我试图显示带有收藏夹图标的项目列表。长按列表项目时，该项目将添加到收藏夹。当我们说将项目添加到收藏夹时，我们实际上将项目列表存储在映射到键的共享首选项中。
Android SharedPreferences 类不提供存储 List 或 ArrayList 或任何集合的方法。为了在键下存储列表（甚至是数组、集合或任何集合），因此使用 Gson 将列表转换为 JSON 格式并将其存储为字符串。检索时将 JSON 字符串转换回列表并返回。
但我遇到了意外的崩溃和错误！
主要活动
package com.company.wishlist_2;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentActivity;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentTransaction;
import android.view.Menu;
import android.view.MenuItem;
public class MainActivity extends FragmentActivity {
private Fragment contentFragment;
ProductListFragment pdtListFragment;
FavoriteListFragment favListFragment;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
FragmentManager fragmentManager = getSupportFragmentManager();
if (savedInstanceState != null) {
if (savedInstanceState.containsKey("content")) {
String content = savedInstanceState.getString("content");
if (content.equals(FavoriteListFragment.ARG_ITEM_ID)) {
if (fragmentManager.findFragmentByTag(FavoriteListFragment.ARG_ITEM_ID) != null) {
setFragmentTitle(R.string.favorites);
contentFragment = fragmentManager
.findFragmentByTag(FavoriteListFragment.ARG_ITEM_ID);
}
}
}
if (fragmentManager.findFragmentByTag(ProductListFragment.ARG_ITEM_ID) != null) {
pdtListFragment = (ProductListFragment) fragmentManager
.findFragmentByTag(ProductListFragment.ARG_ITEM_ID);
contentFragment = pdtListFragment;
}
} else {
pdtListFragment = new ProductListFragment();
setFragmentTitle(R.string.app_name);
switchContent(pdtListFragment, ProductListFragment.ARG_ITEM_ID);
}
}
@Override
protected void onSaveInstanceState(Bundle outState) {
if (contentFragment instanceof FavoriteListFragment) {
outState.putString("content", FavoriteListFragment.ARG_ITEM_ID);
} else {
outState.putString("content", ProductListFragment.ARG_ITEM_ID);
}
super.onSaveInstanceState(outState);
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
getMenuInflater().inflate(R.menu.main, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
switch (item.getItemId()) {
case R.id.menu_favorites:
setFragmentTitle(R.string.favorites);
favListFragment = new FavoriteListFragment();
switchContent(favListFragment, FavoriteListFragment.ARG_ITEM_ID);
return true;
}
return super.onOptionsItemSelected(item);
}
public void switchContent(Fragment fragment, String tag) {
FragmentManager fragmentManager = getSupportFragmentManager();
while (fragmentManager.popBackStackImmediate());
if (fragment != null) {
FragmentTransaction transaction = fragmentManager
.beginTransaction();
transaction.replace(R.id.content_frame, fragment, tag);
if (!(fragment instanceof ProductListFragment)) {
transaction.addToBackStack(tag);
}
transaction.commit();
contentFragment = fragment;
}
}
protected void setFragmentTitle(int resourseId) {
setTitle(resourseId);
getActionBar().setTitle(resourseId);
}
@Override
public void onBackPressed() {
FragmentManager fm = getSupportFragmentManager();
if (fm.getBackStackEntryCount() > 0) {
super.onBackPressed();
} else if (contentFragment instanceof ProductListFragment
|| fm.getBackStackEntryCount() == 0) {
finish();
}
}
}
产品
package com.company.wishlist_2;
public class Product {
private int id;
private String name;
private String description;
private double price;
public Product() {
super();
}
public Product(int id, String name, String description, double price) {
super();
this.id = id;
this.name = name;
this.description = description;
this.price = price;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getDescription() {
return description;
}
public void setDescription(String description) {
this.description = description;
}
public double getPrice() {
return price;
}
public void setPrice(double price) {
this.price = price;
}
@Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + id;
return result;
}
@Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
Product other = (Product) obj;
if (id != other.id)
