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Stack Overflow (stackoverflow.com): Stack Overflow is a community-driven Q&A platform where developers can ask questions, share knowledge, and learn from each other. It's an excellent resource for troubleshooting and finding solutions to specific coding problems.
GitHub (github.com): GitHub is a code hosting and collaboration platform that's essential for version control and collaborative development. You can find numerous open-source mobile app projects to study and contribute to on GitHub.
Google Developers (developer.android.com): If you're interested in Android development, Google Developers provides comprehensive documentation, tutorials, and resources for Android app development, including the official Android Developer Guide.