Use Intent

public class FirstActivity extends Activity
{
   
 
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

          Intent miTOSecondActivity = new Intent(FirstActivity.this,SecondActivity.class);

          miTOSecondActivity.putExtra("v1", "one" );

          startActivity(miTOSecondActivity);

  }

}

public class SecondActivity extends Activity
{

 @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

          Intent i = getIntent();

          String s = i.getStringExtra("v1");

          Toast.makeText(SecondActivity.thiss , s ,Toast.LENGTH_SHORT).show();

   }

}