Copy the properties from one instance of an Apex Class to another

Question

I am attempting to copy the properties from one Apex class instance to another (without hard coding the property names). See the below example:

public class Post{      

    public String name          { get; set; }
    public String topic         { get; set; }
    public String description   { get; set; }

    public Post(String jsonString){

        Post self = this;

        Post post = (Post)JSON.deserialize(jsonString, Post.class);

        self = post;

        //this = post; "Error: Expression cannot be assigned"

        System.debug(self); // {"name":"John Smith", "topic":"Test Topic", "description":"Test descriptions."}

        System.debug(this); // {"name":null, "topic":null, "description":null}

    }

}   

Post post = new Post('{"name":"John Smith", "topic":"Test Topic", "description":"Test descriptions."}');

//post is equal to {"name":null, "topic":null, "description":null}

My question is, how can I set the class properties en masse without explicitly referencing the class properties.

If that is not possible, is it possible to call a wrapper class' constructor during JSON deserialization?

For example:

public class Post{      

    public String apiKey { get; set; }

    public String name          { get; set; }
    public String topic         { get; set; }
    public String description   { get; set; }

    public Post(){
        this.apiKey = 'abcd1234';
    }

}   

Post post = (Post)JSON.deserialize('{"name":"John Smith", "topic":"Test Topic", "description":"Test descriptions."}', Post.class);

//post.apiKey == null;

Answer 1

No, you can't call the constructor after deserialization. You might want to look into the Factory Pattern. It would look something like:

public class Post
{
    public static Post factory(String payload)
    {
        Post instance = (Post)JSON.deserialize(payload, Post.class);
        instance.apiKey = 'someValue';
        return instance;
    }

    public String apiKey { get; private set; }
    public String someOtherProperty { get; private set; }
    public Post()
    {
        // do stuff
    }
}

You could even add an instance clone method if you want that looks something like:

public Post clone()
{
    return factory(JSON.serialize(this));
}

Answer 2

Based on your second example, I wrote up a snippet you could use to set some fields via a constructor, and the rest through a JSON String.

Using a DummyObject, with two fields included, one, set via a no argument constructor, and the other, set using the Copy method, which could take either a JSON string or a concrete object.

public class DummyObject {

    private String dummyField { get; set; }
    public String someField { get; set; }

    public DummyObject() {
        dummyField = 'Constructor Only'; 
    }

    public void Copy(DummyObject dummy) {
        this.someField = dummy.someField; 
    }
}

The snippet takes a JSON string with a field value for someField, then creates a Type based on the expected type (likely obtained from the JSON), before creating a new instance of DummyObject, setting half the fields. We then call Copy, using the JSON data provided, and deserializing it into a type.

String someJson = '{"someField":"Json Only"}';

Type typeOf = Type.forName('DummyObject');

DummyObject dummy = (DummyObject)typeOf.newInstance();

dummy.Copy(
    (DummyObject)JSON.deserialize(someJson, typeOf)
); 

System.debug(JSON.serializePretty(dummy)); 

I'm not sure how useful this is (a no argument constructor is kind of a pain, but you can have other constructors as well), and the real downside is that unless your properties are stored in a Map<String, Object>, you won't be able to get at them dynamically. However, by moving the Copy from the caller to the object itself, we can at least keep changes local to the file instead of requiring the parser to dig through the JSON and set field values,