Android : Why must a callback to a method in an Android activity be static?

In a barebones Android application, I want to call a method in the MainActivity instance, from an asynchronous callback in the instance of another class. Why does Java force me to call a static method in the class, rather than a non-static method in the instance itself?

My app has a MainActivity class and a TextToSpeech (TTS) class. From the main activity, I instantiate the class, passing a pointer to the MainActivity instance. Instantiation of the TTS engine is an asynchronous operation. I cannot interact with the TTS instance until it has triggered an onInit() method.

Below is code that works. However, I had imagined that I would be able to call a non-static method in the MainActivity instance, and this appears not to be possible. The code below uses a static call to the MainActivity class itself, so no instance variables are available.

Here are the changes I made to a basic Hello World application in Android Studio.

//MainActivity.java
package com.example.callback;

import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.util.Log;


public class MainActivity extends ActionBarActivity {

  private static TTS tts; // apparently this has to be static

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

  public void onResume() {
    super.onResume();
    tts = new TTS();
    Log.d("onResume", this.toString());
    // D/Main﹕ com.example.callback.MainActivity@4a014e50

    tts.init(this);
  }

  // Apparently this method has to be static
  public static void ttsReady() {
    tts.speakText("Hello world");
  }
}

Custom class.

// TTS.java
package com.example.callback;

import android.app.Activity;
import android.content.Context;
import android.speech.tts.TextToSpeech;
import android.util.Log;

import java.util.HashMap;
import java.util.Locale;

public class TTS implements TextToSpeech.OnInitListener {

  private TextToSpeech tts;
  private Activity activity;

  public void init(Activity currentActivity) {
    activity = currentActivity;
    Context context = activity.getApplicationContext();
    tts = new android.speech.tts.TextToSpeech(context, this);
  }

  @Override
  public void onInit(int status) {
    if (status == TextToSpeech.SUCCESS) {
      tts.setLanguage(Locale.UK);
      Log.d("onInit", activity.toString());
      // D/onInit﹕ com.example.callback.MainActivity@4a014e50

      // activity.ttsReady();
      // The line above does not compile. ttsReady() is not recognized as a
      // method of activity, regardless of whether the tts variable and the
      // ttsReady() method in the MainActivity class are made static or not,
      // Making the activity variable static here has no effect either.

      // The commented line below throws Error:(31, 35)
      // error: non-static method toString() cannot be referenced from a static context
      // So presumably MainActivity is a static variable.
      // Log.d("onInit", MainActivity.toString())

      // This works, if the tts variable and the ttsReady() method in the
      // MainActivity class are made static. Is there a non-static alternative?
      MainActivity.ttsReady();
    }
  }

  // Deprecated signature for speak() used for compatibility with API 20
  // and earlier
  public void speakText(String toSpeak) {
    int mode = android.speech.tts.TextToSpeech.QUEUE_FLUSH;
    // Object hashMap = null; // Causes a "no suitable method error".
    // How is HashMap null not the same as Object null? Using just plain null
    // instead of hashMap also works with no problems.
    HashMap hashMap = null;
    tts.speak(toSpeak, mode, hashMap);
  }
}