Android fragments Android片段未显示在我的应用程序中

我对我的片段有一个奇怪的问题，我在运行时没有出错（我使用的是Android Studio v8.7，Gradle编译得很好），但是当我的应用程序在我的手机上运行时，它现在显示出来了，它看起来像一个空的活动

活动类

package android.bignerdranch.com.criminalintent;

import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentActivity;
import android.support.v4.app.FragmentManager;
import android.os.Bundle;
import android.view.MenuItem;


public class CrimeActivity extends FragmentActivity {

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

        // These are Fragments from the app.v4 package, it's needed to make the app compatible with API < 11
        FragmentManager fm = getSupportFragmentManager();
        Fragment fragment = fm.findFragmentById(R.id.fragmentContainer);

        if(fragment == null) {
            fragment = new Fragment();
            fm.beginTransaction().add(R.id.fragmentContainer, fragment).commit();
        }
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        int id = item.getItemId();
        if (id == R.id.action_settings) {
            return true;
        }
        return super.onOptionsItemSelected(item);
    }
}

Activity.xml

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/fragmentContainer"
    android:layout_width="match_parent"
    android:layout_height="match_parent" />

Fragment.xml

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:orientation="vertical">

    <TextView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="@string/crime_title_label"
        style="?android:listSeparatorTextViewStyle" />

    <EditText
        android:id="@+id/crime_title"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="@string/crime_title_hint" />

    <TextView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="@string/crime_details_label"
        style="?android:listSeparatorTextViewStyle" />

    <Button
        android:id="@+id/crime_date"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginLeft="16dp"
        android:layout_marginRight="16dp" />

    <CheckBox
        android:id="@+id/crime_solved"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginLeft="16dp"
        android:layout_marginRight="16dp"
        android:text="@string/crime_solved_label" />

</LinearLayout>

---

有人能给我指出正确的方向，告诉我什么能解决我的问题吗

好的，我发现了错误，我实例化了错误的片段。 而不是

if(fragment == null) {
            fragment = new Fragment();
            fm.beginTransaction().add(R.id.fragmentContainer, fragment).commit();

应该是

if(fragment == null) {
            fragment = new CrimeFragment();
            fm.beginTransaction().add(R.id.fragmentContainer, fragment).commit();

if(fragment == null) {
            fragment = new CrimeFragment();
            fm.beginTransaction().add(R.id.fragmentContainer, fragment).commit();