Android my first app by android studio

Interface

package com.example.helloworld;

import android.app.Activity;
import android.os.Bundle;
import android.view.Menu;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;



public class MainActivity extends Activity {
    private Button button,
                  Reset;
    private EditText editText;
    private TextView Result;
    public String str;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        button = (Button)findViewById(R.id.button);
        Reset = (Button)findViewById(R.id.Reset);
        editText = (EditText)findViewById(R.id.editText);
        Result = (TextView)findViewById(R.id.Result);
        button.setOnClickListener(buttonOnClick);
        Reset.setOnClickListener(ResetOnClick);

    }

    private Button.OnClickListener buttonOnClick = new
     Button.OnClickListener(){
        public void onClick(View v){
             str = editText.getText().toString();
             Result.setText(str);
        }
    };

    private Button.OnClickListener ResetOnClick = new
       Button.OnClickListener(){
           public void onClick(View v){
               str=" ";
               Result.setText(str);
           }
       };
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }
    
}