How to get a javascript function's return value from android webview to application - Java

I want to get the return value of a javascript function inside Android webview to be made accessible to the android application and get it converted to a string.

So I put this code:

String fileName = MyWebView.loadUrl("javascript:android.onData(returnFileName())");

But then it shows this error: Incompatible types. Found: 'void', required: 'java.lang.String'.

How to fix this?

Edit:
Actually I have an input field where the user can enter the name of a file generated as base64(data:)(which was actually a blob but I converted it). On Firefox and Chrome the downloaded file have the name user entered (I used a.download=document.getElementById.value for that) but on webview while downloading(by setting Download listener) the file I had to process the (data:) to a working URI. Now I would like to add the name entered by User to the downloaded file. For that I need to get the value of input field inside webview to Android application.

Thanks in advance.

The prototype of WebView's loadUrl() is:

public void loadUrl(String url)

As such, loadUrl() will never return anything, certainly not the filename as you expect.

Instead, you can use the evaluateJavascript() method of the WebView:

• Create a JavaScript interface in your Java application by extending the WebViewClient class and overriding its onPageFinished() method. In there, inject JavaScript into the WebView and get its return value:

webView.setWebViewClient(new WebViewClient() {
    @Override
    public void onPageFinished(WebView view, String url) {
        webView.evaluateJavascript("yourJavaScriptFunction()", new ValueCallback<String>() {
            @Override
            public void onReceiveValue(String value) {
                // value is the return value of your JavaScript function
            }
        });
    }
});

• In your JavaScript function, make sure to return a value:

function yourJavaScriptFunction() {
    return "hello world";
}

• Once the WebView has finished loading the page, the onReceiveValue() method of the ValueCallback interface will be called, and you can get the return value of your JavaScript function from the value parameter.

Note: evaluateJavascript() is only available on Android 4.4 (API level 19) and above. If you're targeting an earlier version of Android, you'll need to use the loadUrl() method to execute your JavaScript and get its return value.

Are you sure you enabled JavaScript?

getSettings().setJavaScriptEnabled(true);

Also try visiting <a href="https://www.digitalocean.com/community/tutorials/android-webview-example-tutorial">this site</a>. They have some useful information on Android WebView.

So this is what worked for me after experimenting with the answer by @Introspective; I am posting here because any future visitors may find it helpful.
Instead of onPageFinished(), I put evaluateJavascript() inside download listener. A JsonReader was required as the result is a JSON value.

MyWebView.setDownloadListener((url, userAgent, contentDisposition, mimeType, contentLength) -> {
    if (url.startsWith("data:")) {
       MyWebView.evaluateJavascript("returnfileName()", string -> {
           JsonReader jsonReader = new JsonReader(new StringReader(string));
           jsonReader.setLenient(true);//this is needed because I am returning  a single value
    if(jsonReader.peek() != JsonToken.NULL) {
           String fileName = jsonReader.nextString();
           //Now I can use the String fileName as I wish.
           //Rest of the code
   }
}

Javascript function returnFileName()

function returnfileName(){
  var fileName;
  if (document.getElementById("id of input field").value===""){ 
  fileName = "Default file name in case user didn't enter any filename";
  }
  else{
  fileName = document.getElementById("id of input field").value;}
  }
  return fileName;
}

Since evaluateJavascript() is a void, I had to put rest of the code to download the file inside the evaluateJavascript() itself, because otherwise String fileName will not be accessible for them (void doesn't have a return value).