<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <TextView
        android:id="@+id/textView1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Hello World!"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <Button
        android:id="@+id/button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:onClick="bt1_click"
        android:text="Button"
        tools:layout_editor_absoluteX="162dp"
        tools:layout_editor_absoluteY="394dp"
        tools:ignore="MissingConstraints" />

</androidx.constraintlayout.widget.ConstraintLayout>

package com.stackof.calljni;

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.widget.TextView;
import android.view.View;

public class MainActivity extends AppCompatActivity {
    String Tag = "StackOF";
    TextView textview1;
    int c=0;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        textview1=(TextView)findViewById(R.id.textView1);
    }
    public void bt1_click(View view){
        c = c+1;
        textview1.setText("click:"+c+"\n"+JniLib.callToCpp());
    }
}

package com.stackof.calljni;

public class JniLib {
    static {
        System.loadLibrary("JniLib");
    }

    public static native String callToCpp();
}