Question

自从我开始在android项目上使用java编程时，我没有遇到问题：非静态字段无法从静态上下文中引用。

这是我的代码：

package com.esmad.pdm.friendlymanager;

import android.app.DatePickerDialog;
import android.app.Dialog;
import android.app.TimePickerDialog;
import android.content.DialogInterface;
import android.content.Intent;
import java.util.Calendar;
import android.media.Image;
import android.os.Build;
import android.support.annotation.IdRes;
import android.support.annotation.RequiresApi;
import android.support.design.widget.TextInputLayout;
import android.support.v4.app.DialogFragment;
import android.support.v7.app.AlertDialog;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.text.format.DateFormat;
import android.util.Log;
import android.view.View;
import android.widget.DatePicker;
import android.widget.EditText;
import android.widget.ImageView;
import android.widget.RadioGroup;
import android.widget.TextView;
import android.widget.TimePicker;


public class NewMatch extends AppCompatActivity {

    ImageView questionMark;
    RadioGroup radioGroup;
    EditText gameEventNameTxt;
    TextInputLayout textInputLayout;
    TextView dateHourTxt;
    public static boolean changedData = false;
    public static boolean changedHour = false;


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

        questionMark = (ImageView) findViewById(R.id.questionMark);
        questionMark.setClickable(true);

        gameEventNameTxt = (EditText)findViewById(R.id.gameEvent);
        textInputLayout = (TextInputLayout)findViewById(R.id.textInputLayout);

        radioGroup = (RadioGroup) findViewById(R.id.radioGroup);
        int checkedRadioButtonId = radioGroup.getCheckedRadioButtonId();

        dateHourTxt = (TextView)findViewById(R.id.dateHour);

        radioGroup.setOnCheckedChangeListener(new RadioGroup.OnCheckedChangeListener(){

            @Override
            public void onCheckedChanged(RadioGroup group, @IdRes int checkedId) {

                if (checkedId == R.id.event) {
                    textInputLayout.setHint("Event name");
                }
                else{
                    textInputLayout.setHint("Match name");
                }
            }
        });


    }


            public void imageClick(View view) {
                Log.d("builderH","im in the builder");
                AlertDialog.Builder builder = new AlertDialog.Builder(NewMatch.this);
                builder.setTitle("What is a event?");
                builder.setMessage("You can create a Event and a game, a Event is a list of games that can be repeated without the need to create every week a new game!");
                builder.setCancelable(false);
                builder.setPositiveButton("I got it!", new DialogInterface.OnClickListener() {
                    @Override
                    public void onClick(DialogInterface dialog, int which) {
                        dialog.cancel();
                    }
                });
                builder.show();
            }

    public static class TimePickerFragment extends DialogFragment
            implements TimePickerDialog.OnTimeSetListener {

        @RequiresApi(api = Build.VERSION_CODES.N)
        @Override
        public Dialog onCreateDialog(Bundle savedInstanceState) {
            // Use the current time as the default values for the picker
            final Calendar c = Calendar.getInstance();
            int hour = c.get(Calendar.HOUR_OF_DAY);
            int minute = c.get(Calendar.MINUTE);

            // Create a new instance of TimePickerDialog and return it
            return new TimePickerDialog(getActivity(), this, hour, minute,
                    DateFormat.is24HourFormat(getActivity()));
        }

        public void onTimeSet(TimePicker view, int hourOfDay, int minute) {
            changedHour = true;
            if(changedHour && changedData){
                dateHourTxt
            }
        }
    }

    public void hourPicker(View view){
        DialogFragment newFragment = new TimePickerFragment();
        newFragment.show(getSupportFragmentManager(), "timePicker");
    }

    public static class DatePickerFragment extends DialogFragment
            implements DatePickerDialog.OnDateSetListener {

        @Override
        public Dialog onCreateDialog(Bundle savedInstanceState) {
            // Use the current date as the default date in the picker
            final Calendar c = Calendar.getInstance();
            int year = c.get(Calendar.YEAR);
            int month = c.get(Calendar.MONTH);
            int day = c.get(Calendar.DAY_OF_MONTH);

            // Create a new instance of DatePickerDialog and return it
            return new DatePickerDialog(getActivity(), this, year, month, day);
        }

        public void onDateSet(DatePicker view, int year, int month, int day) {
            changedData = true;
        }
    }

    public void datePicker(View view){
        DialogFragment newFragment = new DatePickerFragment();
        newFragment.show(getSupportFragmentManager(), "datePicker");
    }


    }

正如你们上面所见，我在初始化时将一些数据更改为静态，但在其他活动中我并不需要这样做，而且我的onTimeSet方法中的dateHourTxt也发生了同样的错误：S

你们知道为什么会这样吗？

Answer 1

您从一个完全独立的类引用dateHourTxt，该类不包含dateHourTxt。

您的onTimeSet()方法位于TimePickerFragment。那是一个static类。虽然它的行实际位于NewMatch内，但它无法访问NewMatch上的常规字段。 TimePickerFragment可以很容易地成为一个单独的Java类，在它自己的文件中。事实上，在您获得更多Java经验之前，我建议您这样做，以帮助您更好地可视化类之间的分离。

如果您希望TimePickerFragment与NewMatch一起使用，则需要使用典型技术，例如在片段中使用getActivity()来获取NewMatch实例，所以你可以调用方法。

Answer 2

当您使嵌套类静态时，它不再能够访问外部类非静态字段。它只能访问静态字段。在您的示例解决方案中，使用Fragment.getActivity方法访问NewMatch。它应该如下所示，

        if(changedHour && changedData){
            if (getActivity() != null && getActivity() instanceof NewMatch) {
              ((NewMatch)getActivity()).dateHourTxt = ...
            }
        }

为什么它被认为是非静态场？

2 个答案: