Resolving "R cannot be resolved" Error in Android Development with Eclipse

Introduction

When developing an Android application using Eclipse, developers often encounter a common error: "R cannot be resolved." This issue typically arises when Eclipse fails to generate or recognize the R.java file, which serves as a bridge between your Java code and the XML resources in your project. Understanding why this happens and how to resolve it is crucial for smooth Android application development.

Understanding R.java

The R.java file is automatically generated during the build process of an Android application. It contains resource IDs for all the resources defined in the /res/ directory, such as layouts, strings, images, etc. These IDs allow your Java code to reference these resources by their names rather than relying on hardcoded values.

Common Causes and Solutions

1. Erroneous Import Statements:

   • Eclipse may incorrectly add import android.R; at the top of your activity files. This import causes your project to use Android’s default resource class instead of the one generated from your project’s /res/ directory.
   • Solution: Remove any incorrect import android.R; statements from your Java files.

2. Issues in XML Files:

   • Errors or inconsistencies in your XML layout files can prevent the generation of R.java.
   • Solution: Check all XML files in the /res/ directory for errors. Pay attention to syntax issues, missing closing tags, and incorrect references.

3. Resource Naming Conventions:

   • Resource file names must adhere to specific conventions (only lowercase letters, numbers, underscores, and periods are allowed).
   • Solution: Ensure all resource files in directories like res/drawables-mdpi comply with these naming rules.

4. Android SDK and ADT Configuration:

   • Outdated or improperly configured Android Development Tools (ADT) can lead to issues where the gen directory is not created, which includes R.java.
   • Solution: Update your ADT plugin and ensure that you have the latest Android SDK Build Tools installed via the Android SDK Manager.

5. Organizing Imports:

   • Using commands like Ctrl + Shift + O in Eclipse to organize imports can sometimes add incorrect import statements.
   • Solution: Manually verify and correct any erroneous imports after using such commands.

Best Practices

• Regularly check your project’s build path settings to ensure that the Android library is correctly configured.
• Clean and rebuild your project frequently, especially after making changes to XML files or adding new resources.
• Stay updated with the latest versions of Eclipse ADT and Android SDK Build Tools to avoid compatibility issues.

Example Code

Here’s a simple example of an activity file referencing resources correctly:

package eu.mauriziopz.gps;

import android.app.Activity;
import android.os.Bundle;

public class GpsActivity extends Activity {
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main); // Ensure R.layout.main references a valid layout file in /res/layout/
    }
}

Conclusion

The "R cannot be resolved" error can be frustrating, but understanding its causes and knowing how to address them will streamline your Android development process. By ensuring proper import statements, adhering to resource naming conventions, and keeping your tools up-to-date, you can avoid this common pitfall and focus on building great applications.