return false;
return true;
}
@Override
public String toString() {
return "Product [id=" + id + ", name=" + name + ", description="
+ description + ", price=" + price + "]";
}
}
收藏列表片段
package com.company.wishlist_2;
import java.util.List;
import android.app.Activity;
import android.app.AlertDialog;
import android.content.DialogInterface;
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.AdapterView;
import android.widget.AdapterView.OnItemClickListener;
import android.widget.AdapterView.OnItemLongClickListener;
import android.widget.ImageView;
import android.widget.ListView;
import android.widget.Toast;
public class FavoriteListFragment extends Fragment {
public static final String ARG_ITEM_ID = "favorite_list";
ListView favoriteList;
SharedPreference sharedPreference;
List<Product> favorites;
Activity activity;
ProductListAdapter productListAdapter;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
activity = getActivity();
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View view = inflater.inflate(R.layout.fragment_product_list, container,
false);
sharedPreference = new SharedPreference();
favorites = sharedPreference.getFavorites(activity);
if (favorites == null) {
showAlert(getResources().getString(R.string.no_favorites_items),
getResources().getString(R.string.no_favorites_msg));
} else {
if (favorites.size() == 0) {
showAlert(
getResources().getString(R.string.no_favorites_items),
getResources().getString(R.string.no_favorites_msg));
}
favoriteList = (ListView) view.findViewById(R.id.list_product);
if (favorites != null) {
productListAdapter = new ProductListAdapter(activity, favorites);
favoriteList.setAdapter(productListAdapter);
favoriteList.setOnItemClickListener(new OnItemClickListener() {
public void onItemClick(AdapterView<?> parent, View arg1,
int position, long arg3) {
}
});
favoriteList
.setOnItemLongClickListener(new OnItemLongClickListener() {
@Override
public boolean onItemLongClick(
AdapterView<?> parent, View view,
int position, long id) {
ImageView button = (ImageView) view
.findViewById(R.id.imgbtn_favorite);
String tag = button.getTag().toString();
if (tag.equalsIgnoreCase("grey")) {
sharedPreference.addFavorite(activity,
favorites.get(position));
Toast.makeText(
activity,
activity.getResources().getString(
R.string.add_favr),
Toast.LENGTH_SHORT).show();
button.setTag("red");
button.setImageResource(R.drawable.heart_red);
} else {
sharedPreference.removeFavorite(activity,
favorites.get(position));
button.setTag("grey");
button.setImageResource(R.drawable.heart_grey);
productListAdapter.remove(favorites
.get(position));
Toast.makeText(
activity,
activity.getResources().getString(
R.string.remove_favr),
Toast.LENGTH_SHORT).show();
}
return true;
}
});
}
}
return view;
}
public void showAlert(String title, String message) {
if (activity != null && !activity.isFinishing()) {
AlertDialog alertDialog = new AlertDialog.Builder(activity)
.create();
alertDialog.setTitle(title);
alertDialog.setMessage(message);
alertDialog.setCancelable(false);
alertDialog.setButton(AlertDialog.BUTTON_POSITIVE, "OK",
new DialogInterface.OnClickListener() {
public void onClick(DialogInterface dialog, int which) {
dialog.dismiss();
// activity.finish();
getFragmentManager().popBackStackImmediate();
}
});
alertDialog.show();
}
}
@Override
public void onResume() {
getActivity().setTitle(R.string.favorites);
getActivity().getActionBar().setTitle(R.string.favorites);
super.onResume();
}
}
产品列表适配器
package com.company.wishlist_2;
import java.util.List;
import android.app.Activity;
import android.content.Context;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.ImageView;
import android.widget.TextView;
public class ProductListAdapter extends ArrayAdapter<Product> {
private Context context;
List<Product> products;
SharedPreference sharedPreference;
public ProductListAdapter(Context context, List<Product> products) {
