<CheckBox
    android:id="@+id/checkbox"
    android:radius="5dp"
    android:text="記住我"
    android:layout_marginLeft="20dp"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"/>

public class MainActivity extends AppCompatActivity implements View.OnClickListener{

 private CheckBox checkBox;

 @Override
 protected void onCreate(Bundle savedInstanceState) {

  super.onCreate(savedInstanceState);
  setContentView(R.layout.activity_main);
  checkBox = findViewById(R.id.checkbox);
  SharedPreferences sp = getSharedPreferences("items",MODE_PRIVATE);

  /**
   * getBoolean(name,defaultValue) 若沒有name,則返回defaultValue
   * 獲取標志位，若flag為true，則有數據
   */
  if( sp.getBoolean("flag",false)==true ){
   getData();
   //模擬點擊
   checkBox.performClick();
  }

  //設置checkbox的監聽事件
  checkBox.setOnCheckedChangeListener(new CompoundButton.OnCheckedChangeListener() {
   @Override
   public void onCheckedChanged(CompoundButton compoundButton, boolean isCheck) {

    //若選中，則保存數據，否則，清除數據
    if(isCheck){
     saveData();
     Toast.makeText(MainActivity.this,"已保存",Toast.LENGTH_LONG).show();
    }
    else{
     clearData();
    }
   }
  });
 }
  /**
  * SharedPreferences為輕量級的存儲方式
  * Context.MODE_PRIVATE:為默認操作模式,代表該文件是私有數據,只能被應用本身訪問。
  * 在該模式下,寫入的內容會覆蓋原文件的內容
  * 存儲位置:Device File Explorer（idea 或Android studio右下角）
  * data/data/項目的包名/shared_prefs
  */
 //清除數據
 private void clearData() {
  SharedPreferences sp = getSharedPreferences("items",MODE_PRIVATE);
  SharedPreferences.Editor editor = sp.edit();
  editor.clear();
  editor.commit();
  et1.setText("");
  et2.setText("");
 }
 /**
  * 獲取SharedPreferences數據
  */
 private void getData(){
  SharedPreferences sp = getSharedPreferences("items", Context.MODE_PRIVATE);
  String username = sp.getString("username","");
  String password = sp.getString("password","");
  et1.setText(username);

  //密文設置
  TransformationMethod method = PasswordTransformationMethod.getInstance();
  et2.setText(password);
  et2.setTransformationMethod(method);

 }

 /**
  *保存數據
  * android:onClick響應的方法 只能用修飾符public
  */
 public void saveData(){


  SharedPreferences sharedPreferences = getSharedPreferences("items", Context.MODE_PRIVATE);
  SharedPreferences.Editor editor = sharedPreferences.edit();

  String username = et1.getText().toString();
  String password = et2.getText().toString();

  if(!username.equals("") && !password.equals("")) {

   editor.putString("username", username);
   editor.putString("password", password);
   editor.putBoolean("flag", true); //標志位
   editor.commit();
  }

 }
}