第二个参数类型错误。找到：“ android.app.Fragment”，必需：“ android.support.v4.app.Fragment”

Wrong 2nd argument type. Found: 'android.app.Fragment', required: 'android.support.v4.app.Fragment'

我正在YouTube上观看教学视频。我从一开始就做过两次相同的事情，但仍然停留在那儿。我将继续收到此错误，但他没有收到错误。

代码在这里。错误在私有void DisplayFragment类下。

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package com.example.kenpachi.navigation_drawer;

import android.app.Fragment;
import android.app.FragmentTransaction;
import android.os.Bundle;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.NavigationView;
import android.support.design.widget.Snackbar;
import android.support.v4.view.GravityCompat;
import android.support.v4.widget.DrawerLayout;
import android.support.v7.app.ActionBarDrawerToggle;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;

public class MainActivity extends AppCompatActivity
implements NavigationView.OnNavigationItemSelectedListener {

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);

FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
fab.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
Snackbar.make(view,"Replace with your own action", Snackbar.LENGTH_LONG)
.setAction("Action", null).show();
}
});

DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(
this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close);
drawer.setDrawerListener(toggle);
toggle.syncState();

NavigationView navigationView = (NavigationView) findViewById(R.id.nav_view);
navigationView.setNavigationItemSelectedListener(this);
}

@Override
public void onBackPressed() {
DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
if (drawer.isDrawerOpen(GravityCompat.START)) {
drawer.closeDrawer(GravityCompat.START);
} else {
super.onBackPressed();
}
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.main, menu);
return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
// Handle action bar item clicks here. The action bar will
// automatically handle clicks on the Home/Up button, so long
// as you specify a parent activity in AndroidManifest.xml.
int id = item.getItemId();

//noinspection SimplifiableIfStatement
if (id == R.id.action_settings) {
return true;
}

return super.onOptionsItemSelected(item);
}

private void DisplayFragment(int id) {

Fragment fragment = null;
switch (id) {
case R.id.nav_one:
fragment = new FragmentOne();
break;
case R.id.nav_two:
fragment = new FragmentTwo();
break;
case R.id.nav_three:
fragment = new FragmentThree();
}

if (fragment != null) {

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

ft.replace(R.id.MyFrameLayout, fragment); //"fragment" gives me this error.

ft.commit();
}
}

@SuppressWarnings("StatementWithEmptyBody")
@Override
public boolean onNavigationItemSelected(MenuItem item) {
// Handle navigation view item clicks here.
int id = item.getItemId();

if (id == R.id.nav_camera) {
// Handle the camera action
} else if (id == R.id.nav_gallery) {

} else if (id == R.id.nav_slideshow) {

} else if (id == R.id.nav_manage) {

} else if (id == R.id.nav_share) {

} else if (id == R.id.nav_send) {

}

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

这是给我错误的行(在DisplayFragment中)：

1	ft.replace(R.id.MyFrameLayout, fragment);

谢谢。如果有人在这里有趣的话，我正在尝试做视频指南：https://youtu.be/iZ8Y7W2jbv8?t=386