super(context, R.layout.product_list_item, products);
this.context = context;
this.products = products;
sharedPreference = new SharedPreference();
}
private class ViewHolder {
TextView productNameTxt;
TextView productDescTxt;
TextView productPriceTxt;
ImageView favoriteImg;
}
@Override
public int getCount() {
return products.size();
}
@Override
public Product getItem(int position) {
return products.get(position);
}
@Override
public long getItemId(int position) {
return 0;
}
@Override
public View getView(int position, View convertView, ViewGroup parent) {
ViewHolder holder = null;
if (convertView == null) {
LayoutInflater inflater = (LayoutInflater) context
.getSystemService(Activity.LAYOUT_INFLATER_SERVICE);
convertView = inflater.inflate(R.layout.product_list_item, null);
holder = new ViewHolder();
holder.productNameTxt = (TextView) convertView
.findViewById(R.id.txt_pdt_name);
holder.productDescTxt = (TextView) convertView
.findViewById(R.id.txt_pdt_desc);
holder.productPriceTxt = (TextView) convertView
.findViewById(R.id.txt_pdt_price);
holder.favoriteImg = (ImageView) convertView
.findViewById(R.id.imgbtn_favorite);
convertView.setTag(holder);
} else {
holder = (ViewHolder) convertView.getTag();
}
Product product = (Product) getItem(position);
holder.productNameTxt.setText(product.getName());
holder.productDescTxt.setText(product.getDescription());
holder.productPriceTxt.setText(product.getPrice() + "");
if (checkFavoriteItem(product)) {
holder.favoriteImg.setImageResource(R.drawable.heart_red);
holder.favoriteImg.setTag("red");
} else {
holder.favoriteImg.setImageResource(R.drawable.heart_grey);
holder.favoriteImg.setTag("grey");
}
return convertView;
}
public boolean checkFavoriteItem(Product checkProduct) {
boolean check = false;
List<Product> favorites = sharedPreference.getFavorites(context);
if (favorites != null) {
for (Product product : favorites) {
if (product.equals(checkProduct)) {
check = true;
break;
}
}
}
return check;
}
@Override
public void add(Product product) {
super.add(product);
products.add(product);
notifyDataSetChanged();
}
@Override
public void remove(Product product) {
super.remove(product);
products.remove(product);
notifyDataSetChanged();
}
}
产品列表片段
package com.company.wishlist_2;
import java.util.ArrayList;
import java.util.List;
import android.app.Activity;
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.AdapterView;
import android.widget.AdapterView.OnItemClickListener;
import android.widget.AdapterView.OnItemLongClickListener;
import android.widget.ImageView;
import android.widget.ListView;
import android.widget.Toast;
public class ProductListFragment extends Fragment implements
OnItemClickListener, OnItemLongClickListener {
public static final String ARG_ITEM_ID = "product_list";
Activity activity;
ListView productListView;
List<Product> products;
ProductListAdapter productListAdapter;
SharedPreference sharedPreference;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
activity = getActivity();
sharedPreference = new SharedPreference();
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View view = inflater.inflate(R.layout.fragment_product_list, container,
false);
findViewsById(view);
setProducts();
productListAdapter = new ProductListAdapter(activity, products);
productListView.setAdapter(productListAdapter);
productListView.setOnItemClickListener(this);
productListView.setOnItemLongClickListener(this);
return view;
}
private void setProducts() {
Product product1 = new Product(1, "Dell XPS", "Dell XPS Laptop", 60000);
Product product2 = new Product(2, "HP Pavilion G6-2014TX",
"HP Pavilion G6-2014TX Laptop", 50000);
Product product3 = new Product(3, "ProBook HP 4540",
"ProBook HP 4540 Laptop", 45000);
Product product4 = new Product(4, "HP Envy 4-1025TX",
"HP Envy 4-1025TX Laptop", 46000);
Product product5 = new Product(5, "Dell Inspiron",
"Dell Inspiron Laptop", 48000);
