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 {@Overrideprotected 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));    }    });