java - UUID 返回空指针异常

Question

我正在编写 big nerd ranch 指南的 Android 编程的第二部分。出于某种原因，当我尝试运行该应用程序时，它在启动时崩溃，日志 cat 显示在线抛出了一个空指针异常:

UUID crimeID = (UUID)getArguments().getSerializable(EXTRA_CRIME_ID);

为什么 EXTRA_CRIME_ID 返回空值？

这是抛出异常的我的犯罪列表 fragment :

package com.bignerdranch.android.criminalintent;

import java.util.UUID;

import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.text.Editable;
import android.text.TextWatcher;
import android.text.format.DateFormat;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.CheckBox;
import android.widget.CompoundButton;
import android.widget.CompoundButton.OnCheckedChangeListener;
import android.widget.EditText;

public class CrimeFragment extends Fragment {
    public static final String EXTRA_CRIME_ID =
           "com.bignerdranch.android.criminalintent.crime_id";

    private Crime mCrime;
    private EditText mTitleField;
    private Button mDateButton;
    private CheckBox mSolvedCheckBox;

    @Override 
    public void onCreate(Bundle savedInstanceState){
        super.onCreate(savedInstanceState);
        **UUID crimeID = (UUID)getArguments().getSerializable(EXTRA_CRIME_ID);**  
        mCrime = CrimeLab.get(getActivity()).getCrime(crimeID);   

    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup parent, Bundle savedInstanceState){
        View v =inflater.inflate(R.layout.fragment_crime, parent, false);
        mTitleField = (EditText)v.findViewById(R.id.crime_title);
        mTitleField.setText(mCrime.getTitle());
        mTitleField.addTextChangedListener(new TextWatcher() {
            public void onTextChanged(CharSequence c, int start, int before, int count){
            mCrime.setTitle(c.toString());
        }

        public void beforeTextChanged(CharSequence c, int start, int count, int after){
            //left  blank
        }

        public void afterTextChanged(Editable c){
            //this one to
        }
    });

    mDateButton = (Button)v.findViewById(R.id.crime_date);
    mDateButton.setText(DateFormat.format("hh:mm:ss", mCrime.getDate()));
    mDateButton.setEnabled(false);

    mSolvedCheckBox = (CheckBox)v.findViewById(R.id.crime_solved);
    mSolvedCheckBox.setChecked(mCrime.isSolved());
    mSolvedCheckBox.setOnCheckedChangeListener(new OnCheckedChangeListener() {
        public void onCheckedChanged(CompoundButton buttonView, boolean isChecked) {
            //set the crimes solved property
            mCrime.setSolved(isChecked);
        }
    });

        return v;
    }

}

Answer 1

我还没有定义任何参数，所以我的 getArguments() 返回 null。我应该用

UUID crimeId = (UUID)getActivity().getIntent().getSerializableExtra(EXTRA_CRIME_ID);

相反。

----2014 年 12 月 16 日更新----

正如 njzk2 所指出的，我将 fragment 链接到 Activity ，以防止它被用于另一个 Activity 。 BigNerdRanch 书中出于简单原因将其链接到前面章节中的 Activity ，然后让读者在后面更改它并添加方法:

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;
}

并更改 UUID crimeId = (UUID)getActivity().getIntent().getSerializableExtra(EXTRA_CRIME_ID);到

 UUID crimeId = (UUID)getArguments().getSerializable(EXTRA_CRIME_ID);