JsonObject();
+    primitive1.addProperty("string", "foo");
+    primitive1.addProperty("intValue", 11);
+
+    JsonObject obj = new JsonObject();
+    obj.add("primitive1", primitive1);
+    obj.add("primitive2", array);
+    
+    try {
+      gson.fromJson(obj, Nested.class);
+      fail("Nested has field BagOfPrimitives which is not an array");
+    } catch (IllegalStateException expected) { }
+  }
+
   public void testChangingCustomTreeAndDeserializing() {
     StringReader json = 
       new StringReader("{'stringValue':'no message','intValue':10,'longValue':20}");

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

@@ -473,8 +473,10 @@ public class ParameterizedTypesTest extends TestCase {
   private interface Immutable {    
   }
   
-  public static final class Amount<Q extends Quantity> implements
+  public static final class Amount<Q extends Quantity> 
-      Measurable<Q>, Field<Amount<?>>, Serializable, Immutable {
+      implements Measurable<Q>, Field<Amount<?>>, Serializable, Immutable {
+    private static final long serialVersionUID = -7560491093120970437L;
+
     int value = 30;
   }