Java 当我动态添加片段时，它会显示错误

每当我动态添加片段时，它在主活动中显示错误，它不会解析FragmentManager类

public class MainActivity extends AppCompatActivity {

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

    FragmentManager fragmentManager=getFragmentManager();
    FragmentTransaction fragmentTransaction=fragmentManager.beginTransaction();
    FirstFragment ff= new FirstFragment();
    fragmentTransaction.add(R.id.fragment_container,ff);
    fragmentTransaction.commit();
}

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提前感谢您的帮助。

混合使用fragment和v4。fragment

使用getSupportFragmentManager（）和v4库的fragment。

    FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();
    transaction.add(R.id.container, MyFragment);  
    transaction.show(mapFragment);
    transaction.commit();