java - 变量作用域[JAVA]

Question

在 if 语句中记录时，我可以成功获取“startTime”变量的值。当我尝试从 if 语句外部访问此变量时，我的问题就出现了。

public class MainActivity extends AppCompatActivity {
String startTime = "";
String endTime = "";
....
....
....
public void dailyHours(){
    //Retrieve clockInId (ObjectID for clockInTime)
    ParseQuery<ParseObject> startQuery = ParseQuery.getQuery("clockIn");
    startQuery.getInBackground(clockInId, new GetCallback<ParseObject>() {
        public void done(ParseObject clockIn, ParseException e) {
            if (e == null) {
                startTime = clockIn.getString("Time");
                Log.d(TAG, "StartTime: " + startTime);
            } else {
                // something went wrong
            }
        }
    });

    //Retrieve clockOutId (ObjectID for clockOutTime)
    ParseQuery<ParseObject> endQuery = ParseQuery.getQuery("clockOut");
    endQuery.getInBackground(clockOutId, new GetCallback<ParseObject>() {
        public void done(ParseObject clockOut, ParseException e) {
            if (e == null) {
                endTime = clockOut.getString("Time");
                Log.d(TAG, "EndTime: " + endTime);
            } else {
                // something went wrong
            }
        }
    });

}

例如，当 Log.d() 移至 ParseQuery 外部时，startTime 返回空。

public void dailyHours(){
    //Retrieve clockInId (ObjectID for clockInTime)
    ParseQuery<ParseObject> startQuery = ParseQuery.getQuery("clockIn");
    startQuery.getInBackground(clockInId, new GetCallback<ParseObject>() {
        public void done(ParseObject clockIn, ParseException e) {
            if (e == null) {
                startTime = clockIn.getString("Time");
            } else {
                // something went wrong
            }
        }
    });
    Log.d(TAG, "StartTime: " + startTime); //startTime returns an empty string when moved here.

Answer 1

GetCallback<ParseObject>()方法是异步调用的(在另一个线程中)，所以实际上赋值 startTime = clockIn.getString("Time")可以在Log.d(TAG, "StartTime: " + startTime)之后执行.

这里有“竞争条件”。