在获取NPE的Activity中调用Fragment方法

Question

我想从MainFragment.java

中调用MainActivity.java的方法

我在这里尝试过：

MainActivity.java

MainFragment mainFragment;

........

mainFragment = new MainFragment();

.........

mainFragment.deom();

MainFragment.java

public void deom() {

    Toast.makeText(getActivity(), "Demo", Toast.LENGTH_LONG).show();

}

日志

java.lang.NullPointerException
at android.widget.Toast.<init>(Toast.java:93)
at android.widget.Toast.makeText(Toast.java:241)
at com.example.x.b(Unknown Source)
at com.example.MainActivity.onOptionsItemSelected(Unknown Source)
at android.app.Activity.onMenuItemSelected(Activity.java:2633)
at android.support.v4.b.ao.onMenuItemSelected(Unknown Source)
at android.support.v7.app.t.onMenuItemSelected(Unknown Source)
at android.support.v7.view.n.onMenuItemSelected(Unknown Source)
at android.support.v7.view.n.onMenuItemSelected(Unknown Source)

我可以知道我在做错了吗？我错过了什么？我为什么要获得NPE。

Answer 1

您必须初始化MainFargment。

MainFragment mainFragment = (MainFragment) getFragmentManager.findFragmentById(R.id.theId);

mainFragment.deom();