将ArrayList <customobject>传递给另一个活动，获取NullPointerException </customobject>

Question

我正在尝试使用Parcelable接口将对象的ArrayList从一个活动传递到另一个活动，但是当我输入我传递给我的ArrayList的活动时，我一直得到一个NullPointerException。

Vitamin.java - 定义一个带有字符串和两个String []数组的对象，实现Parcelable

HealthBenefits.java - 显示字符串列表。选择字符串时，将搜索维生素对象列表，并将包含所选字符串的对象放入另一个ArrayList中。然后通过putParcelableArrayListExtra将此ArrayList传递给VitaminDisplay活动。

VitaminDisplay.java - 显示与传递给活动的维生素名称对应的字符串列表。

Vitamin.java

public class Vitamin implements Parcelable{

String theName;
String[] theFoods, theBenefits;

public Vitamin(String name, String[] benefits, String[] foods)
{
    theName = name;
    theBenefits = benefits;
    theFoods = foods;
}

private Vitamin(Parcel in){
    theName = in.readString();
    ArrayList<String> tempBenefits = new ArrayList<String>();
    tempBenefits = (ArrayList<String>) in.readSerializable();
    ArrayList<String> tempFoods = new ArrayList<String>();
    tempFoods = (ArrayList<String>) in.readSerializable();

    //convert from ArrayList<String> to String[]
    theBenefits = new String[tempBenefits.size()];
    theFoods = new String[tempFoods.size()];
    theBenefits = tempBenefits.toArray(theFoods);
    theFoods = tempFoods.toArray(theFoods);
}

public String getName(){
    return theName;
}

public String[] getBenefit(){
    return theBenefits;
}

public String[] getFoods(){
    return theFoods;
}

@Override
public int describeContents() {
    // TODO Auto-generated method stub
    return 0;
}

@Override
public void writeToParcel(Parcel out, int flags) {
    // TODO Auto-generated method stub
    out.writeString(theName);
    out.writeStringArray(theBenefits);
    out.writeStringArray(theFoods);
}

public static final Parcelable.Creator<Vitamin> CREATOR = new Parcelable.Creator<Vitamin>() {
    public Vitamin createFromParcel(Parcel in) {
        return new Vitamin(in);
    }

    public Vitamin[] newArray(int size) {
        return new Vitamin[size];
    }
};
}

HealthBenefits.java

public class HealthBenefits extends Activity{

@Override
protected void onCreate(Bundle savedInstanceState) {

    super.onCreate(savedInstanceState);
    setContentView(R.layout.health_benefits_layout);

    ListView benefits = (ListView) findViewById(R.id.listView1);
    benefits.setTextFilterEnabled(true);
    ArrayAdapter<CharSequence> aa = ArrayAdapter.createFromResource(this, R.array.benefits, android.R.layout.simple_list_item_1);
    benefits.setAdapter(aa);

    final ArrayList<Vitamin> vitamins = new ArrayList<Vitamin>();
    String[] foods1 = {"Pork", "Whole grain", "Legumes", "Nuts and seeds"};
    String[] vitaBenefits1 = {"Improve nerve function", "Improve metabolism"};
    vitamins.add(new Vitamin("Vitamin B1", vitaBenefits1, foods1));

    String[] vitaBenefits13 = {"Improve circulatory function"};
    String[] foods13 = {"Leafy green vegetables"};
    vitamins.add(new Vitamin("Vitamin K", vitaBenefits13, foods13));

        //...more vitamin objects added to the list, omitted for brevity

    benefits.setOnItemClickListener(new OnItemClickListener() {
        public void onItemClick(AdapterView<?> parent, View view,
            int position, long id) {
            // When clicked, show a toast with the TextView text
            ArrayList<Vitamin> listToDisplay = new ArrayList<Vitamin>();

            if(position == 1)
            {
            //code specific to first list item    
                    for(int i = 0;i < vitamins.size();i++){
                        if(vitamins.get(i).getBenefit().equals("Improve immune system")) {
                            listToDisplay.add(vitamins.get(i));
                        }

                    }
                     Intent myIntent = new Intent(view.getContext(), VitaminDisplay.class);
                     myIntent.putParcelableArrayListExtra("VITAMINS", listToDisplay);
                     startActivityForResult(myIntent, 0);
            }

            if(position == 2)
            {
            //code specific to 2nd list item    
            }
        }
  });
}
}

VitaminDisplay.java

public class VitaminDisplay extends Activity{

ArrayList<Vitamin> vitaminsList;
ArrayList<String> displayList;

@Override
protected void onCreate(Bundle savedInstanceState) {

    super.onCreate(savedInstanceState);
    setContentView(R.layout.health_benefits_layout);

    vitaminsList = getIntent().getExtras().getParcelableArrayList("VITAMINS");
            displayList = new ArrayList<String>();

    for(int i = 0;i < vitaminsList.size();i++){
        displayList.add(vitaminsList.get(i).getName());
        Log.d("VitaminDisplay", vitaminsList.get(i).getName());
    }

    //displayList = new ArrayList<String>();
    //displayList.add("test");

    ListView benefits = (ListView) findViewById(R.id.listView1);
    benefits.setTextFilterEnabled(true);
    ArrayAdapter<String> aa = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, displayList);
    benefits.setAdapter(aa);
}

