コード内の何が問題なのか教えてください。

Question

WorkoutDetailFragmentがキャストされているMainActivity.javaの行は、実行できないというエラーを示します。なぜ動作していないのか教えてください。私は本からコードをコピーしました。

MainActivity

package gangster.workout; 

import android.support.v7.app.AppCompatActivity; 
import android.os.Bundle; 

public class MainActivity extends AppCompatActivity { 

@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_main); 
    WorkoutDetailFragment frag = (WorkoutDetailFragment) getFragmentManager().findFragmentById(R.id.detail_frag); 
    frag.setWorkout(1); 
} 
} 

Workout.Java

package gangster.workout; 


public class Workout { 
private String name; 
private String description; 

public static final Workout[] workouts = { 
     new Workout("The Limb Loosener" , " Pta nhi kya ksgdrna hai isme") , 
     new Workout("The Timb Loosener" , " Pta nhi kya asdfasasfasasdkrna hai isme") , 
     new Workout("The Pimb Loosener" , " Pta nhi kyaasf krna hai isme") , 
     new Workout("The Zimb Loosener" , " Pta nhi kyasfasa krna hai isme") , 
}; 
private Workout(String name , String description){ 
    this.name = name; 
    this.description = description; 
} 

public String getDescription(){ 
    return description; 
} 
public String getName(){ 
    return name; 
} 
public String toString(){ 
    return this.name; 
} 
} 

WorkoutDetailFragment.java

package gangster.workout; 


import android.os.Bundle; 
import android.support.v4.app.Fragment; 
import android.view.LayoutInflater; 
import android.view.View; 
import android.view.ViewGroup; 
import android.widget.TextView; 


public class WorkoutDetailFragment extends Fragment { 
private long workoutId; 

public void setWorkout(long id) { 

    this.workoutId = id; 
} 

@Override 
public void onStart() { 
    super.onStart(); 
    View view = getView(); 
    if (view != null) { 
     TextView title = (TextView) view.findViewById(R.id.textTitle); 
     Workout workout = Workout.workouts[(int) workoutId]; 
     title.setText(workout.getName()); 
     TextView description = (TextView) view.findViewById(R.id.textDescription); 
     description.setText(workout.getDescription()); 
    } 

} 

@Override 
public View onCreateView(LayoutInflater inflater, ViewGroup container, 
         Bundle savedInstanceState) { 
    // Inflate the layout for this fragment 
    return inflater.inflate(R.layout.fragment_workout_detail, container, false); 
} 

} 

Answer 1

代わりgetFragmentManager()