How to call method and return its value with UnityPlayer.UnitySendMessage

Question

How can I call this C# method from Java then return the string value?

public string getTest () { return "test"; }

This is what I've tried:

String str = UnityPlayer.UnitySendMessage("ProfileSave", "getTest","");

I am getting the below error

String str=UnityPlayer.UnitySendMessage("ProfileSave", "getTest","");                                 
^
 required: String  
 found:    void

Answer 1

UnityPlayer.UnitySendMessage is a void function and does not return a value. Take a look at the UnitySendMessageExtension function implementation below. It is similar to turnipinindia's answer but it returns a value. It only returns a string so you have to convert that string to whatever variable type you are working with.

Java:

public final class MyPlugin
{
    //Make class static variable so that the callback function is sent to one instance of this class.
     public static MyPlugin testInstance;

     public static MyPlugin instance()
     {
         if(testInstance == null)
         {
             testInstance = new MyPlugin();
         }
         return testInstance;
     }

    string result = "";


    public string UnitySendMessageExtension(string gameObject, string functionName, string funcParam)
    {
        UnityPlayer.UnitySendMessage(gameObject, functionName, funcParam);
        string tempResult = result;
        return tempResult;
    }

    //Receives result from C# and saves it to result  variable
    void receiveResult(string value)
    {
        result = "";//Clear old data
        result = value; //Get new one
    }
}

C#:

class TestScript: MonoBehaviour
{
    //Sends the data from PlayerPrefs to the receiveResult function in Java
    void sendResultToJava(float value)
    {
        using(AndroidJavaClass javaPlugin = new AndroidJavaClass("com.company.product.MyPlugin"))
        {
             AndroidJavaObject pluginInstance = javaPlugin.CallStatic("instance");
             pluginInstance.Call("receiveResult",value.ToString());
        }
    }

    //Called from Java to get the saved PlayerPrefs
    void testFunction(string key)
    {
        float value = PlayerPrefs.GetFloat(key) //Get the saved value from key
        sendResultToJava(value); //Send the value to Java
    }
}

Usage from Java:

String str = UnitySendMessageExtension("ProfileLoad", "testFunction","highScore");  

This will work with any C# function. Just make sure that you call sendResultToJava function at the end of the C# function like we did in the testFunction function, if you want it to return value.