Android Studio模拟器打开应用程序白屏

Question

Whenever I install and run my app in the Android Studio emulator all I get is a white screen and the tool bar above it even though in the preview it works completely fine. Heres the xml code:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="fill_parent"
    android:background="@drawable/rectanglestemlogo"
    android:orientation="horizontal">

    <Button
        android:id="@+id/Home_Button"
        android:layout_width="0dp"
        android:layout_weight="0.33"
        android:layout_height="wrap_content"
        android:onClick="goToStemAcademy"
        android:text="@string/Home_Button"
        android:textAllCaps="false"
        android:textColor="#000000" />

    <Button
        android:id="@+id/StemAcademy_Button"
        android:layout_width="0dp"
        android:layout_weight="0.33"
        android:layout_height="wrap_content"
        android:layout_marginEnd="0dp"
        android:text="@string/Stem_Academy"
        android:textAllCaps="false"
        android:textColor="#000000"
        android:visibility="visible" />

    <Button
        android:id="@+id/Competitions_Button"
        android:layout_width="0dp"
        android:layout_weight="0.33"
        android:layout_height="wrap_content"
        android:layout_marginEnd="2dp"
        android:text="@string/Competitions"
        android:textAllCaps="false"
        android:textColor="#000000" />

</LinearLayout>

then the java:

package com.example.dmssteamapp6;

import android.content.Intent;
import android.view.View;

import androidx.appcompat.app.AppCompatActivity;

public class MainActivity extends AppCompatActivity {
    public void goToStemAcademy(View v) {
        startActivity(new Intent(MainActivity.this, StemAcademy.class));
    }
}

then the new xml activity:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical" android:layout_width="match_parent"
    android:layout_height="match_parent">

    <Button
        android:id="@+id/button"
        android:layout_width="match_parent"
        android:layout_height="138dp"
        android:onClick="goToHome"
        android:text="Button" />
</LinearLayout>

and the java that goes with it:

package com.example.dmssteamapp6;

import android.content.Intent;
import android.view.View;

import androidx.appcompat.app.AppCompatActivity;

public class StemAcademy extends AppCompatActivity {
    public void goToHome(View v) {
        startActivity(new Intent(StemAcademy.this, MainActivity.class));
    }
}

also here is the androidmanifest.xml code if applicable:

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.dmssteamapp6">

    <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>
        <activity
            android:name=".StemAcademy"
            android:label="@string/app_name"
            android:theme="@style/AppTheme.NoActionBar">
        </activity>
    </application>
</manifest>

Answer 1

Just Create onCreate method in every java class

public class StemAcademy extends AppCompatActivity {

 @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_stemacademy);
}

 public void goToHome(View v) {
        startActivity(new Intent(StemAcademy.this, MainActivity.class));
    }

}

Activity main class

public class MainActivity extends AppCompatActivity {

 @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_main);
    }

    public void goToStemAcademy(View v) {
        startActivity(new Intent(MainActivity.this, StemAcademy.class));
    }
}