java - MainActivity 中带有 saveText 的 Android Java NullPointerException

Question

尝试在 MainActivity 中使用 saveText 时出现 NullPointerException 我是 OOP 和 Java 的菜鸟，我不明白自己做错了什么。 感谢您的帮助。

public class FileIO extends Activity{

    FileInputStream fin;
    FileOutputStream fos;

    public void SaveFile(String FileName, String FileData) {
        try {
            fos = openFileOutput(FileName, MODE_PRIVATE);
            fos.write(FileData.getBytes());
            Toast.makeText(this, "Файл "+FileName+" сохранен", Toast.LENGTH_SHORT).show();
        }
        catch(IOException ex) {
            Toast.makeText(this, ex.getMessage(), Toast.LENGTH_SHORT).show();
        }
        finally{
            try{
                if(fos!=null) {
                    fos.close();
                }
            }
            catch(IOException ex){
                Toast.makeText(this, ex.getMessage(), Toast.LENGTH_SHORT).show();
            }
        }
    }

    public String ReadFile(String FileName) {
        String text = "";

        try {
            fin = openFileInput(FileName);
            byte[] bytes = new byte[fin.available()];
            fin.read(bytes);
            text = new String (bytes);
        }
        catch(IOException ex) {
            Toast.makeText(this, ex.getMessage(), Toast.LENGTH_SHORT).show();
        }
        finally {
            try {
                if (fin != null) {
                    fin.close();
                }
            } catch (IOException ex) {
                Toast.makeText(this, ex.getMessage(), Toast.LENGTH_SHORT).show();
            }
        }

        return text;
    }
}


public class MainActivity extends ActionBarActivity {

    FileInputStream fin;
    FileOutputStream fos;

    FileIO FL = new FileIO();

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }
    // сохранение файла
    public void saveText(View view){
        try {
            EditText textBox = (EditText) findViewById(R.id.save_text);
            String text = textBox.getText().toString();

            FL.SaveFile("content.txt", text);
        }
        catch(Exception ex) {
            Toast.makeText(this, "Ошибка записи файла", Toast.LENGTH_SHORT).show();
            Log.d("MyLog", ex.toString() + "\n" + ex.getMessage() + "\n" + ex.getCause());
        }
    }
    // открытие файла
    public void openText(View view){

        TextView textView = (TextView) findViewById(R.id.open_text);
        try {
            fin = openFileInput("content.txt");
            byte[] bytes = new byte[fin.available()];
            fin.read(bytes);
            String text = new String (bytes);
            textView.setText(text);
        }
        catch(IOException ex) {

            Toast.makeText(this, ex.getMessage(), Toast.LENGTH_SHORT).show();
        }
        finally{

            try{
                if(fin!=null)
                    fin.close();
            }
            catch(IOException ex){

                Toast.makeText(this, ex.getMessage(), Toast.LENGTH_SHORT).show();
            }
        }
    }
}

Answer 1

不要从 Activity 扩展类 FileIO。

更改您的代码

public class FileIO extends Activity{

到

public class FileIO{

在创建对象时将“MainActivity”上下文传递给构造函数中的 FileIO 类，并使用它来显示 Toast 消息。

 FileIO fileIo= new FileIO(MainActivity.this);

希望对你有帮助