我已经仔细检查了StackOverflow上的所有答案,但没有运气找出我为什么会出错:引起原因:java.lang.NullPointerException:尝试调用虚拟方法'java.lang.String android.content.Context.getPackageName() '在空对象引用上。我下载了gson2.2.2,还将编译行放入build.gradle中。
这是我保存对象(帐户)的类的代码:
public class CreateAccount extends AppCompatActivity {
SharedPreferences accountsPref = getPreferences(MODE_PRIVATE);
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_create_account);
setTitle("Create New Account");
getSupportActionBar().setDisplayHomeAsUpEnabled(true);
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.menu_create_account, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
switch (item.getItemId()) {
//back button
case android.R.id.home:
// app icon in action bar clicked; go home
Intent intent = new Intent(this, MainScreen.class);
intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
startActivity(intent);
return true;
//save button
case R.id.action_menu_save:
// get EditText by id
EditText inputTxt1 = (EditText) findViewById(R.id.AccountName);
// Store EditText in Variable
String name = inputTxt1.getText().toString();
EditText inputTxt2 = (EditText) findViewById(R.id.StartingBalance);
double balance = Double.parseDouble(inputTxt2.getText().toString());
Account newAccount = new Account(name, balance);
SharedPreferences.Editor editor = accountsPref.edit();
Gson gson = new Gson();
String json = gson.toJson(newAccount);
editor.putString("newAccount", json);
editor.commit();
Intent intent2 = new Intent(this, MainScreen.class);
intent2.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
startActivity(intent2);
return true;
default:
return super.onOptionsItemSelected(item);
}
}
}
这是我尝试加载对象的Activity:
package com.cashtrack.kennethlee.cashtrack;
import android.content.Context;
import android.content.Intent;
import android.content.SharedPreferences;
import android.os.Bundle;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.Snackbar;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.View;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.TextView;
import com.google.gson.Gson;
import java.io.FileInputStream;
import java.io.StringWriter;
import java.util.ArrayList;
public class MainScreen extends AppCompatActivity {
//array list of accounts
ArrayList<Account> accounts = new ArrayList<>();
SharedPreferences accountsPref = getPreferences(MODE_PRIVATE);
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main_screen);
Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
fab.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
startActivity(new Intent(MainScreen.this, CreateAccount.class));
}
});
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.menu_main_screen, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
// Handle action bar item clicks here. The action bar will
// automatically handle clicks on the Home/Up button, so long
// as you specify a parent activity in AndroidManifest.xml.
int id = item.getItemId();
//noinspection SimplifiableIfStatement
if (id == R.id.action_settings) {
return true;
}
return super.onOptionsItemSelected(item);
}
@Override
public void onResume() {
super.onResume();
Gson gson = new Gson();
String json = accountsPref.getString("newAccount", "");
Account newAccount = gson.fromJson(json, Account.class);
//accounts.add(newAccount);
}
}
最佳答案
您需要在两个活动的SharedPreferences accountsPref
中初始化onCreate()
。
将此代码行拆分为两个语句:SharedPreferences accountsPref = getPreferences(MODE_PRIVATE);
SharedPreferences accountsPref
在您当前拥有的全球范围内。accountsPref = getPreferences(MODE_PRIVATE);
内的onCreate()
。onCreate()
是Android框架告诉您活动已初始化的地方。