一个简单的记事本   

外部存储需要内存卡读取写入权限

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:padding="10dp"
    tools:context="com.ccyumo.demo.MainActivity">

    <EditText
        android:id="@+id/content"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:layout_weight="9"
        android:background="#ffa200"
        android:gravity="top|left"
        android:scrollbars="vertical"
        android:text="alertDialog"
        android:padding="10dp"
        android:layout_marginBottom="10dp"/>
    <Button
        android:id="@+id/save_btn"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:layout_weight="1"
        android:gravity="center"
        android:background="@color/colorPrimary"
        android:text="保存"/>


</LinearLayout>

MainActivity.java

public class MainActivity extends AppCompatActivity {
    EditText content;
    Button save_btn;
    //File file=; 外部存储代码
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        content = (EditText) findViewById(R.id.content);
        save_btn = (Button) findViewById(R.id.save_btn);
        //file=new File(Environment.getExternalStorageDirectory(),"text.text");
        byte buffer[]=null;
        FileInputStream fi = null;
        save_btn.setOnClickListener(new myClick());
        try {
            fi = openFileInput("Demo");//外部存储改为 fi = new FileInputStream(file);
            buffer = new byte[fi.available()];
            fi.read(buffer);

        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            if (fi != null) {
                try {
                    fi.close();
                    String cont=new String(buffer);
                    content.setText(cont);
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }

    }

    //保存按钮监听事件
    public class myClick implements View.OnClickListener {

        @Override
        public void onClick(View v) {
            String con = content.getText().toString();
            FileOutputStream fileOutputStream = null;
            //保存文件
            try {
                fileOutputStream = openFileOutput("Demo", MODE_PRIVATE);//外部存储改为  fileOutputStream = new FileOutputStream(file);
                fileOutputStream.write(con.getBytes());
                fileOutputStream.flush();
            } catch (FileNotFoundException e) {
                e.printStackTrace();
            } catch (IOException e) {
                e.printStackTrace();
            } finally {
                try {
                    if (fileOutputStream != null) {
                        fileOutputStream.close();
                    }
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }


}