Intent to go to another Activity

Button btnNext = (Button) findViewById(R.id.btn_next);
Intent mIntentToParty = new Intent(MainActivity.this, SecondActivity.class);

View.OnClickListener listener = new View.OnClickListener() {
   @Override
   public void onClick(View v) {
       Log.i("MAIN", "Click btn_next");
       startActivity(mIntentToParty);
   }
};

btnNext.setOnClickListener(listener);

Example

public class SecondActivity extends AppCompatActivity {

    private Intent mIntentToBeLame;
    private Intent mIntentToLogin;

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

        Button btnBack = (Button) findViewById(R.id.btn_back);
        Button btnLogin = (Button) findViewById(R.id.btn_sketch);
        mIntentToBeLame = new Intent(SecondActivity.this, MainActivity.class);
        mIntentToLogin = new Intent(SecondActivity.this, LoginActivity.class);

        View.OnClickListener listener = new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Log.i("SECOND", "click btn_back");
                startActivity(mIntentToBeLame);
            }
        };

        View.OnClickListener sketchyLoginListener = new View.OnClickListener() {
          @Override
            public void onClick(View v) {
              startActivity(mIntentToLogin);
          }
        };

        btnBack.setOnClickListener(listener);
        btnLogin.setOnClickListener(sketchyLoginListener);
    }
}

Sending Data to Main (and protecting against errors)

Intent fromSecondActivity = getIntent();
if (fromSecondActivity != null) {
    String msgFromPartyActivity;
    try {
        msgFromPartyActivity = fromSecondActivity.getStringExtra("MSG");
        txtHiThere.setText(msgFromPartyActivity);
    } catch (Exception ex) {
        ex.printStackTrace();
    }
}

Using Intents to Send SMS (example)

private void sharePoem(String poem) {
    Intent sendTextIntent = new Intent();
    sendTextIntent.setAction(Intent.ACTION_SEND);
    sendTextIntent.putExtra(Intent.EXTRA_TEXT, poem);
    sendTextIntent.setType("text/plain");
    startActivity(sendTextIntent);
}