Initializing SharedPreferences gives: Attempt to invoke virtual method on a null object reference

Question

When I launch my app it just crashes and gives me this error code.:

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

This is my main 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;
        }
    }
}

This is the second 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

Generally speaking, you cannot call methods that you inherit from Activity (or subclasses, like AppCompatActivity) until inside of the onCreate() method. Until then, things are not set up yet.

Hence, change:

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

to:

SharedPreferences sharedPreferences;
SharedPreferences.Editor editor;

and initialize those fields in onCreate():

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

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