Product product6 = new Product(6, "Dell Vostro", "Dell Vostro Laptop",
50000);
Product product7 = new Product(7, "IdeaPad Z Series",
"Lenovo IdeaPad Z Series Laptop", 40000);
Product product8 = new Product(8, "ThinkPad X Series",
"Lenovo ThinkPad X Series Laptop", 38000);
Product product9 = new Product(9, "VAIO S Series",
"Sony VAIO S Series Laptop", 39000);
Product product10 = new Product(10, "Series 5",
"Samsung Series 5 Laptop", 50000);
products = new ArrayList<Product>();
products.add(product1);
products.add(product2);
products.add(product3);
products.add(product4);
products.add(product5);
products.add(product6);
products.add(product7);
products.add(product8);
products.add(product9);
products.add(product10);
}
private void findViewsById(View view) {
productListView = (ListView) view.findViewById(R.id.list_product);
}
@Override
public void onItemClick(AdapterView<?> parent, View view, int position,
long id) {
Product product = (Product) parent.getItemAtPosition(position);
Toast.makeText(activity, product.toString(), Toast.LENGTH_LONG).show();
}
@Override
public boolean onItemLongClick(AdapterView<?> arg0, View view,
int position, long arg3) {
ImageView button = (ImageView) view.findViewById(R.id.imgbtn_favorite);
String tag = button.getTag().toString();
if (tag.equalsIgnoreCase("grey")) {
sharedPreference.addFavorite(activity, products.get(position));
Toast.makeText(activity,
activity.getResources().getString(R.string.add_favr),
Toast.LENGTH_SHORT).show();
button.setTag("red");
button.setImageResource(R.drawable.heart_red);
} else {
sharedPreference.removeFavorite(activity, products.get(position));
button.setTag("grey");
button.setImageResource(R.drawable.heart_grey);
Toast.makeText(activity,
activity.getResources().getString(R.string.remove_favr),
Toast.LENGTH_SHORT).show();
}
return true;
}
@Override
public void onResume() {
getActivity().setTitle(R.string.app_name);
getActivity().getActionBar().setTitle(R.string.app_name);
super.onResume();
}
}
共享偏好
package com.company.wishlist_2;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import android.content.Context;
import android.content.SharedPreferences;
import android.content.SharedPreferences.Editor;
import com.google.gson.Gson;
public class SharedPreference {
public static final String PREFS_NAME = "PRODUCT_APP";
public static final String FAVORITES = "Product_Favorite";
public SharedPreference() {
super();
}
public void saveFavorites(Context context, List<Product> favorites) {
SharedPreferences settings;
Editor editor;
settings = context.getSharedPreferences(PREFS_NAME,
Context.MODE_PRIVATE);
editor = settings.edit();
Gson gson = new Gson();
String jsonFavorites = gson.toJson(favorites);
editor.putString(FAVORITES, jsonFavorites);
editor.commit();
}
public void addFavorite(Context context, Product product) {
List<Product> favorites = getFavorites(context);
if (favorites == null)
favorites = new ArrayList<Product>();
favorites.add(product);
saveFavorites(context, favorites);
}
public void removeFavorite(Context context, Product product) {
ArrayList<Product> favorites = getFavorites(context);
if (favorites != null) {
favorites.remove(product);
saveFavorites(context, favorites);
}
}
public ArrayList<Product> getFavorites(Context context) {
SharedPreferences settings;
List<Product> favorites;
settings = context.getSharedPreferences(PREFS_NAME,
Context.MODE_PRIVATE);
if (settings.contains(FAVORITES)) {
String jsonFavorites = settings.getString(FAVORITES, null);
Gson gson = new Gson();
Product[] favoriteItems = gson.fromJson(jsonFavorites,
Product[].class);
favorites = Arrays.asList(favoriteItems);
favorites = new ArrayList<Product>(favorites);
} else
return null;
return (ArrayList<Product>) favorites;
}
}
我的错误
01-17 14:14:53.599 8584-8584/com.company.wishlist_2 E/AndroidRuntime: FATAL EXCEPTION: main
