运行时程序崩溃，在“ openFileOutput”上引发错误

Question

我开始在Android设备下学习Java，并尝试编写一个简单的示例。单击按钮后，程序应写入文件。单击按钮后，主类MainActivity从fileWrite类中调用方法ReadWriteFile，但出现错误。 ReadWriteFile类在单独的文件中描述。我做错了什么？

MainActivity.java：

public class MainActivity extends AppCompatActivity implements View.OnClickListener {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Button button = (Button) findViewById(R.id.button);
        button.setOnClickListener(this);
    }
    @Override
    public void onClick(View view) {
        ReadWriteFile readWriteFile = new ReadWriteFile();
        readWriteFile.fileWrite("test.txt","test text");
    }
}

ReadWriteFile.java：

public class ReadWriteFile {
    Context context;
    public void ReadWriteFile (Context context){
        this.context = context;
    }
    public void fileWrite (String fileName, String inputData){
        try {
            FileOutputStream fos;
            fos = context.openFileOutput(fileName,Context.MODE_PRIVATE);
            OutputStreamWriter osw = new OutputStreamWriter(fos);
            BufferedWriter bw = new BufferedWriter(osw);
            bw.write(inputData);
            bw.close();
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

错误：

D/AndroidRuntime: Shutting down VM
E/AndroidRuntime: FATAL EXCEPTION: main
    Process: ru.test.test, PID: 8751
    java.lang.NullPointerException: Attempt to invoke virtual method 'java.io.FileOutputStream android.content.Context.openFileOutput(java.lang.String, int)' on a null object reference
        at ru.test.test.ReadWriteFile.fileWrite(ReadWriteFile.java:22)
        at ru.test.test.MainActivity.onClick(MainActivity.java:20)
        at android.view.View.performClick(View.java:5610)
        at android.view.View$PerformClick.run(View.java:22265)
        at android.os.Handler.handleCallback(Handler.java:751)
        at android.os.Handler.dispatchMessage(Handler.java:95)
        at android.os.Looper.loop(Looper.java:154)
        at android.app.ActivityThread.main(ActivityThread.java:6077)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:866)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:756)
Application terminated.