MainActivity.java

package pro.test.localweb;

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.util.Log;
import android.webkit.WebView;
import android.webkit.WebViewClient;


public class MainActivity extends AppCompatActivity {

    private  WebView myWebView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        // webView に関する試行錯誤
        myWebView = (WebView) findViewById(R.id.localWeb);
        myWebView.loadUrl("file:///android_asset/index.html"); // ローカルのhtmlファイルを指定
        myWebView.setWebViewClient(new WebViewClient());
        // WebView内のJavaScriptの実行を許可
        myWebView.getSettings().setJavaScriptEnabled(true);
        // WebChromeClientを設定する
        // ※コレを設定しないとJSのalertは表示されない
        myWebView.addJavascriptInterface(new JavaScript(this), "android");

    }

}

app\src\main\assets\index.html

<html>
<head>
    <script src="https://ajax.googleapis.com/ajax/libs/jquery/3.4.1/jquery.min.js"></script>
</head>
<body>
test3
<div class="a"></div>
<br>
<button class="check" >Toastを表示</button>
</body>
<script>

$('.a').html("a")

$('.check').on('click',function(){
     android.hogeMethod('ほげほげ')
     //$('.a').html("testb")
});

</script>
</html>

pro/test/localweb/JavaScript.java


package pro.test.localweb;

import android.content.Context;
import android.webkit.JavascriptInterface;
import android.widget.Toast;

public class JavaScript {
    private Context c;

    private String IdString = "shop_admin1";

    public JavaScript(Context c) {
        this.c = c;
    }

    @JavascriptInterface
    public void hogeMethod(String s) {
        Toast toast = Toast.makeText(c, s, Toast.LENGTH_LONG);
        toast.show();
    }

    @JavascriptInterface
    public String GetLoginId(){
        return IdString;
    }

}