Activity以外から更新とSharedPreferencesの使い方

MainActivity.java

import android.content.Context;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity {

    Context context;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        context = this;
        TextView textview1 = findViewById(R.id.textView1);
        textview1.setText("かきくけこ");
        final TextChange text_change = new TextChange(context);

        findViewById(R.id.button1).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                text_change.textChange();
            }
        });
    }

    public static void testTest() {
        Log.d("@@@@@::", "testTest");
    }

}