Android Studio显示旧项目界面并报错，新手寻求解决方案

Hey there, let's tackle each of your problems one by one to get you back on track:

1. Layout Positions Don't Match Your Design #

It's totally normal for new Android Studio users to hit this snag. Here's how to fix it:

• Check your layout type: Stick with ConstraintLayout (Android Studio's default for new projects)—it’s built to handle screen adaptation way better than outdated layouts like AbsoluteLayout. If you’re using LinearLayout, double-check the orientation attribute and use layout_weight for flexible sizing instead of hardcoding pixel values.
• Match preview device to your running device: In the Design tab of activity_main.xml, use the device dropdown at the top to select the exact same model as your emulator or physical phone. This ensures what you see in preview matches what runs on the device.
• Avoid hardcoded dp values: Use match_parent, wrap_content, or proper constraints (like linking views to parent edges or other views) instead of fixed numbers to make layouts adapt to different screen sizes.

2. "package androidx.test.ext.junit.runners does not exist" Error (No Code Written) #

This error almost always comes from missing or unsynced test dependencies. Try these steps:

• Open your app module’s build.gradle file (the one inside the app folder), scroll to the dependencies block. Look for a line like:

  androidTestImplementation 'androidx.test.ext:junit:1.1.5'
  

  If it’s missing, add it (use the latest version suggested by Android Studio). If you don’t need test code right now, you can temporarily comment out androidTest-related dependencies and delete the test files in the androidTest directory.
• Click the Sync Project with Gradle Files button (the elephant icon in the top right) to sync your dependencies.
• If that fails, go to File > Project Structure > SDK Location and make sure your Android SDK Build Tools and Platform Tools are up to date.

3. Always Seeing the First Project's Interface #

This is likely a cache or run configuration issue. Let’s fix it:

• Verify your run configuration: Click the dropdown next to the Run button (▶️) at the top of Android Studio. Make sure it’s set to your current project’s module, not the first one you created.
• Clear app data on your device: On your emulator or physical phone, go to Settings > Apps > [Your App Name] > Storage, then tap Clear Cache and Clear Data. This wipes old cached layouts.
• Invalidate caches and restart: Go to File > Invalidate Caches..., check the box for Invalidate and Restart, and let Android Studio restart. This clears deep caches that Clean/Rebuild might not touch.

内容的提问来源于stack exchange，提问作者user12170668