java - Activity 'MainActivity' 未在 AndroidManifest.xml 中声明，尽管它存在

Question

直到昨天为止一切正常，但今天当我尝试运行该应用程序时，它说“运行应用程序时出错:未找到默认 Activity ”

1. 尝试通过在“编辑配置”选项中指定 Activity 路径来解决该问题，但随后显示

   The activity 'MainActivity' is not declared in AndroidManifest.xml

2. 尝试过文件->使缓存无效/重新启动但不起作用

3. 彻底检查 list 以查找语法错误，但显然 list 中没有此类错误

4. 确保 build.gradle 文件中的版本相同

AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.myapplication">

<application
    android:allowBackup="true"
    android:icon="@mipmap/ic_launcher"
    android:label="@string/app_name"
    android:roundIcon="@mipmap/ic_launcher_round"
    android:supportsRtl="true"
    android:theme="@style/AppTheme">
    <activity
        android:name=".MainActivity"
        android:label="@string/app_name"
        android:theme="@style/AppTheme.NoActionBar">
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>
</application>

编辑

MainActivity.java

    package com.example.myapplication;

    import android.os.Bundle;

    import com.google.android.material.floatingactionbutton.FloatingActionButton;
    import com.google.android.material.snackbar.Snackbar;

    import androidx.appcompat.app.AppCompatActivity;
    import androidx.appcompat.widget.Toolbar;

    import android.view.View;
    import android.view.Menu;
    import android.view.MenuItem;

     public class MainActivity extends AppCompatActivity {

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    Toolbar toolbar = findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);

    FloatingActionButton fab = findViewById(R.id.fab);
    fab.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
                            Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                                    .setAction("Action", null).show();
                        }
                    });
                }

                @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_main, 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);
        }
       }

尝试卸载并重新安装 Android Studio 3.4 但仍然不起作用

Answer 1

听起来像是 Android Studio 的错误。我自己也遇到过几次。删除缓存并重新启动工作室。在此处查找更多引用:' https://stackoverflow.com/a/54321295/5182150