在 SharedPreferences 中保存对象并使用 GSON 从 Android Studio 中的 Activity 中读取它

Question

我查看了 StackOverflow 上的所有答案，但没有弄清楚为什么会出现错误：Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.content .Context.getPackageName()' 在空对象引用上。我下载了 gson2.2.2 并将编译行放在 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);
        }
    }
}

这是我试图加载对象的 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);
    } 
}

Answer 1

您需要在两个活动的onCreate() 中初始化SharedPreferences accountsPref。

将这行代码拆分成两条语句： SharedPreferences accountsPref = getPreferences(MODE_PRIVATE);

• SharedPreferences accountsPref 在您当前拥有的全局范围内。

• accountsPref = getPreferences(MODE_PRIVATE); 内onCreate()。

onCreate() 是 Android 框架告诉您活动已初始化的地方。