Working activity for https://stackoverflow.com/questions/38796319/injecting-javascript-bridge-in-webview/

MainActivity.java

import android.graphics.Bitmap;
import android.os.Build;
import android.os.Bundle;
import android.os.Handler;
import android.support.v7.app.AppCompatActivity;
import android.util.Log;
import android.view.View;
import android.webkit.JavascriptInterface;
import android.webkit.WebView;
import android.webkit.WebViewClient;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity {
  WebView webView;
  Handler handler;

  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    handler = new Handler();
    setContentView(R.layout.activity_main);
    if (BuildConfig.DEBUG && Build.VERSION.SDK_INT >= Build.VERSION_CODES.KITKAT) {
      // Make it possible to debug on your PC using "chrome://inspect/#devices"
      WebView.setWebContentsDebuggingEnabled(true);
    }
    webView = new WebView(getApplicationContext());
    webView.getSettings().setJavaScriptEnabled(true);
    webView.setWebViewClient(new WebViewClient() {
      @Override
      public void onPageStarted(WebView view, String url, Bitmap favicon) {
        webView.setVisibility(View.GONE);
      }

      @Override
      public void onPageFinished(final WebView view, String url) {
        Log.e("checking", "MYmsg");
        webView.loadUrl("javascript:(function() { " +
                "document.body.innerHTML = '<p>test<p>';" + "})()");
        webView.loadUrl("javascript: window.CallToAnAndroidFunction.setVisible()");
      }
    });
    webView.setVisibility(View.INVISIBLE);
    webView.addJavascriptInterface(new myJavaScriptInterface(), "CallToAnAndroidFunction");
    webView.loadUrl("https://example.com");
    // if you need to debug visually. Then uncomment the next line.
    //setContentView(webView);
  }

  @Override
  protected void onDestroy() {
    // Destroy the webview, otherwise it's kept in memory.
    webView.destroy();
    super.onDestroy();
  }

  public class myJavaScriptInterface {
    @JavascriptInterface
    public void setVisible() {
      Runnable runnable = new Runnable() {
        @Override
        public void run() {
          // The nested `MainActivity.this.runOnUiThread` isn't needed.
          webView.setVisibility(View.VISIBLE);
          Log.e("Testing", "no");
          Toast.makeText(MainActivity.this, "Reached JS", Toast.LENGTH_LONG).show();
        }
      };
      // Show the toast and log 2 seconds from now.
      // If you want it to run as soon as possible. 
      // Then replace the handler.postDelayed with `MainActivity.this.runOnUiThread(runnable)` or set the delay to 0.
      handler.postDelayed(runnable, 2000);
    }
  }
}