}

我认为异常可能来自于我如何在Vitamin.java中实现Parcelable类，但我还没有找到解决方案。有什么建议吗？

这是完整的logcat：

E/AndroidRuntime(1958): FATAL EXCEPTION: main

04-21 21:55:08.963: E/AndroidRuntime(1958): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.heartsmart/com.example.heartsmart.VitaminDisplay}: java.lang.NullPointerException
04-21 21:55:08.963: E/AndroidRuntime(1958):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2180)
04-21 21:55:08.963: E/AndroidRuntime(1958):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)
04-21 21:55:08.963: E/AndroidRuntime(1958):     at android.app.ActivityThread.access$600(ActivityThread.java:141)
04-21 21:55:08.963: E/AndroidRuntime(1958):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)
04-21 21:55:08.963: E/AndroidRuntime(1958):     at android.os.Handler.dispatchMessage(Handler.java:99)
04-21 21:55:08.963: E/AndroidRuntime(1958):     at android.os.Looper.loop(Looper.java:137)
04-21 21:55:08.963: E/AndroidRuntime(1958):     at android.app.ActivityThread.main(ActivityThread.java:5041)
04-21 21:55:08.963: E/AndroidRuntime(1958):     at java.lang.reflect.Method.invokeNative(Native Method)
04-21 21:55:08.963: E/AndroidRuntime(1958):     at java.lang.reflect.Method.invoke(Method.java:511)
04-21 21:55:08.963: E/AndroidRuntime(1958):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
04-21 21:55:08.963: E/AndroidRuntime(1958):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
04-21 21:55:08.963: E/AndroidRuntime(1958):     at dalvik.system.NativeStart.main(Native Method)
04-21 21:55:08.963: E/AndroidRuntime(1958): Caused by: java.lang.NullPointerException
04-21 21:55:08.963: E/AndroidRuntime(1958):     at java.io.ByteArrayInputStream.<init>(ByteArrayInputStream.java:60)
04-21 21:55:08.963: E/AndroidRuntime(1958):     at android.os.Parcel.readSerializable(Parcel.java:2139)
04-21 21:55:08.963: E/AndroidRuntime(1958):     at com.example.heartsmart.Vitamin.<init>(Vitamin.java:23)
04-21 21:55:08.963: E/AndroidRuntime(1958):     at com.example.heartsmart.Vitamin.<init>(Vitamin.java:20)
04-21 21:55:08.963: E/AndroidRuntime(1958):     at com.example.heartsmart.Vitamin$1.createFromParcel(Vitamin.java:62)
04-21 21:55:08.963: E/AndroidRuntime(1958):     at com.example.heartsmart.Vitamin$1.createFromParcel(Vitamin.java:1)
04-21 21:55:08.963: E/AndroidRuntime(1958):     at android.os.Parcel.readParcelable(Parcel.java:2103)
04-21 21:55:08.963: E/AndroidRuntime(1958):     at android.os.Parcel.readValue(Parcel.java:1965)
04-21 21:55:08.963: E/AndroidRuntime(1958):     at android.os.Parcel.readListInternal(Parcel.java:2235)
04-21 21:55:08.963: E/AndroidRuntime(1958):     at android.os.Parcel.readArrayList(Parcel.java:1655)
04-21 21:55:08.963: E/AndroidRuntime(1958):     at android.os.Parcel.readValue(Parcel.java:1986)
04-21 21:55:08.963: E/AndroidRuntime(1958):     at android.os.Parcel.readMapInternal(Parcel.java:2226)
04-21 21:55:08.963: E/AndroidRuntime(1958):     at android.os.Bundle.unparcel(Bundle.java:223)
04-21 21:55:08.963: E/AndroidRuntime(1958):     at android.os.Bundle.containsKey(Bundle.java:271)
04-21 21:55:08.963: E/AndroidRuntime(1958):     at com.example.heartsmart.VitaminDisplay.onCreate(VitaminDisplay.java:23)
04-21 21:55:08.963: E/AndroidRuntime(1958):     at android.app.Activity.performCreate(Activity.java:5104)
04-21 21:55:08.963: E/AndroidRuntime(1958):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1080)
04-21 21:55:08.963: E/AndroidRuntime(1958):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2144)
04-21 21:55:08.963: E/AndroidRuntime(1958):     ... 11 more

Answer 1

您正在使用Parcel.writeStringArray编写数组，但尝试使用Parcel.readSerializable来阅读它们。更改您的Vitamin构造函数以匹配您编写writeToParcel的方式。这应该这样做：

private Vitamin(Parcel in){
    theName = in.readString();
    theBenefits = in.createStringArray();
    theFoods = in.createStringArray();
}

或者，修改writeToParcel以将字符串数组写为Serializable个对象。或者您可以将数组放入Bundle并编写然后读取包。

更好的是，切换到使用ArrayList<String>，我认为可以为此提供更好的支持。

Answer 2

可能会有更多，这可能不是您目前遇到的问题，但这是一个问题。在VitaminDisplay中，您在尝试添加ArrayList之前永远不会初始化<{1}}

ArrayList<String> displayList;


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

vitaminsList = getIntent().getParcelableArrayListExtra("VITAMINS");
for(int i = 0;i < vitaminsList.size();i++){
    displayList.add(vitaminsList.get(i).getName());   // displayList isn't initialized

将其更改为

ArrayList<String> displayList = new ArrayList<String>();  // Change this here

@Override
protected void onCreate(Bundle savedInstanceState) {

super.onCreate(savedInstanceState);
setContentView(R.layout.health_benefits_layout);

vitaminsList = getIntent().getParcelableArrayListExtra("VITAMINS");
for(int i = 0;i < vitaminsList.size();i++){
    displayList.add(vitaminsList.get(i).getName());

Answer 3

我的头脑。当你试图获得数组时，有一件事是缺失的。

而不是做

vitaminsList = getIntent().getParcelableArrayListExtra("VITAMINS");

试

vitaminsList = getIntent().getExtras.getParcelableArrayList("VITAMINS");

您应该添加更多支票，例如......

Bundle b = getIntent().getExtras();
if(b!=null && b.containsKey("VITAMINS")){
   vitaminsList = b.getParcelableArrayList("VITAMINS");
}