android - 初始化 SharedPreferences 给出 : Attempt to invoke virtual method on a null object reference

Question

当我启动我的应用程序时，它崩溃了，并给我这个错误代码。

Attempt to invoke virtual method 'android.content.SharedPreferences android.content.Context.getSharedPreferences(java.lang.String, int)' on a null object reference

这是我的主要 Activity :

package com.awplicity.testappshared;

import android.content.Context;
import android.content.Intent;
import android.content.SharedPreferences;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;

public class MainActivity extends AppCompatActivity {
    SharedPreferences sharedPreferences = getSharedPreferences("", Context.MODE_PRIVATE);
    SharedPreferences.Editor editor = sharedPreferences.edit();
    Button button1;

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

    private void onClick(View v) {
        switch(v.getId()) {
            case R.id.nextAct:
                editor.putString("mystring", "Hi");
                editor.commit();
                startActivity(new Intent("com.awplicity.testappshared.Main2Activity"));
                break;
        }
    }
}

这是第二个 Activity :

package com.awplicity.testappshared;

import android.content.Context;
import android.content.SharedPreferences;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.TextView;

public class Main2Activity extends AppCompatActivity {

    TextView tv;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main2);
        tv = (TextView) findViewById(R.id.tv_main2);
        setText();
    }

    public void setText() {
        SharedPreferences sharedPreferences = getSharedPreferences("", Context.MODE_PRIVATE);
        tv.setText(sharedPreferences.getString("mystring", ""));
    }
}

Answer 1

一般来说，在 onCreate() 内部之前，您不能调用从 Activity(或子类，如 AppCompatActivity)继承的方法方法。到那时，事情还没有准备好。

因此，改变:

SharedPreferences sharedPreferences = getSharedPreferences("", Context.MODE_PRIVATE);
SharedPreferences.Editor editor = sharedPreferences.edit();

到:

SharedPreferences sharedPreferences;
SharedPreferences.Editor editor;

并在 onCreate() 中初始化这些字段:

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

    sharedPreferences = getSharedPreferences("", Context.MODE_PRIVATE);
    editor = sharedPreferences.edit();
}