如何计算饼图中显示的日期与当前日期之间的差异

Question

I have an SQLite database containing a number of tasks and the dates on which these tasks were completed. I am then adding these dates as PieEntries in a piechart. I would like to find the difference between the dates on the chart and the current date.

The idea would be to set different colors to different segments of the chart depending on the difference between the dates.

I've found a few posts on here such as this one that I've been using : Android difference between Two Dates . But so far nothing seems to be working.

Below is my cursor cycling through the database and using that to get the dates and place them in to PieEntry List. There is also the code I'm using to calculate the current date and the code to calculate the difference between those that populate the chart and the current date.

final ArrayList<TaskObject> taskObjects = new ArrayList<>();

    if (mCursor != null) {
        mCursor.moveToFirst();
        do {
            TaskObject taskObject = new TaskObject();
            taskObject.setTask(mCursor.getString(mCursor.getColumnIndex(TaskEntry.COLUMN_TASK)));
            taskObject.setObserver(mCursor.getString(mCursor.getColumnIndex(TaskEntry.COLUMN_OBSERVER)));
            taskObject.setDate(mCursor.getString(mCursor.getColumnIndex(TaskEntry.COLUMN_DATE)));
            taskObjects.add(taskObject);
        } while (mCursor.moveToNext());

        float distribution = 100 / taskObjects.size();

        List<PieEntry> pieEntries = new ArrayList<>();
        for (int i = 0; i < taskObjects.size(); i++) {
            pieEntries.add(new PieEntry(distribution, (taskObjects.get(i).getTask()) + "\n" + taskObjects.get(i).getDate()));

            SimpleDateFormat simpleDateFormat = new SimpleDateFormat("dd/MM/yyyy");
            String pastDateString = taskObjects.get(i).getDate();
            Date currentDate = Calendar.getInstance().getTime();
            Date pastDate = null;
            try {
                pastDate = simpleDateFormat.parse(pastDateString);
            } catch (ParseException e) {
                e.printStackTrace();
            }

            //Comparing dates
            long difference = Math.abs(currentDate.getTime() - pastDate.getTime());
            long differenceDates = difference / (24 * 60 * 60 * 1000);

            //Convert long to String
            String dayDifference = Long.toString(differenceDates);

            Log.e("HERE","HERE: " + dayDifference);
        }

        PieDataSet pieDataSet = new PieDataSet(pieEntries, "Label");
        pieDataSet.setColors(ColorTemplate.MATERIAL_COLORS);
        PieData pieData = new PieData(pieDataSet);

        mPieChart.setData(pieData);
        mPieChart.invalidate();
    }
    mCursor.close();

The idea is to subtract the pastDate from the currentDate to reveal an integer value that could then be used to change the color of the piechart segment.

EDIT:

            List<PieEntry> pieEntries = new ArrayList<>();
            for (int i = 0; i < taskObjects.size(); i++) {
                pieEntries.add(new PieEntry(distribution, (taskObjects.get(i).getTask()) + "\n" + taskObjects.get(i).getDate()));

                mPieDataSet = new PieDataSet(pieEntries, "Versatility Matrix & Qualification Status");

                String pastDateString = taskObjects.get(i).getDate();
                SimpleDateFormat simpleDateFormat = new SimpleDateFormat("dd/MM/yyyy");
                Date d = null;
                try {
                    d = simpleDateFormat.parse(pastDateString);//catch exception
                } catch (ParseException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }

                Calendar pastDate = Calendar.getInstance();
                pastDate.setTime(d);

                Calendar currentDate = Calendar.getInstance();

                long diff = currentDate.getTimeInMillis() - pastDate.getTimeInMillis();
                long days = diff/(24*60*60*1000);
                int daysDifference = (int) days;
                Log.d(TAG, "days in difference = " + daysDifference);


                if(daysDifference >= 90) {
                    mPieDataSet.setColor(Color.RED);
                } else if (daysDifference < 90 && daysDifference >= 60) {
                    mPieDataSet.setColor(Color.YELLOW);
                } else {
                    mPieDataSet.setColor(Color.GREEN);
                }
            }

            PieData pieData = new PieData(mPieDataSet);
            mPieChart.setData(pieData);
            mPieChart.invalidate();

This new code doesn't return an integer value for the difference in days and all chart segments are showing as red currently.

Answer 1

It's a guess: When you do mPieDataSet.setColor(Color.RED); , I think you are setting the colour on the entire pie, not just on the individual slice/pie entry. In this way if the last entry you are processing in your loop is 90 days old, all of your pie chart gets the red colour.

I have not been able to reproduce a wrong calculation in your code where the count of days would go over 90 when it shouldn't. If I set pastDateString to 4/1/2019 , I get 88 days, and your if / else construct chooses the YELLOW way through.

Disclaimer: I don't know MPAndroidChart.

General recommendations

If your database has a date datatype, use it for dates. Don't store them as strings. Second, consider not using SimpleDateFormat , Calendar and Date . They are poorly designed and long outdated. You may add ThreeTenABP to your Android project and use LocalDate and other classes from java.time, the modern Java date and time API. It is so much nicer to work with. And gives much better support for calculation of differences between dates:

    long days = ChronoUnit.DAYS.between(pastLocalDate, LocalDate.now(yourTimeZone));

Links

• Oracle tutorial: Date Time , explaining how to use java.time .
• ThreeTen Backport project
• ThreeTenABP , Android edition of ThreeTen Backport
• Question: How to use ThreeTenABP in Android Project , with a very thorough explanation.
• Java Specification Request (JSR) 310 .