具体内容参考下面代码的 save 和 load 部分：

import android.app.Activity;
import android.content.Context;
import android.os.Bundle;
import android.view.View;
import android.widget.EditText;

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;

public class MainActivity extends Activity {

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

        ActivitiesList.add(this);
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        ActivitiesList.remove(this);
    }

    public void save(View view) {
        FileOutputStream outputStream = null;
        BufferedWriter writer = null;

        try {
            outputStream = openFileOutput("data", Context.MODE_PRIVATE);
            writer = new BufferedWriter(new OutputStreamWriter(outputStream));
            EditText editText = findViewById(R.id.showText);
            if (editText != null) {
                writer.write(editText.getText().toString());
            }
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            try {
                if (writer != null) {
                    writer.close();
                }
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }

    public void load(View view) {
        FileInputStream inputStream = null;
        BufferedReader reader = null;
        StringBuilder content = new StringBuilder();
        try {
            inputStream = openFileInput("data");
            reader = new BufferedReader(new InputStreamReader(inputStream));
            String line;
            while ((line=reader.readLine()) != null) {
                content.append(line);
            }
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            if (reader != null) {
                try {
                    reader.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }

        EditText editText = findViewById(R.id.showText);
        if (editText != null) {
            editText.append(content.toString());
        }
    }
}