Error "Must Override a Superclass Method" after importing project into Eclipse

Hello everyone! I'm a beginner in Java and Android development. I've been working on a project in Eclipse for a week, everything worked perfectly. Today I decided to move the project to another computer — copied the folder, imported it via File → Import → Existing Projects into Workspace. And then it started: almost all methods with the @Override annotation are underlined in red, and when hovering Eclipse says "The method ... must override a superclass method". \n\nHere's an example of my code:\n\n```java\npublic class MyActivity extends Activity {\n @Override\n protected void onCreate(Bundle savedInstanceState) {\n super.onCreate(savedInstanceState);\n setContentView(R.layout.activity_main);\n }\n\n @Override\n public boolean onCreateOptionsMenu(Menu menu) {\n getMenuInflater().inflate(R.menu.main, menu);\n return