问题

I'm developing an Android app that uses a database, every time that the user insert a new register the current data and time is save in the db using

Calendar cal = Calendar.getInstance();

So, When I retrieve the data from the db, got a String like this:

java.util.GregorianCalendar[time=1496007575129,areFieldsSet=true,lenient=true,zone=America/Mexico_City,firstDayOfWeek=1,minimalDaysInFirstWeek=1,ERA=1,YEAR=2017,MONTH=4,WEEK_OF_YEAR=22,WEEK_OF_MONTH=5,DAY_OF_MONTH=28,DAY_OF_YEAR=148,DAY_OF_WEEK=1,DAY_OF_WEEK_IN_MONTH=4,AM_PM=1,HOUR=4,HOUR_OF_DAY=16,MINUTE=39,SECOND=35,MILLISECOND=129,ZONE_OFFSET=-21600000,DST_OFFSET=3600000]

The problem comes when I try convert that String using SimpleDateFormat.parse to display it in a RecyclerView, I get always the same date: 09/04/2017.

This is the code in my RecViewAdapter.java:

@Override
public void onBindViewHolder(ViewHolder holder,int position){
    items.moveToPosition(position);

    String s,d,p,f;


    s = items.getString(ConsultaTomas.SISTOLICA);
    holder.systolica.setText(s);

    d = items.getString(ConsultaTomas.DIASTOLICA);
    holder.diastolica.setText(d);

    p = items.getString(ConsultaTomas.PULSO);
    holder.pulso.setText(p);

    f = items.getString(ConsultaTomas.FECHA);
    SimpleDateFormat sdf = new SimpleDateFormat("dd/MM/yyyy");

    try {
        holder.fecha.setText(sdf.format(sdf.parse(f)));

    }catch (ParseException e){
        Log.d("PARSINGFECHA","Error al parcear fecha");
    }


}

The other data is showed correctly in the RecView and the Calendar String are all diferent, so there is not the same date/hour in these strings. So, the question is:

How can I Convert Calendar.toString() into date using SimpleDateFormat.parse()?

This is the result running the app in a real device: two muppets

回答1:

You need to modify the way you store the Calendar, call getTime() and format it as desired to begin with. For example,

Calendar cal = Calendar.getInstance();
DateFormat sdf = new SimpleDateFormat("MM/dd/yyyy");
System.out.println(sdf.format(cal.getTime()));

回答2:

tl;dr

( GregorianCalendar ) myCal                    // Cast the more general `Calendar` to subclass `GregorianCalendar`. 
.toZonedDateTime()                             // Convert from terrible legacy class `GregorianCalendar` to its modern replacement `ZonedDateTime`. 
.toLocalDate()                                 // Extract just the date, without the time-of-day and without the time zone. Returns a `LocalDate` object.
.format(                                       // Generate text representing the value of this `LocalDate` object.
    DateTimeFormatter
    .ofLocalizedDate( FormatStyle.SHORT )      // Automatically localize the text.
    .withLocale( new Locale( "es" , "ES" ) )   // Specify a `Locale` to determine the human language and cultural norms to use in localization.
)

java.time

The modern approach uses the java.time classes that years ago supplanted the terrible legacy classes Calendar & GregorianCalendar.

When you encounter a Calendar object, immediately convert to its replacement, ZonedDateTime, assuming your object is actually a GregorianCalendar.

if( myCal instanceOf GregorianCalendar )
{
    GregorianCalendar gc = ( GregorianCalendar ) myCal ;  // Cast from more general class to the specific class.
    ZonedDateTime zdt = gc.toZonedDateTime() ;
}

You apparently are interested only in the date portion, without the time-of-day and without the time zone. So extract a LocalDate.

LocalDate ld = zdt.toLocalDate() ;  // Ignore time-of-day and time zone.

Generate text in your desired format.

DateTimeFormatter f = DateTimeFormatter.ofPattern( "dd/MM/uuuu" ) ;
String output = ld.format( f ) ;

Or better: Let java.time automatically localize for you.

Locale locale = new Locale( "es" , "ES" ) ;
DateTimeFormatter f = DateTimeFormatter.ofLocalizedDate( FormatStyle.SHORT ).withLocale( locale ) ;

About java.time

The java.time framework is built into Java 8 and later. These classes supplant the troublesome old legacy date-time classes such as java.util.Date, Calendar, & SimpleDateFormat.

To learn more, see the Oracle Tutorial. And search Stack Overflow for many examples and explanations. Specification is JSR 310.

The Joda-Time project, now in maintenance mode, advises migration to the java.time classes.

You may exchange java.time objects directly with your database. Use a JDBC driver compliant with JDBC 4.2 or later. No need for strings, no need for java.sql.* classes. Hibernate 5 & JPA 2.2 support java.time.

Where to obtain the java.time classes?

Java SE 8, Java SE 9, Java SE 10, Java SE 11, and later - Part of the standard Java API with a bundled implementation.
- Java 9 adds some minor features and fixes.
Java SE 6 and Java SE 7
- Most of the java.time functionality is back-ported to Java 6 & 7 in ThreeTen-Backport.
Android
- Later versions of Android bundle implementations of the java.time classes.
- For earlier Android (<26), the ThreeTenABP project adapts ThreeTen-Backport (mentioned above). See How to use ThreeTenABP….

来源：https://stackoverflow.com/questions/44361292/how-can-i-convert-calendar-tostring-into-date-using-simpledateformat-parse

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