01-17 14:14:53.599 8584-8584/com.company.wishlist_2 E/AndroidRuntime: java.lang.RuntimeException: Unable to start activity ComponentInfo{com.company.wishlist_2/com.company.wishlist_2.MainActivity}: java.lang.NullPointerException
01-17 14:14:53.599 8584-8584/com.company.wishlist_2 E/AndroidRuntime: at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2255)
01-17 14:14:53.599 8584-8584/com.company.wishlist_2 E/AndroidRuntime: at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2309)
01-17 14:14:53.599 8584-8584/com.company.wishlist_2 E/AndroidRuntime: at android.app.ActivityThread.access$700(ActivityThread.java:157)
01-17 14:14:53.599 8584-8584/com.company.wishlist_2 E/AndroidRuntime: at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1289)
01-17 14:14:53.599 8584-8584/com.company.wishlist_2 E/AndroidRuntime: at android.os.Handler.dispatchMessage(Handler.java:99)
01-17 14:14:53.599 8584-8584/com.company.wishlist_2 E/AndroidRuntime: at android.os.Looper.loop(Looper.java:176)
01-17 14:14:53.599 8584-8584/com.company.wishlist_2 E/AndroidRuntime: at android.app.ActivityThread.main(ActivityThread.java:5317)
01-17 14:14:53.599 8584-8584/com.company.wishlist_2 E/AndroidRuntime: at java.lang.reflect.Method.invokeNative(Native Method)
01-17 14:14:53.599 8584-8584/com.company.wishlist_2 E/AndroidRuntime: at java.lang.reflect.Method.invoke(Method.java:511)
01-17 14:14:53.599 8584-8584/com.company.wishlist_2 E/AndroidRuntime: at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1102)
01-17 14:14:53.599 8584-8584/com.company.wishlist_2 E/AndroidRuntime: at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:869)
01-17 14:14:53.599 8584-8584/com.company.wishlist_2 E/AndroidRuntime: at dalvik.system.NativeStart.main(Native Method)
01-17 14:14:53.599 8584-8584/com.company.wishlist_2 E/AndroidRuntime: Caused by: java.lang.NullPointerException
01-17 14:14:53.599 8584-8584/com.company.wishlist_2 E/AndroidRuntime: at com.company.wishlist_2.MainActivity.setFragmentTitle(MainActivity.java:100)
01-17 14:14:53.599 8584-8584/com.company.wishlist_2 E/AndroidRuntime: at com.company.wishlist_2.MainActivity.onCreate(MainActivity.java:46)
01-17 14:14:53.599 8584-8584/com.company.wishlist_2 E/AndroidRuntime: at android.app.Activity.performCreate(Activity.java:5326)
01-17 14:14:53.599 8584-8584/com.company.wishlist_2 E/AndroidRuntime: at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1097)
01-17 14:14:53.599 8584-8584/com.company.wishlist_2 E/AndroidRuntime: at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2218)
01-17 14:14:53.599 8584-8584/com.company.wishlist_2 E/AndroidRuntime: at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2309)
01-17 14:14:53.599 8584-8584/com.company.wishlist_2 E/AndroidRuntime: at android.app.ActivityThread.access$700(ActivityThread.java:157)
01-17 14:14:53.599 8584-8584/com.company.wishlist_2 E/AndroidRuntime: at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1289)
01-17 14:14:53.599 8584-8584/com.company.wishlist_2 E/AndroidRuntime: at android.os.Handler.dispatchMessage(Handler.java:99)
01-17 14:14:53.599 8584-8584/com.company.wishlist_2 E/AndroidRuntime: at android.os.Looper.loop(Looper.java:176)
01-17 14:14:53.599 8584-8584/com.company.wishlist_2 E/AndroidRuntime: at android.app.ActivityThread.main(ActivityThread.java:5317)
01-17 14:14:53.599 8584-8584/com.company.wishlist_2 E/AndroidRuntime: at java.lang.reflect.Method.invokeNative(Native Method)
01-17 14:14:53.599 8584-8584/com.company.wishlist_2 E/AndroidRuntime: at java.lang.reflect.Method.invoke(Method.java:511)
01-17 14:14:53.599 8584-8584/com.company.wishlist_2 E/AndroidRuntime: at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1102)
01-17 14:14:53.599 8584-8584/com.company.wishlist_2 E/AndroidRuntime: at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:869)
01-17 14:14:53.599 8584-8584/com.company.wishlist_2 E/AndroidRuntime: at dalvik.system.NativeStart.main(Native Method)