Deserialize a List<T> object with Gson

I want to transfer a list object via Google Gson, but I don't know how to deserialize generic types. What I tried after looking at this answer:

MyClass mc = new Gson().fromJson(result, new List<MyClass>() {}.getClass());

But then I get an error in Eclipse:

"The type new List<MyClass>() {} must implement the inherited abstract method..."

And if I use a quick fix I get over 20 method stubs. I am sure there is an easier solution. Now I have this:

Type listType = new TypeToken<List<MyClass>>() {}.getType();

MyClass mc = new Gson().fromJson(result, listType);

I get the following exception at the fromJson line:

java.lang.NullPointerException
at org.apache.harmony.luni.lang.reflect.ListOfTypes.length(ListOfTypes.java:47)
at org.apache.harmony.luni.lang.reflect.ImplForType.toString(ImplForType.java:83)
at java.lang.StringBuilder.append(StringBuilder.java:203)
at com.google.gson.JsonDeserializerExceptionWrapper.deserialize(JsonDeserializerExceptionWrapper.java:56)
at com.google.gson.JsonDeserializationVisitor.invokeCustomDeserializer(JsonDeserializationVisitor.java:88)
at com.google.gson.JsonDeserializationVisitor.visitUsingCustomHandler(JsonDeserializationVisitor.java:76)
at com.google.gson.ObjectNavigator.accept(ObjectNavigator.java:106)
at com.google.gson.JsonDeserializationContextDefault.fromJsonArray(JsonDeserializationContextDefault.java:64)
at com.google.gson.JsonDeserializationContextDefault.deserialize(JsonDeserializationContextDefault.java:49)
at com.google.gson.Gson.fromJson(Gson.java:568)
at com.google.gson.Gson.fromJson(Gson.java:515)
at com.google.gson.Gson.fromJson(Gson.java:484)
at com.google.gson.Gson.fromJson(Gson.java:434)

I catch JsonParseExceptions and result is not null. I checked listType with the debugger and got:

- list Type
   - args = ListOfTypes
       - list = null
       - resolvedTypes = Type[ 1 ]
   - loader = PathClassLoader
   - ownerType0 = null
   - ownerTypeRes = null
   - rawType = Class (java.util.ArrayList)
   - rawTypeName = "java.util.ArrayList"

So it seems the getClass invocation didn't work. The Gson User Guide mentions a runtime exception that should happen during parsing a generic type to JSON. I did it wrong (not shown above) just as in the example, but didn't get that exception. So I changed the serialization as the user guide suggested. Didn't help though.

Solution

Method to deserialize generic collection:

import java.lang.reflect.Type;
import com.google.gson.reflect.TypeToken;

...

Type listType = new TypeToken<ArrayList<YourClass>>(){}.getType();
List<YourClass> yourClassList = new Gson().fromJson(jsonArray, listType);

Since several people in the comments have mentioned it, here's an explanation of how the TypeToken class is being used. The construction new TypeToken<...>() {}.getType() captures a compile-time type (between the < and >) into a runtime java.lang.reflect.Type object. Unlike a Class object, which can only represent a raw (erased) type, the Type object can represent any type in the Java language, including a parameterized instantiation of a generic type.

The TypeToken class itself does not have a public constructor, because you're not supposed to construct it directly. Instead, you always construct an anonymous subclass (hence the {}, which is a necessary part of this expression).

Due to type erasure, the TypeToken class is only able to capture types that are fully known at compile time. (That is, you can't do new TypeToken<List<T>>() {}.getType() for a type parameter T.)

For more information, see the documentation for the TypeToken class.