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删除以前的数据后，应用程序在第一次运行时运行良好。它在第二次运行时崩溃，并且必须从sharedprefs中获取已保存的信息（我使用HashSets保存了它）。尝试保存数据时出错
SharedPreferences sharedPreferences = getApplicationContext().getSharedPreferences("com.example.adam.vaistai", Context.MODE_PRIVATE);
HashSet<String> set = (HashSet<String>) sharedPreferences.getStringSet("names", null);
HashSet<String> numberSet = (HashSet<String>) sharedPreferences.getStringSet("numbers", null);
HashSet<String> descSet = (HashSet<String>) sharedPreferences.getStringSet("desc", null);
if (set == null){
vaistai.add("Pavyzdys 2017-08-18");
numbers.add(String.valueOf(0));
description.add("Nuo persalimo, vartoti 3 kartus dienoje");
} else {
vaistai = new ArrayList<>(set);
//error
numbers = new ArrayList<>(numberSet);
description = new ArrayList<>(descSet);
}
使用日志我发现，线（vaistai =新的ArrayList <>（组）;）之后的应用程序崩溃
08-26 17:11:00.745 7730-7730/com.example.adam.vaistai E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.adam.vaistai, PID: 7730
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.adam.vaistai/com.example.adam.vaistai.MainActivity}: java.lang.NullPointerException: Attempt to invoke interface method 'java.lang.Object[] java.util.Collection.toArray()' on a null object reference
logcat的示出了该
那的代码MainActivity.java
package com.example.adam.vaistai;
import android.content.Context;
import android.content.DialogInterface;
import android.content.Intent;
import android.content.SharedPreferences;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteStatement;
import android.support.v7.app.AlertDialog;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.Menu;
import android.view.MenuInflater;
import android.view.MenuItem;
import android.view.View;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import java.io.IOException;
import java.util.ArrayList;
import java.util.HashSet;
import java.util.Set;
public class MainActivity extends AppCompatActivity {
ListView listView;
static ArrayList<String> vaistai = new ArrayList<>();
static ArrayList<String> numbers = new ArrayList<>();
static ArrayList<String> description = new ArrayList<>();
static ArrayAdapter arrayAdapter;
public void nextActivity (int pos){
Intent intent = new Intent(this, SecondActivity.class);
intent.putExtra("index", pos);
startActivity(intent);
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
MenuInflater menuInflater = getMenuInflater();
menuInflater.inflate(R.menu.main_menu, menu);
return super.onCreateOptionsMenu(menu);
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
super.onOptionsItemSelected(item);
if (item.getItemId() == R.id.newNote){
vaistai.add("");
numbers.add(String.valueOf(0));
description.add("");
arrayAdapter.notifyDataSetChanged();
nextActivity(vaistai.size()-1);
}
return false;
}
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
listView = (ListView) findViewById(R.id.listView);
SharedPreferences sharedPreferences = getApplicationContext().getSharedPreferences("com.example.adam.vaistai", Context.MODE_PRIVATE);
HashSet<String> set = (HashSet<String>) sharedPreferences.getStringSet("names", null);
HashSet<String> numberSet = (HashSet<String>) sharedPreferences.getStringSet("numbers", null);
HashSet<String> descSet = (HashSet<String>) sharedPreferences.getStringSet("desc", null);
if (set == null){
vaistai.add("Pavyzdys 2017-08-18");
numbers.add(String.valueOf(0));
description.add("Nuo persalimo, vartoti 3 kartus dienoje");
} else {
vaistai = new ArrayList<>(set);
numbers = new ArrayList<>(numberSet);
description = new ArrayList<>(descSet);
}
arrayAdapter = new ArrayAdapter(this, android.R.layout.simple_list_item_1, vaistai);
listView.setAdapter(arrayAdapter);
listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
@Override
public void onItemClick(AdapterView<?> adapterView, View view, int i, long l) {
nextActivity(i);
}
});
listView.setOnItemLongClickListener(new AdapterView.OnItemLongClickListener() {
@Override
public boolean onItemLongClick(AdapterView<?> adapterView, View view, int i, long l) {
final int item = i;
new AlertDialog.Builder(MainActivity.this)
.setIcon(android.R.drawable.alert_dark_frame)
.setTitle("Ar norite istrinti?")
.setMessage("Irasas bus pasalintas")
.setPositiveButton("Taip", new DialogInterface.OnClickListener() {
@Override
public void onClick(DialogInterface dialogInterface, int i) {
vaistai.remove(item);
numbers.remove(item);
description.remove(item);
arrayAdapter.notifyDataSetChanged();
SharedPreferences sharedPreferences = getApplicationContext().getSharedPreferences("com.example.adam.vaistai", Context.MODE_PRIVATE);
HashSet<String> numberSet = new HashSet<String>(MainActivity.numbers);
sharedPreferences.edit().putStringSet("numbers", numberSet).apply();
HashSet<String> set = new HashSet<String>(MainActivity.vaistai);
sharedPreferences.edit().putStringSet("names", set).apply();
HashSet<String> descSet = new HashSet<String>(MainActivity.description);
sharedPreferences.edit().putStringSet("desc", descSet).apply();
}
}
)
.setNegativeButton("Ne", null)
.show();
return true;
}
});
}
}