android java代码中的语法错误

Question

我对编码非常陌生，开始学习android。我在Java代码（我的activity.java）中遇到一些小语法错误。请在此方面帮助我。程序是将两个数字相加并显示结果

package com.example.apple.myapplication;

import android.support.v7.app.AppCompatActivity;

import android.os.Bundle;

import android.view.View;

import android.widget.Button;

import android.widget.EditText;

import android.widget.TextView;

public class MainActivity extends AppCompatActivity {

@Override

protected void onCreate(Bundle savedInstanceState) {

    super.onCreate(savedInstanceState);

    setContentView(R.layout.activity_main);

    EditText firstNumber;

    EditText secondNumber;

    TextView addResult;

    Button btnAdd;

    double num1,num2,sum;

    firstNumber = findViewById(R.id.txtNumber1);

    secondNumber = findViewById(R.id.txtNumber2);

    addResult = findViewById(R.id.txtResult);

    btnAdd = findViewById(R.id.btnAdd);

}

 btnAdd.setOnClickListener(new OnClickListener() {

 public void onClick(View v) {

    num1 = Double.parseDouble(firstNumber.getText().toString());

    num2 = Double.parseDouble(secondNumber.getText().toString());

    sum = num1 + num2;

    addResult.setText(Double.toString(sum));

    }

    });