无法解析 Android 开发人员教程网页上的符号"setText"

Question

我正在学习android studio，我正在学习"MyFirstApp"教程。但是，当我尝试运行该应用程序时，出现错误"无法解析符号"setText"。 这是我的代码：

package com.example.myfirstapp;
import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.TextView;
public class DisplayMessageActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_display_message);
    }
    // Get the Intent that started this activity and extract the string
    Intent intent = getIntent();
    String message = intent.getStringExtra(MainActivity.EXTRA_MESSAGE);
    // Capture the layout's TextView and set the string as its text
    TextView textView = findViewById(R.id.textView);
    textView.setText(message);
}

这应该与教程中的代码相同：https://developer.android.com/training/basics/firstapp/starting-activity.html#java

这是activity_display_message.xml文件：

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.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=".DisplayMessageActivity">
    <TextView
        android:id="@+id/textView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="16dp"
        android:text="@string/textview"
        android:textSize="14sp"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />
</android.support.constraint.ConstraintLayout>

我试图检查一个简单的问题，但似乎我没有做任何不同的事情：Android 教程"无法解析符号'setText'"谁能帮我？

Answer 1

此代码必须在 onCreate(( 中：

// Get the Intent that started this activity and extract the string
Intent intent = getIntent();
String message = intent.getStringExtra(MainActivity.EXTRA_MESSAGE);
// Capture the layout's TextView and set the string as its text
TextView textView = findViewById(R.id.textView);
textView.setText(message);

Answer 2

只需使 TextViewpublic 并在 onCreate(( 方法之外声明即可。

package com.example.myfirstapp;
import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.TextView;
public class DisplayMessageActivity extends AppCompatActivity {
    public  TextView textView = findViewById(R.id.textView);
    @Override
    protected void onCreate(Bundle savedInstanceState) {
          super.onCreate(savedInstanceState);
          setContentView(R.layout.activity_display_message);
    }
     // Get the Intent that started this activity and extract the string
     Intent intent = getIntent();
     String message = intent.getStringExtra(MainActivity.EXTRA_MESSAGE);
     // set the string as its text
     textView.setText(message);
}