[android help] Eclipse Android (ADT) How do I use methods from other classes in MainActivity class?

Eclipse Android (ADT) How do I use methods from other classes in MainActivity class?

Classes A, B and C is located only in src and the MainActivity.class in source/com/example/TestApplication.

MainActivity doesnt seem to see the other classes because they are not located in the same location/package. How can I call the other classes' methods to work in MainActivity.

Here is an example of my code: I want to run UseActivity methods in MainActivity

        public class UseActivity extends Activity
        {
        @Override
        protected void onCreate(Bundle savedInstanceState) 
        {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_main);    
        }
            public String sEntries(View view) 
            {
                DatabaseHandler db = new DatabaseHandler(this);
                String sfinal = "";

                // Inserting Contacts
                Log.d("Insert: ", "Inserting ..");
                db.addContact(new Contact("Ravi", "9100000000"));       
                db.addContact(new Contact("Srinivas", "9199999999"));
                db.addContact(new Contact("Tommy", "9522222222"));
                db.addContact(new Contact("Karthik", "9533333333"));
                switch (view.getId()) 
                {
                case R.id.button1:
                    EditText dbText =  (EditText) findViewById(R.id.editText1);

                     // Reading all contacts
                    Log.d("Reading: ", "Reading all contacts..");
                    List contacts = db.getAllContacts();  
                    for (Contact cn : contacts)
                    {
                        sfinal = "Id: "+cn.getID()+" ,Name: " + cn.getName() + " ,Phone: " + cn.getPhoneNumber();
                            // Writing Contacts to sfinal
                        Log.d("Name: ", sfinal);
                        dbText.setText(sfinal);
                    }
                    break;
                }
                return sfinal;
            }      
    }



public class MainActivity extends Activity 
{

    @Override
    protected void onCreate(Bundle savedInstanceState) 
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) 
    {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }

    UseActivity u = new UseActivity();
    {
        u.sEntries(View view);
    }  /////////////////////////// this doesnt work, program underlines UseActivity in red giving error and doesnt see it as another class (maybe because of package)

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