使用多个片段和活动处理backStack

FragmentManager fragmentManager = getSupportFragmentManager();
FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();

// 添加片段到栈中
fragmentTransaction.add(R.id.fragment_container, new MyFragment(), "MY_FRAGMENT_TAG");
fragmentTransaction.addToBackStack(null); // 添加到backStack
fragmentTransaction.commit();

Intent intent = new Intent(this, SecondActivity.class);
intent.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP | Intent.FLAG_ACTIVITY_SINGLE_TOP);
startActivity(intent);

@Override
public void onBackPressed() {
    if (getSupportFragmentManager().getBackStackEntryCount() > 0) {
        getSupportFragmentManager().popBackStack();
    } else {
        super.onBackPressed();
    }
}

public class MainActivity extends AppCompatActivity {

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

        if (savedInstanceState == null) {
            getSupportFragmentManager().beginTransaction()
                .add(R.id.fragment_container, new FirstFragment())
                .commit();
        }
    }

    public void switchToSecondFragment() {
        getSupportFragmentManager().beginTransaction()
            .replace(R.id.fragment_container, new SecondFragment())
            .addToBackStack(null)
            .commit();
    }

    @Override
    public void onBackPressed() {
        if (getSupportFragmentManager().getBackStackEntryCount() > 0) {
            getSupportFragmentManager().popBackStack();
        } else {
            super.onBackPressed();
        }
    }
}