Java – Google Gson – deserialize list object? (generic type)

genericsgsonjavajson

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 (BalusC's answer):

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

but then I get an error in eclipse saying "The type new List(){} must implement the inherited abstract method…" and if I use a quick fix I get a monster of over 20 method stubs.

I am pretty sure that there is an easier solution, but I seem unable to find it!

Edit:

Now I have

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

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

However, I do 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 do catch JsonParseExceptions and "result" is not null.

I checked listType with the debugger and got the following:

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 properly. Any suggestions…?

Edit2:
I'v checked on the Gson User Guide. It 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 at all. So I changed the serialization as in the user guide suggested. Didn't help, though.

Edit3:
Solved, see my answer below.

Best Answer

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.

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You are correct. You can't do new E(). But you can change it to

private static class SomeContainer<E> {
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It's a pain. But it works. Wrapping it in the factory pattern makes it a little more tolerable.

C# – How to clone a generic list in C#

If your elements are value types, then you can just do:

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However, if they are reference types and you want a deep copy (assuming your elements properly implement ICloneable), you could do something like this:

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List<ICloneable> newList = new List<ICloneable>(oldList.Count);

oldList.ForEach((item) =>
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Obviously, replace ICloneable in the above generics and cast with whatever your element type is that implements ICloneable.

If your element type doesn't support ICloneable but does have a copy-constructor, you could do this instead:

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Best Answer

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