为什么我要将上下文作为接口的实例进行降级?

我刚开始在Android上学习fragment API。 我只想给我的包含活动发一条消息(我做了)。现在我想澄清一个关于下压的误解。

这是我的片段:

import android.content.Context;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.EditText;

public class DetailFragment extends Fragment {

private EditText textFirstName, textLastName, textAge;
private FragmentListener mListener;

public DetailFragment() {
}

@Override
public void onAttach(Context context) {
    super.onAttach(context);

    if (!(context instanceof FragmentListener)) throw new AssertionError();
    mListener = (FragmentListener) context;
}

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    View rootView = inflater.inflate(R.layout.fragment_detail, container, false);

    textFirstName = (EditText) rootView.findViewById(R.id.textFirstName);
    textLastName = (EditText) rootView.findViewById(R.id.textLastName);
    textAge = (EditText) rootView.findViewById(R.id.textAge);

    Button doneButton = (Button) rootView.findViewById(R.id.done_button);
    doneButton.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            done();
        }
    });
    return rootView;
}

private void done() {
    if (mListener == null) {
        throw new AssertionError();
    }

    String firstName = textFirstName.getText().toString();
    String lastName = textLastName.getText().toString();
    int age = Integer.valueOf(textAge.getText().toString());

    mListener.onFragmentFinish(firstName, lastName, age);
}

public interface FragmentListener {
    void onFragmentFinish(String firstName, String lastName, int age);
}

}

我不理解这里的贬低:

mListener = (FragmentListener) context;

上下文类如何与我的FragmentListener接口相关?

我发现这与我对向下投射(向下投射是投射到子类型,向下投射到继承树)的了解是矛盾的。