Ne peut pas résoudre la méthode ' show (android.soutien.v4.App.FragmentManager, java.lang.Chaîne)

pour une raison quelconque, quand j'essaie d'afficher une boîte de dialogue, j'obtiens une erreur de la boîte de dialogue.show (fm, DIALOG_DATE); saying Ne peut pas résoudre méthode show(android.soutien.v4.App.FragmentManager, java.lang.Chaîne de caractères)'

Pourquoi ne peut-il résoudre la méthode?

mDateButton.setOnClickListener(new View.OnClickListener() {
        public void onClick(View v) {
            FragmentManager fm = getActivity().getSupportFragmentManager();
            DatePickerFragment dialog = new DatePickerFragment();
            dialog.show(fm, DIALOG_DATE);
        }
    });

Voici le reste de ma classe:

package com.bignerdranch.android.criminalintent;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.text.Editable; 
import android.text.TextWatcher;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.CheckBox;
import android.widget.Button;
import android.widget.CompoundButton;
import android.widget.CompoundButton.OnCheckedChangeListener;
import android.widget.EditText;

import java.util.UUID;


public class CrimeFragment extends Fragment {
//key for the extra
public static final String EXTRA_CRIME_ID = "com.bignerdranch.android.criminalintent.crime_id";

private static final String DIALOG_DATE = "date";

//holds crime
private Crime mCrime;

//widgets
private EditText mTitleField;
private Button mDateButton;
private CheckBox mSolvedCheckBox;

//at start of build
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    //get crime from crime class

    /*Intents
    *There are two ways a fragment can access data in its activity's intent:
    * an easy direct shortcut
    * or a complex flexible implementation
    * First try out the shortcut
    * in the shortcut, CrimeFragment will access CrimeActivity's intent directly
     */

    //retrieve the extra from CrimeActivity's intent and use it to fetch the Crime
    //UUID crimeId = (UUID)getActivity().getIntent().getSerializableExtra   (EXTRA_CRIME_ID); //shortcut removed in chapter 10 and "should feel warm and fuzzy inside for maintaining CrimeFragments Independence"
    //
    UUID crimeId = (UUID)getArguments().getSerializable(EXTRA_CRIME_ID);
    //CrimeLab.get() requires a context object, so CrimeFragment passes the CrimeActivity
    mCrime = CrimeLab.get(getActivity()).getCrime(crimeId);

}

//Create the view and inflate the layout
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    // Inflate the layout for crime Fragment
    //pass false because view will be added in the activitys code
    View v = inflater.inflate(R.layout.fragment_crime, container, false);

    //gets crime_title from fragment_crime.xml
    mTitleField = (EditText)v.findViewById(R.id.crime_title);
    mTitleField.setText(mCrime.getTitle());
    mTitleField.addTextChangedListener(new TextWatcher() {
        @Override
        public void beforeTextChanged(CharSequence s, int start, int count, int after) {
            //not used
        }

        @Override
        public void onTextChanged(CharSequence s, int start, int before, int count) {
              mCrime.setTitle(s.toString());
        }

        @Override
        public void afterTextChanged(Editable s) {
            //also not used
        }
    });


    //find date button from fragment_crime
    mDateButton = (Button)v.findViewById(R.id.crime_date);
    //set mDateButton text to current date and time
    mDateButton.setText(mCrime.getDate().toString());
    //disable button for now...enabled in chapter 12
   // mDateButton.setEnabled(false);


    mDateButton.setOnClickListener(new View.OnClickListener() {
        public void onClick(View v) {
            FragmentManager fm = getActivity().getSupportFragmentManager();
            DatePickerFragment dialog = new DatePickerFragment();
            dialog.show(fm, DIALOG_DATE);
        }
    });




    //find solved checkbox from fragment_crime
    mSolvedCheckBox = (CheckBox)v.findViewById(R.id.crime_solved);
    mSolvedCheckBox.setChecked(mCrime.isSolved());
    //user clicks solved check box
    mSolvedCheckBox.setOnCheckedChangeListener(new OnCheckedChangeListener() {
        public void onCheckedChanged(CompoundButton buttonView, boolean isChecked){
            //set the crime's solved property
            mCrime.setSolved(isChecked);
        }
    });
    //returns the view
    return v;
}

/*The downside to direct retrieval
*can not encapsulate fragment
* CrimeFragment is no longer a reusable building block because it expects that it will always be hosted by an activity whose intent defines extra named "EXTRA_CRIME_ID"
* CrimeFragment cannot be used with just any activity
*
 */

/*Fragment Arguments
*A better solution is to stash the mCrimeId someplace that belongs to CrimeFragment rather than keeping it in CrimeActivity's personal space
* this someplace can be an arguments bundle
* Every fragment instance can have a Bundle object attached to it
* bundle contains key value pairs that work just like the intent extras of an activity
* Pg. 195
 */

/*attaching arguments to a fragment
*Attaching args to frags must be done after the frag is created but before it is added to the activity
* To hit this window use a static class called newInstance()
* This method creates the fragment instance and bundles up and sets its arguments
 */
//for attaching arguments to a fragment
public static CrimeFragment newInstance(UUID crimeId){
    Bundle args = new Bundle();
    args.putSerializable(EXTRA_CRIME_ID, crimeId);

    CrimeFragment fragment = new CrimeFragment();
    fragment.setArguments(args);

    //pass UUID from extra
    return fragment;
}

}