Java android.support.v7.widget.AppCompatTextView无法强制转换为android.widget.RelativeLayout

我正在学习Android，但当我到达显示消息部分时，第6点我有一个错误

android.support.v7.widget.AppCompatTextView cannot be cast to android.widget.RelativeLayout 

我一步一步地做每件事，就像在教程中一样。那为什么它不起作用呢

DisplayMessageActivity.java

    package com.example.flover.hellloworld;

    import android.content.Intent;
    import android.os.Bundle;
    import android.support.v7.app.AppCompatActivity;
    import android.view.Menu;
    import android.view.MenuItem; 
    import android.widget.RelativeLayout;
    import android.widget.TextView;

    public class DisplayMessageActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_display_message);


        Intent intent = getIntent();
        String message = intent.getStringExtra(MyActivity.EXTRA_MESSAGE);
        TextView textView = new TextView(this);
        textView.setTextSize(40);
        textView.setText(message);

        RelativeLayout layout = (RelativeLayout) findViewById(R.id.content);
        layout.addView(textView);
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_display_message, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }
}

活动显示消息.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.example.flover.hellloworld.DisplayMessageActivity">

    <TextView
        android:id="@+id/content"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        ></TextView>

</RelativeLayout>

导致问题的原因。因为您正在定义id为TextView的RelativeLayout

分配

android:id="@+id/content"

---

至

RelativeLayout

。不要将布局更改为XML中的

TextView
，id内容应该转到相对视图，而不是TextView

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/content"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context="com.example.flover.hellloworld.DisplayMessageActivity">

   <TextView
   android:layout_width="wrap_content"
   android:layout_height="wrap_content"/>

</RelativeLayout>

您需要做的就是将Id分配给它应该是什么样的相对布局

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@+id/rl_Container"
android:layout_width="match_parent"
android:layout_height="match_parent">

<TextView
    android:id="@+id/content"
    android:layout_width="match_parent"
    android:layout_height="match_parent"></TextView>

</RelativeLayout>

对我来说，将android:id添加到根布局容器修复了我的问题。
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@+id/rl_Container"
android:layout_width="match_parent"
android:layout_height="match_parent">

<TextView
    android:id="@+id/content"
    android:layout_width="match_parent"
    android:layout_height="match_parent"></TextView>

</RelativeLayout>

public class DisplayMessageActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {

    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_display_message);


    Intent intent = getIntent();
    String message = intent.getStringExtra(MyActivity.EXTRA_MESSAGE);
    TextView textView = new TextView(this);
    textView.setTextSize(40);
    textView.setText(message);

    RelativeLayout layout = (RelativeLayout) findViewById(R.id.rl_Container);
    layout.addView(textView);
}


}