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Accepted
Morozov
Asked:2020-11-04 18:25:02 +0000 UTC2020-11-04 18:25:02 +0000 UTC

过渡到 NavigationDrawer 中的活动

  • 772

除了片段之外，我还想在我的导航抽屉中为活动添加一个过渡

 public boolean onNavigationItemSelected(MenuItem item) {
    int id = item.getItemId();
    Fragment fragment = null;
    if (id == R.id.nav_camera) {
        fragment = new MainFragment();
    } else if (id == R.id.nav_gallery) {
        fragment = new SecondFragment();
    } else if (id == R.id.nav_slideshow) {
        startActivity(new Intent(getApplicationContext(), MainActivity.class));
    }

    android.support.v4.app.FragmentTransaction fragmentTransaction =
            getSupportFragmentManager().beginTransaction();
    fragmentTransaction.replace(R.id.fragment_container, fragment);
    fragmentTransaction.commit();

    DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
    drawer.closeDrawer(GravityCompat.START);
    return true;
}

如果在遍历片段时没有问题，那么在转到 MainActivity 时会抛出错误

              java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Class java.lang.Object.getClass()' on a null object reference
java

1 个回答

  1. Best Answer

    关键是您的方法中的代码在调用startActivity(). 在该方法中，您将 null 分配给片段实例，然后不对其进行初始化（跳过相应的条件分支），然后（在调用第二个活动之后）尝试调用该片段，该片段为 null，您会得到一个错误。

    例如，您可以这样做：

    public boolean onNavigationItemSelected(MenuItem item) {

    android.support.v4.app.FragmentTransaction fragmentTransaction =
            getSupportFragmentManager().beginTransaction();


    switch (item.getItemId()) {
        case R.id.nav_camera :
            fragmentTransaction.replace(R.id.fragment_container, new MainFragment()).commit();
            break;
        case R.id.nav_gallery :
            fragmentTransaction.replace(R.id.fragment_container, new SecondFragment()).commit();
            break;
        case R.id.nav_slideshow :
            startActivity(new Intent(this, MainActivity.class));
            break;
    }


    DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
    drawer.closeDrawer(GravityCompat.START);
    return true;
}
    • 2

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