R.layout.activity_signedin提示“无法解析符号”，ContentView因R.layout.signedin报错

Hey there! Let's work through those two issues you're hitting—they're super common in Android dev, so we'll get this sorted quickly.

First, let's recap your scenario: you're seeing "cannot resolve symbol" for R.layout.activity_signedin, and your setContentView call is throwing errors linked to that layout reference. Here's your code for context:

package com.example.myapplication;
import androidx.appcompat.app.AppCompatActivity;
import android.os.Bundle;
public class signedin extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_signedin);
    }
}

Let's go through the most likely fixes step by step:

1. Double-check your layout file exists and is named correctly #

Android is case-sensitive when it comes to resource filenames. Make sure:

• You have a file named activity_signedin.xml in your res/layout directory (no typos, extra underscores, or wrong capitalization—e.g., Activity_SignedIn.xml won't work)
• The file isn't accidentally placed in a different layout directory (like layout-land or a density-specific folder)

2. Clean and rebuild your project #

Corrupted auto-generated files (like the R class) are a frequent culprit. Do this in Android Studio:

• Go to Build > Clean Project (this deletes cached build files)
• Then go to Build > Rebuild Project (this regenerates the R class from scratch)

3. Ensure you're importing the correct R class #

Sometimes Android Studio accidentally imports the system android.R class instead of your app's custom R class. Check your imports:

• If you see import android.R;, delete it immediately
• Make sure you're either using the full package path (com.example.myapplication.R.layout.activity_signedin) or have an implicit import of your app's R (usually added automatically, but sometimes gets lost)

4. Fix any errors in your layout file #

If your activity_signedin.xml has syntax errors (unclosed tags, invalid attributes, missing resources like drawables/strings), the R class won't generate properly. Open the layout file and:

• Look for red underlines (Android Studio highlights errors here)
• Fix all issues (e.g., closing a <TextView> tag, correcting a typo in an attribute like android:layout_width)
• Rebuild the project after fixing these

5. Verify your package name matches #

Your Activity is in the com.example.myapplication package—make sure your app's R class is also generated under this package. If you recently changed your app's package name, you might need to sync the project with Gradle (File > Sync Project with Gradle Files)

A quick side note: While not related to your current error, Android conventions use PascalCase for Activity class names (e.g., SignedInActivity instead of signedin). It's not required, but it'll make your code easier to read for other devs!

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